Rates of Molecular Evolution of Primates

Russian Journal of Genetics, Vol. 39, No. 7, 2003, pp. 721 736. Translated from Genetika, Vol. 39, No. 7, 2003, pp. 869 887. Original Russian Text Copyright 2003 by Tfitushkin. THEORETICAL PAPERS AND REVIEWS Rates of Molecular Evolution of Primates E. Ya. Tfitushkin Vavilov Institute of General Genetics, Russian Academy of Sciences, Moscow, 119991 Russia Received December 19, 2001 Abstract This review considers the history and the current state of the debatable issue of variation in accumulation rates of mutational substitutions in the phylogeny of humans and other primates. Arguments pro and contra the hypothesis on gradually decreasing tempo of molecular evolution of humans and apes are discussed. It is shown that data on proteins and (nuclear and mitochondrial) DNA confirm this hypothesis. The conclusion is drawn that the total rate of mutation accumulation is determined by a number of interacting factors. The primary of these factors in most cases (including that of hominids) is generation time, which is closely associated with the number of germ cell divisions. At the same time, fixation rates of nonsynonymous substitutions are also affected by various forms of selection. INTRODUCTION Variation in the rate of accumulation of mutational substitutions is currently among the most debatable problems of evolutionary genetics. Until the 1960s these rates were believed to be very unstable. However, in 1962 Zuckerkandl and Pauling (see Russian translation [1]) suggested that the time of divergence of organisms is approximately proportional to the number of amino acid differences in homologous chains of their hemoglobins. A year later a similar suggestion was advanced by Margoliash [2] who conducted a comparative analysis of cytochrome c amino acid sequences. Based on this idea, Zuckerkandl and Pauling formulated the hypothesis on the approximately constant rates of accumulation of point mutations in homologous amino acid sequences of proteins and nucleotide sequences of DNA and RNA [3]. These authors concluded that fixation of mutational substitutions can serve as a molecular evolutionary clock that could be used to determine the time of divergence of taxa, polypeptides, and polynucleotides. This hypothesis (usually referred to as the molecular clock hypothesis) has generated a discussion that has been continuing until now. The opponents of the hypothesis argued that the rates of molecular evolution are very variable and thus the concept of molecular clock is groundless (see [4]). A special place in this dispute is occupied by discussion of the data related to fixation rates of mutational substitutions in the phylogeny of humans and other primates. The interpretation of these data remained a hotly debated issue. In the present review I consider pro and contra argumentation in the discussion of the tempo of molecular evolution and analyze mutation accumulation dynamics in the evolution of hominids and their phylogenetic relatives. A DILEMMA RAISED BY IMMUNOCHEMICAL COMPARISON OF PROTEINS The early immunochemical studies of blood serum proteins of primates conducted in the 1960s [5, 6] have shown that some proteins evolve much faster than others. The proteins with low variability encoded by evolutionary stable genes were termed S (slowly evolving) proteins, and rapidly evolving ones, R (rapidly evolving) proteins. For instance, γ-globulin was assigned to the R type, and albumin, to the S type. The existence of proteins intermediate between these two extreme categories has been suggested. Rapidly evolving (R) proteins were thought to be more species-specific than slowly evolving (S) ones. The results of the same studies demonstrated that the evolution rates of S proteins vary depending on the taxonomic group. It was firmly believed that unequal antigenic distances along with equal divergence times prove inequality of the evolutionary rates of polypeptide sequences. For instance, the divergence of humans with tree shrews occurred at least no later than their divergence with lemurs, but albumins of humans and tree shrews are more similar immunologically than those of humans and lemurs. The splitting of the branches of humans and galagos coincides with that of humans and lemurs. Nevertheless the albumin of galagos was shown to be more similar to the human on than the albumin of lemurs. Therefore, it was concluded that the albumin evolution rate was faster in lemurs than in galagos and tree shrews. In a similar manner, it was shown that the relative evolutionary stability of the human albumin is in contrast with its rapid evolution in artiodactyls [6]. Among the higher primates, the divergence rate of this protein was the highest in platyrrhines (Cebidae), somewhat lower in cercopithecoids, and the lowest in hominoids. These and similar facts laid grounds to the general conclusion on a slowdown of the 1022-7954/03/3907-0721$25.00 2003 MAIK Nauka /Interperiodica

722 T TUSHKIN rates of progressive evolution of the S (but not R) proteins and were used for substantiating the then-popular theory relating evolutionary progress of primates with the evolution of their placenta. According to this theory proposed by Goodman [5 9], the specific features of evolutionary and genetic processes resulting in the appearance of humans and other modern primates were largely caused by the existence of immunochemical differences between homologous proteins in different individuals, i.e., genetic polymorphism of protein molecules. This theory considers the evolution of hominoid proteins as a culmination of the long-term phylogenetic trend appearing at the earliest stages of evolution of the living matter. It postulates that most of the mutations arising in primary organisms were neutral (note that this suggestion was formulated in 1961, that is, long before the neutral theory proposed in 1968). This promoted the accumulation of huge amounts of genetic variation. A small fraction of the neutral alleles turned out to be preadapted to the environmental conditions that appeared later. These alleles were selected for, which increased their frequencies. As a result of adaptation to the changing environments, the living forms have been gradually increasing their complexity. This process also occurred at the molecular level of organization of the living matter. The latter was manifested, in particular, in the formation of numerous interacting protein sites, which enhanced the number of selective constraints on the protein structure. Hence, the proportion of deleterious mutations increased and that of selectively neutral ones, decreased. As a result, deleterious mutational substations became predominant. The reduction in the number of amino acid sites that could accumulate neutral substitutions led to the slow down of the protein evolution. Placental mammals have an additional factor decreasing the rates of protein divergence: immunological selection against fetal antigens differing from the corresponding maternal ones. This selection is particularly strong in higher primates because these animals possess the hemochorial placenta, which is more permeable than the more ancient epitheliochorial placentas. In 1967, employing microcomplement fixation, Sarich and Wilson [10] checked the assumption on the regularity of molecular evolutionary rates. Using the relative rate test * they concluded that the evolutionary rates * This test is based on comparing homologous informational macromolecules of three types (e.g., A, B, and C), for which is known that two of them (say, B and C) diverged after splitting with the third one (A). The test proper consists in comparing the differences between the macromolecules of the A and B types with that differences between the corresponding molecules of A and C. If the molecular clock hypothesis is valid, then these differences, or, more specifically, the number of molecular substitutions causing them, would be approximately equal. Sarich and Wilson [10] showed, in particular, that during the evolution of the cercopithecoids and humans from their nearest common ancestor (having existed, as they believed, about 30 Myr ago), their albumins changed almost to the same extent. Based on this result, the authors concluded that the molecular clock hypothesis was valid. of this protein are equal in the phylogenetic lineages of hominoids (including human), cercopithecoids, and ceboids. According to Sarich and Wison s estimates, humans and African apes have diverged 4 to 5 Myr ago although most authors at that time believed that this event had occurred 15 20 Myr ago. Based on these findings, Sarich criticized the placental hypothesis, which, in his view, should have been checked up by its author, Goodman, on the basis of his own immunological data without involving the fossil and anatomical data [11]. According to Sarich, the albumin of an average monkey evolved even faster that of an average prosimian, which radically contradicts one of the basic tenets of the above hypothesis. However, the results of Sarich and Wilson were soon subjected to criticism by some paleontologists who did not agree with their dating of the primate time divergence and immunogeneticists raising objections to their methodological approaches (see [12, 13] for references). Having compared antigenic determinants of 37 serum proteins, Bauer [12] obtain datings that coincided with the paleontological ones accepted at that time. According to his calculations, the nearest common relatives of humans with pongids, cercopithecoids, platyrrhines, and prosimians existed respectively 13.9 (but not 4 5, as claimed by Sarich and Wilson), 28.3, 54.9, and 63.3 Myr ago. In a later work, other authors [13] obtained (from the data of Sarich and Wilson for albumins) the same or even higher estimates. What is the cause of such great discrepancy in datings based on the same materials? Here some explanations of Sarich and Wilson s methods would be useful. These authors used the technique of microcomplement fixation, which consists in conducting a cross reaction between a purified antiserum and an antigen of the examined species and selecting (by a gradual increase) of such a antiserum concentration at which the intensity of the reaction between heterologous individuals would be the same as between homologous ones. The index by which the initial antiserum concentration is increased is called the dissimilarity index (ID). For homologous individuals ID is 1, and its value increases with the time of divergence from the common ancestor. Knowing the relationship between the dissimilarity index and the time, a corresponding immunological clock can be constructed. Sarich and Wilson ([14 16], cited from [17]) have established this relationship on the basis of the data of Wilson et al. ([18], cited from [17]) on an immunological comparison of several species of birds, reptiles, and amphibians and the known paleontological datings. They concluded that this relationship is best approximated by an exponential function ID = e kt. The same dependence was suggested for primates. The datings obtained by these authors were based on a comparison of only one protein, albumin, while the calculation was founded on an assumption that the lineages of hominoids and cercopithecoids had diverged 30 Myr ago. In

RATES OF MOLECULAR EVOLUTION OF PRIMATES 723 particular, the conclusion that humans and African apes had diverged about 5 Myr ago was drawn from these assumptions. Since the latter estimate of the time of divergence between hominids and pongids was at variance with the existing views, Read and Lestrel [17] made an attempt at checking whether the results of Wilson et al. correspond to the exponential model. Their approximation of the ID time relationship by a linear (ID = kt + b), power (ID = bt k + c), and exponential (ID = e kt ) functions demonstrated that these data are best described by a power function. Using a set of paleontologically acceptable datings of the divergence between humans and cercopithecoids, the power function gives the time of divergence between human and great apes ranging from 13 to 31 Myr. This was absolutely compatible with the views accepted in the 1960s. Nevertheless, Read and Lestrel suggested that these data are not sufficient for making a conclusive choice among the three possible models. Bauer [12] has accepted the linear model because he expected deviations from linearity only in comparisons going beyond the order of primates. Calibration of the time scale is based on the assumption that the last common ancestor of all placental mammals existed 70 Myr ago. Lovejoy et al. [13] have implemented an essentially different approach. These authors cast doubt on the accuracy of the estimations by their colleagues, including Sarich and Wilson. Lovejoy et al. thought it erroneous to use a geological time scale in estimating amino acid substitution rates because these rates primarily depend on generation length. Based on the degree of neocortex development, they determined generation times of extinct primate taxa and recalculated the divergence times using the same immunological data of Sarich and Wilson. It turned out that hominids and pongids have diverged 14 Myr ago or even earlier. In the overviews of the mid-1970s [19, 20], which in essence summarized the immunochemical studies, the proponents of the hypothesis on the constancy of the protein evolutionary rates in primates and the advocates of the concept on their irregularity both remained convinced in their views. The former (see Sarich and Cronin [19]) agreed that the rates of protein evolution are in some cases irregular. Moreover, using their own material, they revealed a number of such cases as a slow rate of albumin evolution in the lineage of owl monkey (Aotus) and a rapid rate, in anthropoids and phaners (squirrel lemurs) as well as slow evolution of transferrin in the phaner lineage and rapid, in loris. However, all these examples were regarded as exceptions to the rule of constant substitution rates in proteins and nucleic acids. Responding to criticisms in line of those considered above, these authors reasonably argued that the data of Sarich could comply to a multitude of equations and thus can easily be used to produce datings acceptable for different groups of paleontologists. In other words, the critics were accused of forcing the model to fit the given results. The latter (Dene et al. [20]) analyzing their impressively extensive data obtained in immunodiffusion comparisons of blood serum proteins, emphasized that the levels of antigenic species divergence within the genera of catarrhines is substantially lower than that in other mammals including primates. The same is observed in comparisons of larger taxa within superfamilies. Hence, it is concluded that one of two explanations is possible: either the catarrhines are evolutionarily younger than thought before or their antigen evolution slowed down. Having considered some paleontological and morphological data, the authors have chosen the second alternative, i.e., came to the conclusion that the serum proteins in catarrhines evolved slower than in nonanthropoid primates or nonprimate mammals. RATES OF EVOLUTION OF AMINO ACID SEQUENCES The comparative studies of amino acid sequencing were summed up in the mid-1980s since shortly after researchers in the reviewed field have almost completely shifted their attention to direct comparisons of nucleotide sequences. Analysis of the primary protein structure has extended the discussion of the proponents and opponents of the molecular clock theory. The key argumentation in the debate on neutrality of molecular evolution has been formed on the basis of the same material and in relation to this discussion. The estimation of the number of differences at amino acid sites permits evaluation of the number of amino acid, and later, nucleotide, substitutions that led to these differences. The possibility appeared to express the rates of molecular evolution directly in the number of mutational substitutions in unit time. Scoring of the amino acid differences implies detection of homologous sequences and collation of these sequences with localization of deletions and insertions. To do this, special methods of sequence alignment are used (see [21] for review). Based on some assumptions, the rate of molecular evolution can be determined from the estimated proportion of amino acid differences or the so-called amino acid distance d = n/n, where n is the number of homologous sites with differing amino acids and N is the total number of the amino acid pairs under comparison. Most authors accept the assumption that the number of amino acid substitutions follows the Poisson distribution. As early as in the late 1960s on the basis of these assumption Kimura [22] has estimated the evolutionary rates of the α- and β-hemoglobin chains in some lineages of highly diverged vertebrate species and found these rate strikingly constant. He believed that this constancy confirms his neutral theory of molecular evo-

724 T TUSHKIN lution (extensive substantiation of this thesis is presented in [23 25] and critique, in review [4]). In their polemics with Kimura, Langley and Fitch [26] conducted a detailed statistical analysis of the evolutionary rates of the α- and β-chains of hemoglobin, cytochrome c, fibrinopeptide A and concluded that they are not constant. These authors employed the assumption of minimum evolution and the maximum likelihood method. They found that the variability of evolutionary rates expressed as nucleotide substitution numbers was about twice as high as expected. However, the same authors in the same work stated that the estimates of the divergence time obtained from the calculated nucleotide substitution numbers are in good agreement with paleontological datings taken from an independent source. The calculated numbers of mutational substitutions plotted against the corresponding geological times showed a linear relationship, which, in view of Kimura [25], testified to constant rates of molecular evolution. The points belonging to primates are a notable exception. They are located below the line, which indicates a slowdown in evolutionary rates of proteins in the phylogeny of these animals. At this time, i.e., in the early 1970s, the same set of data was addressed by Goodman. This author used the results of the amino acid sequence comparisons to develop his idea on a slowdown of molecular evolution in humans and other primates [27, 28]. He carried out phylogenetic analysis on the basis of amino acid differences and minimal mutation distances obtained by their transformation. In addition, he implemented the approach based on the maximum parsimony method. In the first case, a direct calculation of homologous positions containing different amino acids was performed for each species pair. The proportion of such positions is a measure of genetic distance. Matrices of these distances termed amino acid differences (AAD) can be used to determine the order of species divergence. A more adequate measure of genetic distance is the minimal mutation distance (MMD), which is based on the values of amino acid differences. To calculate MMD, for each pair of differing homologous amino acids the minimal number of nucleotides is estimated that must be replaced to transform the codon of one amino acid into that of the other. For this, a special conversion table is used [29]. Since the maximum possible number of nucleotide substitutions for one compared amino acid pairs is equal to three, this measure does not take into account multiple and synonymous mutations. The advantage of minimal mutation differences over amino acid differences lies in the fact that the former better fit the additive hypothesis according to which genetic distances must be strictly directly proportional to the number of mutations that are recorded during the divergence of the compared species from the common ancestor [30, 31]. This advantage does not manifest in the case of closely related species and grows with the time of divergence of the amino acid sequences. The most powerful tool of analysis of molecular evolution is, according to Goodman, the maximum parsimony method. This method is based on a hypothesis stating that evolutionary changes in general are implemented by the shortest possible pathway. The procedure consists in constructing maximum parsimonious phylogenetic trees (in which the number of mutations separating the descendants and the ancestors is minimized) and selection of the tree of the minimum total length via iteration. The distance between the species estimated by the lengths of the chosen tree and expressed as the number of mutational substitutions is called maximum parsimonious distance (MPD). On the maximum parsimonious tree, two mutations can be separated by more than one mutational substitution at one nucleotide position. In other words, this method accounts for multiple and reverse mutations. It is clear that MPD cannot be smaller than MMD. Using single polypeptides and a set of them, Goodman and Lasker [32] calculated distances of the three types for a number of mammalian species (mostly primates). The conclusions on the primate phylogeny obtained from analyses of AAD, MMD, and MPD, in general coincide. However, comprehensive phylogenetic analysis was carried out only for the results obtained by the maximum parsimony method. Consideration of the maximum parsimonious trees confirmed the view on significant irregularity of evolutionary rates of homologous proteins. This irregularity manifests both within a lineage and among the branches [27, 28]. This statement can be readily proven since the number of fixed mutations, including recurrent ones, is known for each branch of the parsimonious tree. Knowing these numbers, the rates of molecular evolution can be calculated. These rates can be expressed, e.g., in NR%, that is, in the number of nucleotide replacements per 100 codons in 10 8 years. There are many examples of irregular evolution of certain proteins (see [33 36]). In this context, of far more interest are the cases of changes in rates of molecular evolution in general, i.e., concerning the total set of proteins examined (hemoglobin chains, myoglobins, fibrinopeptides, cytochrome c, crystallins of the eye lens, and carboanhydrases were mainly sequenced). In the human lineage, several periods of acceleration and deceleration of the rates of mutation accumulation were revealed for the past 700 Myr [33]. About 500 Myr ago, at the early stage of the evolution of vertebrates, when they had divided into Agnata and Gnatostomata, the process of mutation fixation became considerably faster. This period of high rate of molecular evolution terminated about 300 Myr ago, at the time when the common ancestor of birds and mammals appeared. The rates dropped and remained low until the time of appearance of placental mammals, i.e., according to the most popular dating, 90 Myr ago. During the

RATES OF MOLECULAR EVOLUTION OF PRIMATES 725 initial differentiation of primates, which commenced about 65 Myr ago, the protein evolution rates were very high. For instance, during this period, crystallins and cytochrome c evolved 10 to 20 times and globins and carboanhydrase I, 2 to 5 times as fast as during the preceding period. However, when higher primates appeared about 40 Myr ago, rates of molecular evolution decreased again, which is particularly apparent in the evolutionary history of hominoids. Thus, proteins mostly underwent changes in two parts of the phylogeny: (1) between the ancestors of all vertebrates and the common ancestors of mammals and birds and (2) between the ancestors of placental mammals (Eutheria) and those of higher primates (Anthropoidea). The rates of protein evolution in closest predecessors of humans should be considered as a special issue. After the appearance of higher primates, these rates decreased at least twofold. In the human and chimpanzee lineages they almost dropped to zero [33, 34]. Since the splitting of their common stem from the orangutan branch, in the genes for six polypeptides (namely, α- and β-hemoglobin chains, myoglobin, fibrinipeptides A and B, carboanhydrase I collectively consisting of 735 amino acids) have accumulated only 11 nucleotide substitutions. At that, human and chimpanzee is separated only by five mutations one of which was fixed in Homo (in carboanhydrase I) and four, in Pan (one in myoglobin and three in carboanhydrase I). If the common ancestor of these organisms existed 5 6 Myr ago, the rate of molecular evolution in the period from its appearance to the modern human is about 2.7 NR%. At the same time, the averaged lineage of the currently existing placental mammals has accumulated 140 nucleotide substitutions during the 80 90 Myr elapsed from the divergence of the last common ancestor. Thus, the characteristic rate of molecular evolution for Eutheria is 22 NR%, which is 7 times higher than in the direct predecessors of Homo sapiens. EVOLUTIONARY RATES OF NUCLEOTIDE SEQUENCES The data on nucleotide sequences are more informative than those on proteins. They permit more extensive and in-depth study of molecular evolution. For instance, comparing nucleotide sequences, one can estimate rates of evolution of both coding and noncoding sequences whereas comparing coding sequences provides material to analyze accumulation of synonymous and nonsynonymous substitutions. Owing to the degeneracy of the genetic code, the frequency of nucleotide substitutions is higher than that of amino acid substitutions. As expected, comparative studies of coding nucleotide sequences have confirmed the conclusions made from analysis of amino acid sequences. Hominoid slowdown (as the discussed phenomenon is often called) in the coding DNA evolution was found already in the earliest of these studies carried out in the mid- 1980s, which compared orthologous and paralogous γ-globin genes in human, chimpanzee, gorilla and two nonallelic α-globin genes in chimpanzee [37, 38] (see [39] for information on globin genes of primates). It was shown that the fixation rate (frequency) of nonsynonymous nucleotide substitutions in the phylogeny of human and great apes during the last 10 Myr was only 0.1 10 9 substitutions per site per year. This is 18 times lower than the corresponding rate in the early placental mammals and 9 times lower than the average fixation rate during the divergence of the mammalian orders (during approximately the last 80 Myr), which is 0.88 10 9 substitutions par site per year for 39 loci [40] and about the same as only for the globin (α, ε, γ, β) genes (in the latter case, the calculations were conducted for four orders Primates, Lagomorpha, Artiodactyla, and Rodentia for which the relevant data were available [38]). The rate of synonymous subsititutions in hominins (human and great apes), equal to 0.9 10 9 substitutions per site per year, is more than 5 times (3 4 times for the globin genes) as low as the average rate at the divergence of mammalian orders and 7.5 times as low as in the early placental mammals. Later Li et al. [41, 42] convincingly confirmed the conclusion on the slowdown of the rates of nucleotide substitutions, particularly synonymous ones. In one of these works [41], the average evolutionary rate of five genes at the divergence of human and chimpanzee was estimates as 1.1 10 9 substitutions per synonymous site per year; in the other [42], the corresponding rate calculated for seven genes was 1.3 10 9. (A nucleotide is considered synonymous if all possible substitutions in this site are synonymous; if one or two possible substitutions are synonymous, the site is considered synonymous for one-third and two-thirds [43].) This is significantly lower than the so-called average synonymous rate in mammals, which is about 5 10 9 substitutions. These extremely low fixation rates in the hominin evolution are doubly remarkable since the duplicated α- and γ-globin genes used for calculations had underwent conversion, which enhances rates of nucleotide sequence change. The average rate of synonymous substitutions at the divergence of humans and Old World monkeys was shown to be equal to 2.2 10 9 2.3 10 9. Thus, even with regard to synonymous sites, genes of hominins evolved nearly two times as slowly as genes of Old World monkeys The hominoid slowdown is apparent also in the evolution of noncoding sequences. For instance, it was shown for ψη pseudogenes belonging to the β-globin gene clusters (in higher primates, these clusters include six loci positioned in the following order: 5'-ε γ 1 γ 2 ψη δ β-3'. One of the earliest studies dealing with this issue was published in 1984 [44] when this loci was referred to as ψβ. This study showed that the evolutionary rate of this pseudogene as well as all its parts taken separately (including introns and terminal parts at the 5' and 3' ends) is at least 2 to 3 times lower than the evolutionary rate of nonfunctional mammal DNA, which

726 T TUSHKIN is, as in the case of synonymous substitutions, 5 10 9 substitutions per site per year. The evolution rates of the ψη(ψβ)-globin genes in the lineages of human, chimpanzee, and gorilla after their divergence from the common ancestor were respectively 1.0 10 9, 2.0 10 9, and 2.2 10 9 substitutions per site per year. Thus, the change of this gene was twice lower in the human lineage than in the lineages of African apes. That is, in humans the slowdown was more marked than in other hominoids. These results were confirmed, supplemented, and verified in a number of later works by different authors. Using the method of maximum parsimony, the teams that included Goodman [45 50] estimated that in the primate phylogeny, the evolutionary rate of the η pseudogene was highest in lemurs (2.7 10 9 substitutions per site per year) and in the early monkeys, which are common ancestors of all primates (2.9 10 9 ) and lowest in the ancestors of the modern hominoids (on average from the appearance of catarrhines, 1.3 10 9 ). According to their estimates, the evolutionary rate of the ψη nucleotide sequence in the human lineage was 30 40% lower than in the chimpanzee and gorilla lineages. Based on the relative rate test, Hasegawa et al. [51, 52] also concluded that this rate was high in early primates and later decreased in the Anthropoidea lineage. In addition, these authors showed that in Hominoidea this evolutionary rate was at least 10% lower than in Cercopithecoidea. In the studies [41, 42] cited above, Li et al. analyzed evolution not only of coding but also of noncoding sequences, namely introns and nontranslated regions of some globin genes. These authors also used the relative rate test. They showed a distinct trend for decreasing nucleotide substitution rates in the human lineage for both pseudogene η and the total set of the nuclear DNA sequences examined. The average substitution rate in this total sequence was 1.5 times as low as in the New World monkeys and 2 times lower than in the Old World monkeys. (Note that the latter proportion coincides with that obtained from the data on synonymous substitutions.) Moreover, this rate was lower than the corresponding estimates for the great ape lineage. Nuclear DNA in the ancestors of orangutan, gorilla and chimpanzee evolved respectively 1.3, 2.2, and 1.2 times faster than in the human ancestors (according to [42], in the short communication on this study [41] the results are somewhat different). After obtaining this conclusive evidence in favor of the hominoid slowdown, the opponents of this theory argued that it is related only to certain parts of the genome. The most ardent advocate of this viewpoint was the Australian geneticist Easteal [53, 54]. In 1996, Herbert and Easteal [54] compared human and several of the Old World monkeys for the sequences of 19 genes and adjacent regions. For noncoding sequences, represented by 21 299 sites, the substitution rate in the Old World monkeys was (in full accordance with the studies cited above) higher than in the human lineage (albeit only by ~43%). However, the Australian authors noted that 83% of the compared sites were in the β-globin gene cluster. These authors did not find significant differences in the mutation accumulation rates in sites not included in this cluster. They also did not detect differences in the evolutionary rates of the coding regions including 1592 synonymous and 5275 nonsynonymous sites belonging to 16 genes of various localization. The synonymous sites of the prion gene, for which the human ancestors evolved faster than those of the Old World monkeys, were the only exception. Based on these results, it was concluded that differences in mutation fixation rates between these two lineages are probably specific only for certain genome regions and thus the phenomenon of hominoid lowdown is not of a general nature. In the same year, evidence contradicting this conclusion was published [55]. These data were earlier presented by Li in a report that became a key event of the symposium Molecular Anthropology: Towards a New Evolutionary Paradigm held on March 12 14 1995 in Detroit in commemoration of the 70th anniversary of Morris Goodman [56]. In addition to the η-globin pseudogenes, this study compared introns of eight different genes as well as flanking and nontranslated regions near two of these genes. Only three of these noncoding sequences belonged to the β-globin cluster. In particular, it was shown that the mean rate of nucleotide substitutions averaged over the total sequence set was significantly lower than that in the Old World monkeys. For introns (8478 nucleotide positions), the latter rate exceeded the former by a factor of 1.3 (despite the fact that for three of them, including the ε-globin gene intron, these rates are equal), and for flanking and nontranslated regions (936 positions), by a factor of 2.3. The estimate of the ratio of these rates for introns virtually coincided with that for the ψη-globin genes (1.4). Note that these results were based on the relative rate tests, which does not depend on paleontological dating. This removes one of the main objections of Easteal et al. [54] who also used this tests and argued that intertaxon differences in the rates of molecular evolution found by other authors are explained by incorrect dating of the divergence times. The data on DNA DNA hybridization were also used as an argument in favor of the nonuniversal character of hominoid slowdown. These data reflect evolution of the total genome or its representative part and thus, in principle, could significantly contribute to the solution of this debatable issue. However, the interpretation of these results also raised contradictions between the opposed sides. Sibley and Ahlquist have obtained most comprehensive data on comparison of unique (single-copy) nuclear DNA of primates using DNA DNA hybridization. In 1984 [57] they came to a conclusion that the data on primates including hominoids are compatible with the relative rate tests and indicate uniformity of their molecular evolution. A year later, the same con-

RATES OF MOLECULAR EVOLUTION OF PRIMATES 727 clusion was made by Benveniste [58] on the basis of his results on DNA DNA hybridization. Although in his relative rate tests DNA of the species with greater generation lengths (i.e., human and great apes) produced somewhat more stable hybrids with DNA of the reference species than DNA of other primates, these differences were insignificant. However, in 1986 Britten [59], having analyzed the results of estimation of DNA heteroduplex stability, detected great (fivefold) differences in nucleotide sequence evolutionary rates between higher primates and some birds, on the one hand, and lower primates, rodents, sea urchins, and fruit flies, on the other. He substantiated a suggestion that in the early primates, before their division into lower and higher primates, the DNA evolution rate was high and then started to slow down but differently in these two groups. This author also showed that the hybridization data are in good agreement with the results of sequencing. He did not consider hominoid slowdown. In 1987, a new work by Sibley and Ahlquist [60] appeared. It was based on far more extensive material than their previous publication on the same topic. No evidence in favor of hominoid slowdown was obtained but the authors changed their opinion about the rates of DNA evolution. Earlier, Sibley and Ahlquist had been convinced that the average rate of genome evolution is the same in all animals but in the new study they admitted that this simple idea has been destoyed by facts. Their previous confidence in the constancy of this rate was based on comparisons of the data for primates and birds: calibrating factors (i.e., coefficients for converting T 50 H values reflecting heteroduplex thermostability into divergence times) obtained in both cases were similar. In their reevaluation of these values made for birds, these authors found that a number of groups of nonpasserine birds had considerably lower rates of genomic evolution than passerines and other nonpasserines. In addition, they took into account the literature results testifying for hominoid slowdown. Having considered all this evidence, Sibley and Ahlquist concluded that the genomic evolutionary rate depends on the age of reproduction onset, which, in turn, in associated with generation length. As follows from the speculative (according to Sibley and Ahlquist) calculations based on the ages of the first reproduction, hominoid DNAs evolved twice al slow as those of cercopithecoids. The authors expressed hope that these hypothetical estimations would be verified in the future. However, two years later, based on their experiments on DNA DNA hybridization, Caccone and Powell [61] stated that hominoid slowdown is nonexistent and their data could serve as a good molecular clock. Thus, the DNA DNA hybridization experiments did not yield conclusive evidence proving one of the alternative explanations. Nevertheless, the view on the universality of hominoid slowdown seems to be more plausible. The statement that it concerns only certain loci rather than the total genome encounters a difficult contradiction. It has been shown that the T 50 H estimates reflecting interspecific differences in unique genomic DNA are in good agreement with the genetic differences based on orthologous nucleotide sequences of the β-like globin gene cluster [47, 62]. The presence of this undeniable directly proportional relationship between these two parameters testifies to the fact that the evolutionary changes of single-copy genome regions, on the one hand, and the β-globin loci, on the other, are synchronous on the geological time scale. The constant rate of evolution of the former implies that the rate of evolution of the latter is also constant. If the evolutionary rates of the latter underwent a slowdown (which now seems to be generally accepted), the evolutionary rates of the former must have decreased proportionally. If we accept the reality of hominoid slowdown for β-globin genes, we must accept it for the set of unique DNA sequences. Ignoring the aforementioned linear relationship, the advocates of the universal molecular clock come in obvious contradiction with the facts. The effectiveness of the relative rate test underlying their results depends on the character and interpretation of the data used [63]. Li, a renowned authority on this problem who coauthored [64] a modern version of this test, in his recent studies confirms his fidelity to the hominoid slowdown concept (see, for example, [65]). Apparently, the arguments of its opponents have not convinced him. * At the same time, an important verification has been made. According to previous results (see above), obtained mainly for the β-globin-like sequences, DNA evolved slower in the human lineages than in those of gorilla and chimpanzee. However, a comparison of many (coding and noncoding) DNA sequences of human, chimpanzee, gorilla, and orangutan conducted by Chen and Li by using the relative rate test demonstrated the lack of statistically significant differences in the total rate of molecular evolution among different phylogenies. The only exception was a region of the Y chromosome (SMCY), which in fact evolved considerably slower in the ancestors of humans than in the ancestors of chimpanzee and gorilla. The rates of evolution of mtdna are of special interest because the mode of inheritance of this DNA differs from that of nuclear DNA. Already the early studies of the mitochondrial genome evolution showed that these rates are extremely high: mtdna evolved 5 to 10 times faster than nuclear DNA [68, 69]. As to variation of these rates, the opinions have again diverged. In their works published in the 1980s, Hasegawa et al. [50, 70 72] expressed a firm conviction that the rate of mtdna evolution is the same in different mammalian lineages and in different periods of their phylogeny. At least this rate is more constant than the evolu- * Probably the most extensive review of the concept of hominoid slowdown and a comprehensive answer to the opponents was presented in the book by Li [66] not available for Russian readers. The most amply documented argumentation of the adversaries of the concept is given in the monograph of the Australian geneticists mentioned above [67], which is also absent in Russian libraries.

728 T TUSHKIN tionary rate of the η-globin pseudogene, which was thought to be significantly (more than by 10%) lower in hominoids than in cercopithecoids [50]. However, the mtdna clock based on sequences of 896 nucleotides in length gave a too recent date of human chimpanzee divergence (2.7 ± 0.6 Myr) [72]. This dating has cast some doubt the hypothesis that was then practically generally accepted. According to that hypothesis, the oldest known human ancestor after branching off the chimpanzee lineage was Australopithecus afarensis that lived about 4 Myr ago and moved on two legs. To eradicate this discrepancy between the paleontological and molecular data, a hypothesis was put forward on a horizontal transfer of mtdna between the protohuman and protochimpanzee, which occurred 2.7 ± 0.6 Myr ago after their divergence having started about 5 Myr ago [50, 72]. Introgressive hybridization of the ancestral forms eliminated the accumulated the reading of the clock bringing it back to the initial point. However, later Hasegawa et al. obtained the results that refuted their conviction in constant rates of mtdna evolution. For instance, it was shown that the mtdna transversion rate is higher in human and mouse than in bovines, and probably lower in human than in mouse [73]. A comparison of mtdna in hominoids demonstrated that transitions are more frequent in human and great apes (chimpanzee, gorilla, orangutan) than in gibbons [74]. In lemurs, transition rate was extremely low. It constituted only 1/10 of the transition rate in the other primates although the rate of transversions in lemurs is the same as in the other members of this order [52, 75]. It is known that in mammalian mtdna, transitions occur significantly more often than transversions [69, 72]. However, in the lemur lineage the substitutions of all type have almost the same rate. Moreover, it was shown that the transition rate was higher in catarrhines (except gibbons) than in tarsiers and platyrrhines, and in gibbons it was higher than in the other catarrhines. Only the rate of transitions (but not transversions) varied. No hominoid slowdown was found. However, other Japanese authors [76] arrived at the conclusion that the nucleotide substitution rate in mtdna of hominins (human, chimpanzee, and gorilla) was lower than in the Old World monkeys (which in their study were represented by macaques). According to this study, variation the evolutionary rates of mitochondrial genes has the same pattern as that of nuclear genes. The evolution of nuclear and mitochondrial genes encoding proteins of the electron transport chains deserve special attention. These chains located on the inner mitochondrial membrane include five enzyme complexes, which contain 83 subunits most of which (71) are encoded by the nuclear genome, and some (13), by the mitochondrial genome [77]. Studies of some subunits essential for the chain functioning showed that they evolved much faster in primates than in other mammals. This difference in evolutionary rates seems to be most pronounced in mtdna-encoded cytochrome c oxidase subunit II ([78, 79] and other refs). However, a close ratio of evolutionary rates in primates and nonprimates was also shown in the case of some functionally associated with this subunit polypeptides encoded both by the mitochondrial (cytochrome b [80], cytochrome c oxidase subunit I [81]) and the nuclear (cytochrome c [78]) genomes. According to the recent estimates, this ratio is approximately equal to 2 (5 by earlier estimations [78]). For other genes of the electron transport chains, this ratio is approximately 1.5 [79]. Conclusions on variation of these evolutionary rates within the primate order are controversial. Some authors do not find any significant differences among them at different stage of phylogeny and in different systematic groups. Only occasional local anomalies are noted (according to [79], orangutans and macaques have elevated rates of evolution of the genes considered). However, other authors claimed the existence of a pronounced heterogeneity and pattern in the tempo of evolution of molecules related to aerobic energy metabolism. The aforementioned Australian geneticists found that nonsynonymous nucleotide substitutions in the genes of cytochrome b and cytochrome c oxidase subunit I occurred about twice as frequently in the simian primates than in the lineages of tarsiers, strepsirins (lemuriforms and loriforms) and nonprimate mammals [80, 81]. The same situation, in view of these authors, is observed in the case of the gene of cytochrome c oxidase subunit II. At the same time, they noted that the transversion frequency in the fourfold degenerate sites (positions where all possible substitutions are synonymous) of the cytochrome b gene is the same in all lineages. Another and more detailed scenario was reconstructed by Detroit researchers with participation of Goodman [77, 82]. It is based on a familiar scheme according to which the early stages of the human phylogeny involved acceleration of evolutionary rates while the late stages were characterized by their slowdown. In the cited works, this statement was supported by data on a number of complexes III and IV subunits as well as cytochrome c that interacts with both of these complexes. The most striking example of this dynamics is provided by cytochrome c oxidase subunit IV, which presumably is the key regulator of complex IV, i.e., of the whole enzyme. Phylogenetic sequence analysis of the encoding gene suggests that the nonsynonymous substitution rate was considerably higher in the early ancestors of the modern catarrhines living 40 18 Myr ago than in their descendants (the platyrrhines are also characterized by these differences though they are less marked). It is particularly remarkable that at the early phylogenetic stages of catarrhines, nonsynonymous substitutions were far more (13.9 times) as frequent as synonymous ones, and at the late stages (e.g., in the direct predecessors of human, chimpanzee and gorilla) this pattern was reversed.

RATES OF MOLECULAR EVOLUTION OF PRIMATES 729 1.8/6.6 ~ 6.5 (?) > ~ 1 (?)/> 3 (?) > ~ 3.5 > 1 (?)/~ 4.5 (?) 4.5 < 1 (?)/1 2 < 1 (?)/~ 2 1.7 1.2(1.0 1.8) > ~ 1 (?)/> 2 (?) < 1 (?)/1 2 (?) ~ 2 1.7 < 1 (?)/ 2.2 ~ 2 < 1 (?)/> ~ 2 (?) ~ 2 ~ 1 (?)/> 2 (?) > ~ 3.4 (4.5?) ~ 1 (?)/~ 4.5 (?) 4.5? /> 2 (?) 2.4 0.1/0.9 1.3 1.1(0.7 1.7) 0.1/0.9 1.3 1.2(1.1 2.3) 0.1/~ 1 1.2(1.1 2.1) Human Chimpanzee Gorillas Orangutans Gibbons Macaques Coatas Tarsiers Galagoes Lemurs 70 60 50 40 30 20 10 0 Time, Myr Phylogenetic tree of the species representing major primate taxa. Its topology reflects the results of numerous molecular phylogenetic studies (see [39] for review). The chronology of branching of the phyletic lineages corresponds to the dates from the last reports (e.g., [86]). Above the tree branches, the rates ( 10 9 ) of nonsynonymous/synonymous substitutions are given. (mostly for the globin genes). The rates of substitutions in noncoding DNA sequences (for the ψη globin pseudogene) are given below them. See text for further details. MUTATION ACCUMULATION DYNAMICS IN THE ANCESTORS OF HUMAN AND OTHER PRIMATES Summarizing, let us visualize the total picture of variation in mutation substitution rates in the primate evolution. From the viewpoint of the proponents of the universal molecular clock hypothesis, this variation manifests only in random fluctuations of mutation fixation rates. They argue that the average rate of molecular evolution is the same in all animals including primates. In the case of coding nuclear DNA of mammals, this rate is about 1 10 9 substitutions per nonsynonymous nucleotide site per year and about 5 10 9 substitutions per synonymous site per year (see above). However, the assumption on the global constancy of the molecular evolutionary rates contradicts a number of convincing results, some of which were mentioned in the present review. The concept of local molecular clocks seems more realistic (see [4] for review). This concept assumes significant differences in nucleotide substitution rates in different taxonomic groups and at different phylogenetic stages. The contested by Kimura [83, 84] (see above) hypothesis that these rates increased and decreased during the entire history of the animal kingdom is in agreement with the results of many studies (see comments on a polemic retort by Kimura in [33, 85]). Primates provide the most interesting and striking example of such heterogeneity in the tempo of molecular evolution. The figure presents the fixation rates of nucleotide substitutions in coding and noncoding regions of primate DNA. The coding sequences are mainly represented by genes of (α- and β-like) globins and noncoding sequences, by the ψη globin pseudogene. The ψη evolutionary rates were calculated for nearly all tree branches, and evolutionary rates of the coding sequences, only for their minor part. Substantiated and plausible assumptions have been made with regard to the unknown values; the hypothetical rates are indicated with question marks. If the rates obtained in different studies are very different, the limits of their range are given; in most cases, the value obtained in the most recent work and/or the most reliable value is also presented. Conclusive evidence indicates that primates of the modern type (euprimates) appeared not earlier than 63 Myr ago (see [86]). The estimates of accumulation rates of nucleotide substitutions in early euprimates are lacking but such estimates exist for their direct predecessors, early placental mammals [38]. Apparently, these rates in early primates were similar to those in their closest ancestors. The corresponding values presented in the figure are produced by such extrapolation. Note that their values are substantially higher for mammals. After the split of two major primate branches, Strepsirhini and Haplorhini, the rates started to diminish, in the former to a greater extent than in the latter. Note also that among the lower primates, lemuriforms evolved slower than loriforms and tarsiers. The most pronounced slowdown was observed in the hominoid phylogeny. The rate estimates are from works [38, 41, 42, 46, 48, 51, 55, 87 94]. The issue of variation in the mtdna evolutionary tempo is even less clear than that of evolutionary rates of nuclear DNA. If we accept that humans and chimpanzees diverged 5 Myr ago, the fixation rate in the

730 T TUSHKIN mitochondrial DNA molecule (without the D-loop that has very heterogeneous evolutionary rates) in the close ancestors of these species is 1.7 10 8 substitutions per site per year [95]. To complete the picture, note that evolution of some regions during the phylogeny of higher primates and human, in particular, did not slow down but was accelerated. For instance, the noncoding immunoglobulin alpha gene in the Old World monkeys evolved 1.5 to 2 times as slow as in hominoids including human [96]. The sequences of the X-chromosomal α-repeat DNA in the human lineage evolved faster than in the lineages of African hominoids [97], which is related to strong intertaxon differences in its recombination rate. In baboons, after their split from macaques, an abnormal rise of nonsynonymous mutations in the α-globin genes was recorded [98]. The frequency of these mutations is at least ten times as high as the frequency of synonymous substitutions characteristic of most higher primates. At the same time, humans exhibited far more synonymous substitutions at this locus than baboons and macaques. In conclusion of this section, I would like to make one more comment. It is clear that the values of the estimates of molecular evolution rates depend on dating of calibration points. In establishing the evolutionary chronology of primates, these points are usually represented by the branching nodes between hominoids and cercopithecoids, between the branch of human, chimpanzees, and gorillas, on the one hand, and orangutans, on the other, and between the Old World and New World monkeys. These events are generally taken to occur respectively 25 30, 12 16,and 35 40 Myr ago. These datings are based on paleontological data, and the latter is, in addition, substantiated by the estimates of the divergence time of Africa and South America as a result of continent drift [99]. Another relatively reliable although disputable calibration point is the appearance of euprimates 63 Myr ago. At that time an asteroid fell on the Earth, which led to a catastrophic devastation of the biosphere. The extinction of many animals, e.g., dinosaurs, provided numerous ecological niches to be filled by mammals. However, several years ago a team of Swedish geneticists cast doubt on the datings based on fossil primates, since their paleontological record is very scarce. Instead, these authors suggested to use for molecular clock calibration more reliable (in their opinion) paleontological data on divergence of mammals from some other orders. To implement their idea, they calibrated an mtdna clock on the basis of the date of divergence between artiodactyls and whales as well as horses and rhinoceroses. These events ostensibly occurred respectively 60 and 50 Myr ago [79, 100]. The divergence times of several primate taxa calculated in this manner turned out to approximately twice exceed the currently accepted ones (see figure). For instance, the time of hominoid cercopithecoid split was more than 50 (instead of 25 30) Myr, and that of human chimpanzee split, 10 13 (instead of 5 6) Myr. These results imply that evolution of both nuclear and mitochondrial DNAs was at least twice slower than is generally thought. However, the general view seems to me better substantiated and more convincing while the works of its opponents are prone to criticism (see [101]). FACTORS AFFECTING THE TEMPO OF MOLECULAR EVOLUTION The establishment of factors that determine the rate of mutation accumulation is a key aim of evolution biology in general and evolutionary genetics in particular. For convenience, they may be classed into factors determining the mutation rate and those affecting the rate of fixation of nucleotide substitutions. However, this classification is arbitrary as all of these factors are directly or indirectly associated and interact with and one another. Mutation Rate The major factors that affect mutation rate include generation length, the number of divisions of germline cells, DNA repair, metabolic rate, and effects of external mutagens. Below, the operation of these factors is considered. Generation length. A hypothesis that related variation of molecular evolution rates to generation length (the generation-time hypothesis) was advanced earlier than others, in the 1960s (see above and refs in [55, 102, 103]), and since then remains the most popular explanation of their variability. According to this hypothesis, nucleotide substitution rate is proportional to the number of generations having appeared during the corresponding time interval. Hence, genomes of species that have long generation times must evolve slower than those of species with shorter generation times. Long-generation species have less replications of germline cells, during which mutations are thought to occur. This hypothesis was to some extent supported by comparisons of the nucleotide substitution numbers in species with strongly differing generation times. For instance, it was shown that DNA evolution rate was much higher in rodents (specifically, in mice and rats) than in humans and other higher primates. According to DNA hybridization data (see above), these differences in evolutionary rates are fivefold [59] or even tenfold [103]. Analysis of sequenced regions of nuclear DNA shows two- to fourfold differences [41, 55, 64, 104, 105]. It was noted that the generation-time effect was stronger for synonymous than for nonsynonymous substitutions [64, 106, 107]. This effect also manifests in comparisons of various mammalian orders [105, 108], Drosophila and mammals [109], and crane species [110]. It was recently found in a comparison of mtdna of rodents and primates [111]. Greater generation length may explain the extremely low mtdna evolu-

RATES OF MOLECULAR EVOLUTION OF PRIMATES 731 tion rate in elephants as compared with other land mammals [112]. At the same time, mtdna of sharks evolved 7 8 times slower than mtdna of primates despite the fact that their generation times are similar [113]. The proponents of the universal molecular clock hypothesis for many years denied the existence of the generation-time effect ([114 117] and others). However, their studies were based mainly on comparisons of amino acid sequences and on immunological data. Thus, they analyzed nonsynonymous substitutions, for which this effect is weak and problematic to detect. During the evolution of the human ancestors from the time of the appearance of the first primates, generation time substantially increased. This may have caused the detected slowdown of mutation accumulation rates. The number of cell divisions. This factor is related to the previous one, but they are not connected by a simple, say, directly proportional, relationship. The number of cell divisions exerts a more direct affect on mutation rate since mutations are mainly generated by errors of DNA replication (recently, some authors contested this accepted view [118]). Therefore, the number of divisions of germline cells seems to be more important as a factor determining nucleotide substitution rate than generation length. This can be illustrated by examining the situation with molecular evolution rates in mouse and human. These rates, as noted above, are five- to tenfold different according to the DNA hybridization data, and at least two- to fourfold different based on comparisons of sequences of DNA regions. However, the mean generation time is 25 years in human and about 4 months in mouse [119] (or, according to a more cautious estimate, 2 8 months [55]). Thus, their generation lengths differ by a factor of 75, i.e., much greater than the DNA evolution rates. The number of cell divisions in spermatogenesis of a man attains about 300 by the age of 25 years while in a mouse for the same 25 years (75 murine generations) this value reaches 2250 {119]. The latter value exceeds the former one by a factor of 7.5, which is in good agreement with the ratio of molecular evolution rates in the murine and human lineages. DNA repair. This factor strongly affects mutation yield since the mutation number is determined by balance between the appearance and correction of DNA alterations. It was suggested that enhanced efficiency of repair was among the key causes of the reduction of de novo mutation frequencies in the phylogeny of higher primates [59, 88] (see also an interview with Goodman in [120]). Some evidence indicates that this efficiency in fact was increasing during primate evolution (refs in [59, 121]). It was also suggested that the differences in DNA repair efficiency explain lower substitution rates in lemurs compared with galagos and tarsiers [122] and higher in rodents compared to humans [59]. Abnormally high mutation rates in non-nuclear (in particular, mitochondrial) genes are explained exactly by inefficient repair. Nevertheless, some authors ([41] and others) believe that this obviously significant factor is secondary to generation length and the number of cell divisions. However, these factors are connected to each other. The increase in generation time (that occurred, in particular, in the human phylogeny) is closely related to the improvement of mechanisms of DNA repair (and replication) (see [108]). This improvement is a necessary condition of the welfare of species that evolved in the direction of longer generation times. Metabolic rate. This factor directly affects the mutation process. Intensity of metabolism determined the concentration of oxygen radicals, which are generated as a side product of aerobic respiration and have mutagenic properties, and intensity of DNA synthesis. The higher the metabolism intensity, the higher must be the nucleotide substitution rate. These notions laid ground for the metabolic rate hypothesis, which was aimed to complement the hypothesis relating molecular evolution rates to generation time [113, 123]. There is evidence indicating that both hypotheses satisfactorily account for intertaxon differences in molecular evolution rates [123]. For instance, the rates of nucleotide substitution in nuclear and mitochondrial DNA of primates (monkeys of the New and Old Worlds, great apes, and human) correlate both with metabolic rate (coefficient of correlation 0.75) expressed in ml O 2 /kg per hour and with generation time (coefficient of correlation 0.75). However, the former hypothesis explain some facts that contradict the latter one. For example, the extremely low evolutionary rates of shark mtdna as compared to primates (although primates and sharks have similar generation lengths; see above) may be caused by the lower metabolic rates of the former (metabolism is more intensive in homoiterms than in poikyloterms). Other authors [108] did not reveal any association between metabolic and molecular evolutionary rates. External mutagens. This factor is unlikely to play any significant role in the genetic evolution of primates including human. However, other viewpoints exist on this matter. According to one of them, supported by studies on bacterial test systems, humans, since they have started to use fire, are subject to the effect of mutagenic products of pyrolysis of amino acids (especially tryptophane), which are formed on the roasted surfaces of meat and fish [124]. In principle, this must have enhanced mutation rate in our ancestors. However, the time period during which these substances (mutagens and comutagens) affected humans is relatively short: the age of the most ancient known fire place left by Homo erectus is about 1.5 Myr [125]. Moreover, these substances are effectively inactivated by antimutagens contained in fruits and vegetables [124] that are included in normal human diet. According to another, purely speculative, assumption, the formation of the human race was mediated by high radiation in the region of the East African breaks of the Earth crust, where key events of anthropogenesis allegedly took place [126]. This raises at least two questions.

732 T TUSHKIN First, can mutation rate be regarded as a limiting factor of anthropogenesis? And, second, can the radiation level in the region of this rift system lead to a noticeable enhancement of the mutation rates? The author of the hypothesis did not answer these principal questions; in fact, he even did not pose them. Furthermore, this assumption is at variance with the reduction in mutation substituion rate in the human lineage. Mutation Fixation Rate Here I consider only two integral factors: genetic drift and selection. Genetic drift. The significance of this factor is determined primarily by population size, if we accept a reasonable assumption that most mutations are slightly deleterious (this assumption is sometimes called Ohta s hypothesis, see [25]). The smaller the size of populations of a species, the higher must be mutation substitution rate caused by genetic drift. For instance, in the model of effectively neutral mutation (see [25]), with the most realistic, according to Kimura, value of the key parameter β, the rate of molecular evolution calculated per generation (k g ) is inversely proportional to N e, when 4N e s' 1 (N e, effective population size; s', mean selective disadvantage of mutations). To explain the approximately constant rate of molecular evolution per year (k = k g /g), it was proposed that species generation length g is also inversely proportional to N e (at the condition that mutation rate is the same in different species). In other words, g N e was assumed to be constant. To explain hominoid slowdown, in the existence of which he was not fully convinced, Kimura suggested that the increase in g in higher primates was not accompanied by a corresponding reduction in effective population sizes. In the case of hominoids, the value of g N e was substantially higher than in other primates, which led to a decrease in mutation substitution rate calculated per year. Selection. The role of natural selection in molecular evolution is a controversial issue. In the 1970s 1980s, the controversy between selectionists and neutralists (as the opponents were then called) was very strong but then diminished. The selectionists stated that tempo and pattern of molecular evolution are determined by selection. For instance, Goodman (who initially was an adherent of neutralism; see above) argued that during adaptive radiation, when organisms adapt to a new environment, their proteins are subjected to strong natural selection. This increases the rate of molecular evolution and augments functional density of protein molecules, i.e., the proportion of amino acid sites involved in specific, especially contact, functions. The acceleration is followed by deceleration caused by negative (stabilizing, purifying) selection aimed at preserving the acquired improvements [127, 128]. This hypothesis underlies the selectionist explanation of hominoid slowdown. By contrast, according to the neutral theory, the number of advantageous mutations is negligibly small and hence positive selection has no effect on the accumulation rates of mutational substitutions. The extended version of this theory assumes that sa significant fraction of these alleles belongs to nearly neutral ones. The latter include alleles whose selective advantage or disadvantage measured by the selection coefficient s satisfy the condition s 1/2N e. In small populations, their fate is determined by random drift; in large populations, they are subject to selection. Modern models developing these ideas of Kimura show that the rate of mutational substitutions negatively correlates with population size of the species (see review [129]), which is in agreement with the hypothesis relating this rate with generation length. As a rule, the latter is positively related to body size of the organism, and larger body size is associated with reduction in population size. Nevertheless, inspite of the views of neutralists, the impact of directional selection on molecular evolution seems to be very substantial. According to recent estimates, obtained by comparing genes of humans and Old World monkeys, the proportion of amino acid substitutions caused by positive selection is 35% [130]. Relative Contribution of Different Factors Which of the factors listed above has a key effect on the rate of mutational substitutions? Is it possible to distinguish the most important one among these factors? I think that it is impossible to identify one leading factor that determines the mutation accumulation rate in all organisms. Instead, we can speak of an hierarchy of these factors, which is different in different phylogenetic lineages. In most (but not all) cases the top position in this hierarchy is occupied by generation length, which is closely related to the division number of germline cells. This factor is in direct causal relationship with mutation rate. Apparently, the slowdown of molecular evolution in human predecessors was mainly caused exactly by an increase in generation length. It brought about a general reduction in mutation accumulation rates in coding and noncoding DNA regions, including nonsynonymous substitutions (although the dynamics of the latter is primarily determined by selection). However, the generation time increase itself was caused by selection directed to delay the age of reproduction, or, more specifically, to increase the period of individual development beneficial for mastering the parental experience. This key adaptation of hominids created cultural continuity of generations. The lengthening of the time interval between birth and reproduction was accompanied by selection for DNA repair improvement, which resulted in further slow down of the molecular evolution of our ancestors. The joint effect of these factors is reflected in the term nucleotide generation length [123] (nucleotide generation is the time interval between

RATES OF MOLECULAR EVOLUTION OF PRIMATES 733 copying at the given nucleotide position during DNA replication and/or repair). However, even remotely precise estimation of the nucleotide generation length is currently out of the question. Unraveling the tangle of relationships between various factors that have affected mutation accumulation rate in the human phylogeny requires further development of the evolutionary genomics of primates. REFERENCES 1. Zuckerkandl, E. and Pauling, L., Molecular Diseases, Evolution, and Gene Diversity, in Gorizonty biokhimii (Frontiers in Biochemistry), Moscow: Mir, 1964, pp. 148 173. 2. Margoliash, E., Primary Structure and Evolution of Cytochrome C, Proc. Natl. Acad. Sci. USA, 1963, vol. 50, pp. 672 679. 3. Zuckerkandl, E. and Pauling, L., Evolutionary Divergence and Convergence in Proteins, Evolving Genes and Proteins, New York: Academic, 1965, pp. 97 166. 4. 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