Language, Nature and Nurture Can genes settle the debate?

Transcription

1 Language, Nature and Nurture Can genes settle the debate? By Richard Kunert, Suzanne Jongman and Tineke Prins Is language innate or learned? Is it nature or nurture? The greatest minds have tried to find answers to this question. For decades the debate has raged between the most prominent scientists from Noam Chomsky and Steven Pinker to B.F. Skinner. Much has changed between the start of the debate and today. We now know much more about the human genome and its role in everyday activities than even ten years ago. This has transformed the nature-nurture question into something different altogether. Let s lead you on a travel back to our distant past, a journey on an imagined island where German children transform Thai and a visit to the household of a family crippled by language problems. Through all of these visits a picture emerges proving neither the nature nor the nurture camp right. Perhaps, we asked the wrong question: genes don t settle the debate. Instead, they transform it into a nature and nurture discussion. But wait, how could genes change the debate? Haven t we already heard all the arguments? Not nearly. The human species shares a surprising amount of genes with our closest evolutionary cousins but they don t talk while we do. How did that happen? Was nature or nurture responsible, or did they even act together? One can ask the same questions about different peoples rather than different species: why are there so many different languages in the world? Did they just occur by random chance through different cultures or are there deeper genetic differences between peoples responsible nurture or also nature? Finally, we all know people better at languages and some who are surprisingly poor at it. A look at one family reveals some surprising patterns: it s not just in the upbringing. Across these three levels species, population, individual one encounters a new set of questions which go beyond the old trenches of the nature-nurture debate. But let s first go on a journey into our evolutionary past. Sure nature is responsible for evolution. Or is it?

2 The making of a language species humans creating humanity Did we, the human species, start the evolution of language? Standard Darwinian evolutionary theory, as we learned in our biology classes in high school, would answer this question with a decisive no. Darwin introduced the concept of natural selection as the key mechanisms for evolution. Organisms are in competition for survival and reproduction and only those that are best adapted to the environment will endure, also known as survival of the fittest. Through this process, genes that code for advantageous traits will be passed on to the next generation whereas less beneficial traits will disappear. One way for variation between organisms to occur is through the process of mutation. Mutations are changes in the DNA sequence that can either have no effect or prove to be beneficial or detrimental for adaptation to the environment. Consider our closest evolutionary cousins, the great apes. Why do we have language and they do not? Some genetic difference must underlie this difference, sometime after the human and chimpanzee lineage diverged a change must have occurred that proved to be the basis for the development of speech and language. This line of thought was sparked by the discovery of the gene FOXP2 that was shown to play a role in successful language use and was soon referred to as the language gene in popular media. FOXP2 is also present in chimpanzees but with two changes in the DNA sequence. It has been suggested that the causal chain of evolution of language started with this mutated DNA sequence leading to the emergence of some type of language which in turn proved to be beneficial for humans in adapting to the environment as it allowed for the development of culture. FOXP2: The language gene FOXP2 (or forkhead box protein P2) is responsible for regulating many genes involved in the development of the brain and lungs. Individuals that have a mutation of the FOXP2 gene suffer from speech and language disorders, and have difficulties expressing and articulating language. FOXP2 was discovered by Anthony Monaco and Simon Fisher. When investigating the KE family, in which half of the members (spread over three generations) suffered from severe language problems, they found that the problems of the affected family members were due to a defect on chromosome 7. The finding was confirmed by another case outside the KE family: patient CS showed similar language problems as the KE family. These problems were caused by the same defect on chromosome 7, exactly where the researchers suspected FOXP2 to be. FOXP2 is also present in animals, but with changes in its DNA sequence compared to that of the human gene. Researchers have looked at the role of FOXP2 in, for example, songbirds and mice, and the results of their research suggest that FOXP2 supports the brain s ability to learn sequences of movements. In humans, it influences the development of the nervous system, and parts of the brain involved in motor skills. This fits with the idea that the human version of FOXP2 has played a vital role in evolving the complex muscle articulation needed for language.

3 One way language could have led to human culture as we know it today is suggested by Dunbar. He argues that language was a necessary trait to allow humans to live in large social groups which left them better adapted to environmental pressures. One way bigger groups are better adapted is for instance that individuals are better protected against predators. To be able to maintain living in a group there has to be some form of social bonding, grooming is used to this extent by other primates. However, grooming is time consuming and puts a limit on the size of the group. Language could have started as a form of gossip for social bonding, vocal grooming instead of manual grooming, allowing for larger communities to live together. So the proposed chain of evolution is: first genes, then language and finally culture. Could it have been the other way around? Odling-Smee and Laland take such a radically different approach to the evolution of language. They argue that adaptations are not just called for by problems posed by environmental change, but that organisms themselves change the environment that then needs to be adapted to. In other words, organisms partly create their own environment and their own natural selection pressures. This is called niche construction and implies that not only genes are passed on from generation to generation but also the environment that is in part created by past generations. One example to show the difference between standard evolutionary theory and niche construction is dam building by beavers. Darwin would argue that dam building was adaptive and genes that favored this behavior were selected and passed on. However, the construction of dams by beavers alters the environment considerably; one instance is the altered river flow resulting in changed nutrient availability. By building a dam, beavers have created a new niche with different selection pressures compared to the previous environment. Not only genes adaptive for dam Cultural niche construction There are several examples of human niche construction where cultural practices have led to altered selection pressures. Yam cultivating cultures in West Africa are such an example. To grow yam, forests needed to be cleared increasing the amount of standing water, which in turn caused a rise in the number of mosquitoes carrying malaria. This increased the frequency of the sickle-cell S gene variant in the yam cultivating population; normally an unfavorable variant as it causes the disease sickle-cell anemia but it also offers protection against malaria disease. No such increase was found in neighboring populations that do not grow crops. Thus, the agriculturalists themselves created a natural selection pressure leading to genetic changes. Another instance of cultural niche construction is dairy farming and lactose tolerance. Most humans become intolerant to lactose after childhood, but there are adults that carry a variant of the lactase LCT gene and remain tolerant. There is a high correlation between the frequency of this gene variant and cultures that have a history of dairy farming and milk drinking. Individuals that could consume the newly available dairy products had an advantage over those individuals that could not, causing the gene variant for lactose tolerance to be passed on to new generations and increasing its frequency.

4 building will be passed on, but also genes adaptive for the newly created environment. Thus, beavers are actively involved in altering their own adaptation requirements. There is increasing evidence that humans also create their own niche, through the development of culture. Odling-Smee and Laland argue that a similar process was at the basis of the emergence of language: humans created a niche that demanded the use of language. They hypothesize that the fast growing advances in technology and cultural complexity led to the necessity for better and faster ways to communicate to transfer knowledge. Humans therefore might have created their own selective environment where language became an adaptive trait through cultural niche construction: having language allowed for faster learning, transmission, and innovation. Thus, those individuals with language were better adapted to the demands of the self-created environment and their genes were passed on to the next generations. Perhaps we need to rethink the causal chain of evolution of language, we ourselves may have been the start. Why my mother tongue is what it is the tone in genes The evolution of language has led to a great linguistic diversity with languages passed on from parents to children. Did genes shape these languages? Did they determine my own mother tongue? Intuitively, one is inclined to answer with a resounding no. Of course not, if you had been adopted by Thai parents, you would speak Thai. But given that this was (presumably) not the case, your mother tongue is just your parental language. Still, there is a growing opinion that genes do nonetheless play a role. Before looking at this opinion, it is worth asking why genes shouldn t play a role in language. A computational model by Andrea Baronchelli of Northeastern University presents a good case. It suggests that the great diversity of languages is due to fast language change even within my own lifetime new words like googling or chillaxing have emerged without anyone raising an eyebrow. This in turn favours generalist language learners who are able to learn any language equally well. Why? Well, genes are slow to change. Language presents a moving target for evolutionary mechanisms. Instead of adapting to one language in particular, people who can learn any language are at an advantage. It is thus crucial to look at the rate of language change: is it slow enough for genes to change in response to it? An examination of the connections between modern languages of the same family gives some clues as to the real rate of language change. These languages emerged out of a common origin and separated millennia ago. Any commonalities are likely remnants of their common origin language which is long dead. For example, the first European visitors to India noticed curious commonalities between Indian languages such as Sanskrit (3 = tráyas ) and European ones such as ancient Greek ( treĩs ) and Latin ( trēs ). Since the time of the split between European and Indian languages these words do not appear to have changed much. Nowadays, this can be extended beyond mere anecdotes. In a 2007 article in Nature, Mark Pagel and colleagues showed that the more often a word

5 is used today the more likely it is to be similar across languages with a common origin, even if this connection lies 7,500 years in the past. Using structural features, such as grammar systems and the inventory of language sounds, one can look even up to 12,000 years into the past. These numbers correspond to up to a quarter of the time the world s languages had in order to differentiate! So, yes, language vocabulary and structural features do indeed change quickly, but still, there are exceptions, for example among the very common words. This opens up the possibility that genes which are quite stable do influence at least those language features which have been found to be consistent for thousands of years. What is missing so far is an actual example of such a gene-language link. It was found by Dan Dediu and Robert Ladd who looked at tone, a feature which is a relatively stable language characteristic. Tone refers to the use of pitch differences to differentiate words. Take this Thai tongue twister, for example: /mǎi mài mâi mái/. The same consonants and vowels get repeated with different pitches notice the diacritics above the a resulting in the sentence Does new silk burn? Figure 1: Geographic distribution of one version of A) ASPM, B) Microcephalin, and C) tone language. Dediu and Ladd noticed a surprising parallel between the location of tone languages and the location of different versions of two genes in the world, the so called ASPM and Microcephalin genes, as can be seen in Figure 1. It turns out that the gene-tone relationship is unusually strong; for hardly any other genes or linguistic phenomenon it is that strong. Therefore, it seems to reflect some fundamental relationship between ASPM, Microcephalin and tone. The most straightforward

6 explanation would be that we are looking at tone genes if you have them in one version you can learn tone, otherwise not. Dediu and Ladd reject such a direct account my own ASPM and Microcephalin versions do not determine whether I will ever be able to learn Thai. Instead, genes could exert a subtle effect, nudging successive generations of language learners in a certain direction. Imagine a bunch of German babies were dropped on a lonely island and they learnt Thai from Thai native teachers. They would probably manage very well and their teachers would be very proud. Without their teachers noticing it, however, the German children struggled a bit with the Thai tone system. Over generations, this struggle would reduce tonality bit by bit. Were Thai teachers to discover this island again a few hundred years later, they would be astonished what an odd version of Thai people spoke on the island. A Thai without tone. So, because language is a not a homogenous everchanging system, but instead a mix of stable and less stable features, the former could potentially be influenced by genes which are known to be stable as well. So, did genes shape my mother tongue? In a sense yes, the combined genetic background of generations of German speakers shaped German. In another sense no, my genes did not determine that German would end up being my mother tongue. Both answers are true. Why some talk and some say nothing It is clear that peoples differ in their genetic and their language backgrounds. But so do different individuals within each language group. We all learn to speak our native language without any formal teaching or instruction. If learning to speak and understand a mother tongue comes natural to us, how is it that people differ in their language proficiency? Why are there people that don t have any trouble speaking a language, whilst others struggle to find the right words? Is this in our environment and upbringing, or might it be in our nature, our genes? It is clear that our environment affects the process of learning a language. A very socially competent and highly educated parent will address her child in a different way than a less socially competent and poorly educated parent. In other words, the socioeconomic status of the parents will determine the input a child receives, and the context within which words are acquired. Psycholinguists Naigles and Hoff-Ginsberg have shown that for children who acquire new verbs, it is the frequency with which each verb occurs in the parental input that has the greatest effect on acquisition speed. Moreover, the way the words are distributed as well as the content of the adult speech play an important role. So the verb go that occurs often in the speech of the parents, and in many different linguistic structures such as questions, commands, and statements, will be learned earlier than the verb run which occurs less often and in less diverse syntactic frames. Does this mean that members from the same family, growing up in the same environment, receiving the same parental input and who live in the same society will show the same pattern in language learning? No it doesn't, because some differences might not occur through nurture, but instead through nature.

7 Genetic variations can cause individual differences, as in the case of the KE family. This family was treated for their severe speech and language disorder at the Institute of Child Health in London. Investigating the language abilities of the KE family members with speech and language problems, Vargha-Khadem and her team discovered that the affected members of the family had the following characteristics in common: problems in processing words according to grammatical rules, difficulties with understanding complex sentences, problems with reading and writing words and troubles moving their mouths. For example, affected members of the KE family have problems with the production of morphological markers. Such members would not know how to fill in the blank in the following sentence: This creature is smaller than this one, but this creature must be the., while an unaffected member would have no trouble giving the answer smallest. How can we explain this difference given the very similar environment of the family members? Simon Fisher and colleagues searched for the damaged gene in the KE family, performing a genome-wide scan of DNA samples taken from the family members. They noticed that the KE family s disorder was inherited in a simple fashion: as the result of a defect in a single gene, the FOXP2 gene. In the KE family, half of the members (spread over three generations) were affected by a mutation of the FOXP2 gene, while the other half was unaffected. In this case, the genes explained the differences in language ability between the family members, not the environment. Language is an impressive feature of the human species, which we tend to forget as it seems that every child learns language with relative ease. It takes much more effort to learn mathematics than it does to learn words. Yet there are differences between people. These can be subtle differences, for instance which specific words we use to tell the same story. These individual differences arise because of environmental diversity. But even when the environment is identical, changes in language performance are found: genetic variation can cause differences too. Why we need both nature and nurture So, nature or nurture? Going from 4 million years ago to the present day, to a hypothetical future, it seems that this is the wrong question to ask. Instead, the question should have been in which ways and to what extent the two interact. Language might well have evolved because humans created their own cultural niche, which resulted in genes necessary for language, which in turn happened to be beneficial. More than just whether language evolved but also which language evolved appears to be linked to genes as in the case of tone languages and upbringing. Finally, how well it is used appears to be an interaction of how you grow up as well as what genes you have inherited. On all three levels nature and nurture are not adversaries claiming to be right while proving the other one wrong. Instead, they complement each other. Each one provides a piece of a bigger picture which neither would be able to provide by itself. The natureversus-nurture debate? More like the nature-andnurture puzzle.

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