EVOLUTION I: EVIDENCES OF EVOLUTION

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1 EVOLUTION I: EVIDENCES OF EVOLUTION Evolution, an important biological principle, is frequently misunderstood by layman, as well as quite a few scientists. Theologians often attack the concept as being atheistic, but most of them don t really understand the mechanism of evolution at all. To understand evolution to the fullest extent an individual should have a good background in biology, chemistry, mathematics, genetics, astronomy, geology, paleontology, and biochemistry. As you can see, few of us have such diversified knowledge, and that includes many scientists. We recognize that most of you don not as yet possess advanced expertise in the above fields. However, we can introduce you to the subject of evolution today without going too deeply into its mechanisms. Our objective for today is to have you gain an appreciation for the evidences of evolution. We will try to present some of the evidences that indicate that evolution has occurred and indeed does occur now. We can look at the study of evolution in much the same way that we view the study of historical events. There are no living eyewitnesses to the civilizations of Rome and Greece. Nevertheless, practically no one doubts that these two cultures did exist because of the records and artifacts that have been preserved which testify of their existence. In much the same way, scientists have pieced together the evidences of evolution (fossils, etc.) in an attempt to understand how life began and how it changed (evolved) over vast expanses of time. The evidences that evolution has is for the most part indirect, as are the statues of ancient Greece. However, this evidence is so overwhelming that one is hard pressed to deny that it exists or that what it implies has not occurred. This is that life began as simple one-celled organisms and slowly evolved into the varied and rich life forms that have and do now inhabit the earth. I. The Meaning of Evolution Evolution involves changes in the gene pool of any given species from one generation to the next one. You must remember that this is usually a fairly slow process because gene frequencies are relatively stable. Recent research however, indicates that some groups undergo rapid evolution during periods of environmental changes. Neo-Darwinian evolution, which is accepted by most biologists today, states that natural selection favors those individuals best suited for survival and thus it is their genes that are perpetuated in future generations within the species. There are a few principles of evolution that you should become familiar with before you examine the evidences. 1. Once a structure is lost in the course of evolution, it can never be re-evolved by that group of organisms possessing it. This is called the Principle of Irreversibility. 2. Extinction is a fact of life. Those organisms that have become extinct have done so because of an inability to adapt to an ever changing environment. Once a group of organisms becomes extinct, they never again reappear in the fossil record in later rock layers (Principle of Irreversibility).

2 Table 1. Geologic Time Table and History of Life ERA PERIOD YEARS TO BIOLOGICAL BEGINNING EVENTS OF PERIOD Cenozoic *Age of Mammals Quaternary 2 Million Evolution of man. Ice Age ends 10,000 years ago. Many large mammals become extinct. Mesozoic *Age of Reptiles Paleozoic *Age of Invertebrates Tertiary 65 Million Mammals and birds evolve into diverse forms and become dominant animals. Flowering plants become dominant plants. Cretaceous 145 Million Flowering plants evolve; insects evolve rapidly. Extinction of dinosaurs at end of period. Jurassic 210 Million First birds appear. Dinosaurs evolve into many diverse forms. Pre-flowering plants. Triassic 245 Million First mammals appear. Conifers are dominant land plants. First dinosaurs. Permian 285 Million Reptiles evolve; mammal-like reptiles appear. Forests of seed ferns and conifers. Carboniferous (Mississippian- Pennsylvanian) 360 Million Age of Amphibians and great coal forests of giant tree ferns. First reptiles appear. First conifers. Devonian 400 Million Age of Fishes. First land vertebrates (amphibians) appear. Land plants evolve. First insects. Silurian 435 Million First land plants and animals (arthropods) appear. Fishes evolve. Ordovician 500 Million First vertebrates (fishes) appear. Algae common. Cambrian 550 Million First arthropods (trilobites). All life is in the oceans. Precambrian 4.7 Billion Simple multicellular and one celled organisms. Photosynthesis evolved. Oldest fossils (bacteria) 3.5 billion years old. Life begins 4 billion years ago.

3 II. The Fossil Record Perhaps the most overwhelming evidence that life on the earth has changed or evolved are fossils. A fossil is the evidence or remains of an organism that once lived on the earth. They may be preserved in various ways and everything from the entire organism may be preserved (as in insects preserved in amber) or only an indication as to what the animal or plant looked like (as in dinosaur tracks). We do not have enough time to go into the aspects of the various ways in which fossils are preserved but we can say that most of them are preserved in the rock layers (strata) that slowly accumulated on the earth s surface. Table 1 presents a simplified Geologic Tim Table and some important biological events that took place during the different Periods. We don not want you to memorize this table, but hopefully you will become familiar with some of the important points contained within the table. We want to emphasize that scientists have determined the limits of each stratum (Period) by the fossils contained within! Notice for example, that not much happened before the Cambrian Period began 550 million year ago. This is not to imply that life did not exist before then, because it did. However, most of the rock layers in the Pre-Cambrian have become distorted because of hat and pressure and as a result many of the fossils contained within were destroyed. An important event occurred during the Cambrian and this was the development of hard parts among animals so that their remains are better preserved. We will try and introduce you to some of the more important common fossil groups and examine the evolutionary trends within each group. A. Trilobites The beginning of the Cambrian Period is determined by the appearance of trilobites. These organisms are related to modern horseshoe crabs and were extremely common during the Paleozoic Era; but all became extinct by the end of that era. How long ago was this? Examine the trilobites, Elrathia kingi from the Cambrian Period on your table and note the platelike cephalon (head) and the large compound eyes on the dorsal surface. Notice that their body (thorax) is divided lengthwise into 3 (tri) regions (lobes), hence their name. Also, note that the last several segments of the tail region may be fused into the pygidium. Below make a drawing of your trilobite and label its following parts: cephalon, compound eyes, thorax, and pygidium. Drawing of trilobite, Elrathia kingi

4 Often, in evolution, things go from simple (primitive) to complex (advanced). However, sometimes it proceeds in an opposite manner so that more advanced organisms may be more simplified. Just such a thing occurred in several groups of trilobites. There was a general trend toward reduction of the eyes, reduction of body spines, and loss of thoracic segments, and development of the pygidium into a well defined structure with a fusion of segments. Figure 1 presents an evolutionary trend in this group of trilobites, but they are not in the correct evolutionary sequence. Using the above evolutionary trends as your guide, put these trilobites in the correct evolutionary sequence below starting with the most primitive at the left proceeding to the most advanced on the right. Figure 1 Evolutionary trends in trilobites What has happened to the eyes in the most advanced trilobite species above? What else is reduced or simplified about that species? B. Cephalopods Cephalopods are Mollusks of the class Cephalopoda, the most familiar of which are squid and octopuses. All cephalopods possess tentacles with suckers and most have a shell or skeleton composed of calcium carbonate (CaCO 3 ) like the shells of clams and oysters. There are two main groups (sub-classes) from the Paleozoic and Mesozoic Eras. The first group is the nautiloids, which are known from the Cambrian to the present. Most of the nautiloids had straight shells, but some, like the only living (extant) form, the chambered Nautilus, possess a coiled shell. Examine the demonstrations specimens of Nautilus and notice that as the animal grew it added new chambers to its shell. These chambers are separated from one another by septa (sing. septum) which are often visible on the external surface of the shell as suture lines.

5 The second major group of cephalopod fossils is the ammonoids, which apparently evolved from the nautiloids during later Silurian times. They became very common during the Mesozoic Era and evolved into many diverse types; however, all of them became extinct by the end of the Cretaceous Period, when the dinosaurs also suffered a similar fate. How long ago was this? Unlike the nautiloids, most ammonoids possessed coiled shells very similar to Nautilus. Examine the ammonoid Hildoceras bifrons, from the Jurassic Period, on your lab table and make a drawing of it below, labeling at least one chamber and one suture. Drawing of ammonoid, Hildoceras bifrons All of the Nautiloids possess rather simple, straight sutures (Fig. 2A). The ammonoids evolved more complex suture patterns (Figs. 2B-D). The most primitive pattern, goniatitic, is characterized by an angular and curved pattern (Fig. 2B). The next pattern to evolved, ceratitic, is curved with an interrupted series of tooth-like indentations (Fig. 2C). The most complex pattern, ammonitic, was the last to evolved and is characterized by extremely convoluted suture lines (Fig. 2D). This increasing complexity of suture patterns from the simple nautiloid type, to the more complex ammonoid type, provides considerable support to the theory of evolution. Figure 2 Suture Pattern Types of Cephalopods

6 On your lab table you will find a box containing 4 cephalopod fossils (# ). Working in groups of 3 or 4, carefully examine these 4 specimens and arrange them in an evolutionary sequence from the most primitive to the most advanced. Check your sequence by matching the fossil number to their description on the inside lid label of your box and then comparing the Periods on the lib with those in Table 1. Then answer the following questions. 1. Are any of these cephalopod fossils a nautiloid? How did you determine this? 2. Which specimen has the most complex sutures? Does it have a straight or coiled shell? 3. Which specimen is the most primitive ammonoid? What type of sutures does it have? C. Evolution of Horses The evolution of horses is perhaps better understood than most other groups of mammals simply because their fossil record is so complete. Most of the major evolutionary developments in this groups occurred in North America during the Tertiary Period, however all horses became extinct on that continent at the end of the Ice Age, about 10,000 years ago. They were then reintroduced to North America by the Spanish during the 1500 s. The earliest known form in this rather varied line is Hyracotherium (#155) (also known as Eohippus), which first appeared during early Tertiary times. It had 4 toes on its front legs, low crowned molars for browsing on leaves, and was the size of a small dog. Mesohippus (#156) appeared later and was a larger animal, about the size of a collie, with 3 front toes and slightly higher crowned teeth. Pliohippus (#157) was the first one toed horse and appeared during later Tertiary times. It looked much like modern horses with high crowned molars suitable for grazing on the grasses that were becoming abundant then. Finally, the modern horse, Equus (#154) appeared by the Quaternary Period, about 2 million years ago, and had even higher crowned teeth. Working in groups of 3 or 4, carefully remove the replicas of fossil horse teeth (# ) from their box and arrange them in an evolutionary sequence from most primitive to most advanced and answer the following questions. 1. Which fossil(s) exhibit teeth best suited for grazing? 2. Which fossil(s) exhibit teeth best suited for browsing? 3. What changes in the horses environment may have been responsible for the changes you observed in these teeth? 4. Is there any correlation between changes in tooth size and shape, and reduction of toes? Explain

7 III. Evidence From Comparative Anatomy By studying the anatomy of organisms, one can often determine to what degree they are related, if any. Essentially what you would look for are homologous structures. These are structures that have the same embryological origin, but not always the same function. Now let us examine the comparative anatomy of three mammals to lean something of their evolution. Examine the demonstration skeletons of the cat and the monkey. Look closely at the way each mammal walks. 1. Which animal walks on the entire flat surface of its feet, a primitive character? 2. Which animal walks only on its toes, an advanced character? 3. How many toes does the monkey have on its hind leg? How many toes does the cat have on its hind leg? How many toes do you have? 4. Which species has lost the inside digit (big toe) on its hind leg, an advanced character? 5. What is happening evolutionarily to the inside digit (thumb) on the cat s front legs? 6. Examine the way your species (man) walks. Is this a primitive or advanced character? 7. Examine the clavicle or collar bone (a primitive character) on the monkey, the human skeleton, and feel it on yourself. Now try to find it on the cat. Does the cat have a collar bone? You should find that the cat has lost its collar bone (an advanced character), a major evolutionary advancement that allows it to run faster and be more flexible than it could be with a collar bone. 8. Structurally then, which animal (monkey, man, or cat) is the most highly evolved?

8 IV. Embryological Evidence Early during the 19 th century it became apparent to certain biologists that during the embryological development of any individual organism, it seemed to pass through stages that resembled more primitive organisms. Some scientists became convinced that highly evolved organisms, like mammals, actually passed through stages in their embryological development during which they were fishlike, amphibian-like, reptile-like, etc. Today, science recognizes that organisms do not actually develop from more primitive to advanced organisms during their embryological development. However, we now understand that they may exhibit certain primitive (ancestral) characteristics during their embryological development. For example, the modern horse (Equus), which you studied is one-toed. However, their young embryos have three toes, an indication that their ancestors also possessed three toes. It then follows that by comparing the embryological development of supposedly related organisms, which we may learn something about their evolution and of how closely related they are to each other. For example, the body plan of vertebrates extends to their embryological development. Examine the model of a one month old human embryo. As in all vertebrates, at sometime during development, humans have a supporting dorsal rod or notochord, and exhibit paired pharyngeal gill arches. In many vertebrates (fishes and amphibians) arches become functional gills. Now, compare the human embryo model with the various embryonic stages of chick and pig embryo (in bioplastic). Use a dissecting microscope to examine the chick and pig embryos. List several primitive characters present on the human embryo that are not present on adult humans. Check with instructor to determine if these are valid primitive characters Why do all these embryos resemble one another so closely?