Neanderthal Man (34 page)
This focus on teaching and learning probably has fundamental consequences for human societies. Whereas apes must learn every skill they eventually acquire through trial-and-error and without a parent or other group member actively teaching them, humans can much more effectively build on the accumulated knowledge of previous generations. As a result, when an engineer improves a car, she need not invent it from scratch. She will build on the inventions of previous generations all the way back to the invention of the combustion engine in the twentieth century and of the wheel in antiquity. To this accumulated wisdom of her ancestors she will merely add some modifications to the design that later generations of engineers will in turn take for granted and continue to build upon. Mike calls this the “ratchet effect.” It is clearly a key to the enormous cultural and technological success of humans.
My fascination with Mike’s work stems from my conviction that there are genetic underpinnings to our propensity for shared attention and the ability to learn complex things from others. In fact, there is ample evidence to suggest that genetic traits are a necessary foundation to these human behaviors. In the past, people sometimes did what we now consider to be unethical experiments in which they raised newborn apes together with their own children in their home. Although apes learned how to do many human-like things—they could construct simple two-word sentences, manipulate household appliances, use bicycles, and smoke cigarettes—they did not learn truly complex skills and they did not engage in communication on the scale that humans do. In essence, they did not become cognitively human. So it’s clear that there is a biological substrate necessary for fully acquiring human culture.
This is not to say that genes alone are sufficient for acquiring human culture, only that they are a necessary substrate. In the imaginary experiment where a human child is raised in the absence of any contact with other human beings, it is very likely that the child would never develop most of the cognitive traits that we associate with humans, including awareness of the interests of others. That unfortunate child would probably also not develop the most sophisticated of cultural traits that emanates from our tendency to share attention with others: language. So I am convinced that social input is necessary for the development of human cognition. However, no matter how early in life and how intensively they are integrated into human society and no matter how much teaching they are subject to, apes do not develop more than rudimentary cultural skills. Social training alone is not enough. A genetic readiness to acquire human culture is necessary. Similarly, I am convinced that a newborn human raised by chimpanzees would fail to become cognitively chimpanzee. There is surely also a genetic substrate necessary to becoming fully chimpanzee that humans lack. But since we are humans, we are more interested in what makes humans human than in what makes chimpanzees chimpanzee. We should not be ashamed of being “humancentric” in our interests. In fact, there is an objective reason to be so parochial. The reason is that humans, and not chimpanzees, have come to dominate much of the planet and the biosphere. We have done so because of the power of our culture and technology; these have allowed us to increase our numbers vastly, to colonize areas of the planet that otherwise would not have been habitable for us, and to have an impact on and even threaten aspects of the biosphere. Understanding what caused this unique development is one of the most fascinating, perhaps even one of the most pressing, problems that scientists face today. One key to the genetic underpinnings of this development may well be found through comparing the genomes of present-day humans with Neanderthals. Indeed, it is this feeling that kept me going during years of struggling with the technical minutiae of retrieving the Neanderthal genome.
According to the fossil record, Neanderthals appeared between 300,000 and 400,000 years ago and existed until about 30,000 years ago. Throughout their entire existence their technology did not change much. They continued to produce much the same technology throughout their history, a history that was three or four times longer than what modern humans have experienced. Only at the very end of their history, when they may have had contact with modern humans, does their technology change in some regions. Over the millennia, they expanded and retracted with the changing climates in the areas they lived in Europe and western Asia, but they didn’t expand across open water to other uninhabited parts of the world. They spread pretty much as other large mammals had done before them. In that, they were similar to other extinct forms of humans that had existed in Africa for the past 6 million years and in Asia and Europe for about 2 million years.
All of this changed abruptly when fully modern humans appeared in Africa and spread around the world in the form of the replacement crowd. In the 50,000 years that followed—a time four to eight times shorter than the entire length of time the Neanderthals existed—the replacement crowd not only settled on almost every habitable speck of land on the planet, they developed technology that allowed them to go to the moon and beyond. If there is a genetic underpinning to this cultural and technological explosion, as I’m sure there is, then scientists should eventually be able to understand this by comparing the genomes of Neanderthals to the genomes of people living today.
Fueled by this dream, I was itching to start looking for crucial differences between Neanderthals and present-day humans once Udo had finally mapped all the Neanderthal fragments in the summer of 2009. But I also realized that I needed to be realistic about what those differences would tell us. The dirty little secret of genomics is that we still know next to nothing about how a genome translates into the particularities of a living and breathing individual. If I sequenced my own genome and showed it to a geneticist, she would be able to say approximately where on the planet I or my ancestors came from by matching variants in my genome with the geographic patterns of variants across the globe. She would not, however, be able to tell whether I was smart or dumb, tall or short, or almost anything else that matters with respect to how I function as a human being. Indeed, despite the fact that most efforts to understand the genome have sprung from efforts to combat disease, for the vast majority of diseases, such as Alzheimer’s, cancer, diabetes, or heart disease, our current understanding allows us only to assign vague probabilities to the likelihood that an individual will develop them. So in my realistic moments, I realized that we would not be able to directly identify the genetic underpinnings of the differences between Neanderthals and modern humans. There would be no smoking gun to be found.
Still, the Neanderthal genome was a tool that would allow us to begin to ask questions about what set Neanderthals and humans apart—a tool that not only we but all future generations of biologists and anthropologists would be able to use. The first step was obviously to make a catalog of all the genetic changes that happened in the ancestors of people living today after they separated from the ancestors of the Neanderthals. These changes would be many, and most of them would be without great consequences, but the crucial genetic events that we were interested in would be hidden among them.
The crucial task of making the first version of such a catalog of all changes unique to modern humans was taken on by Martin Kircher together with his supervisor Janet Kelso. Ideally, such a catalog should contain all genetic changes that are present today in all or nearly all humans and that occurred after modern humans parted ways with the ancestors of Neanderthals. The catalog should thus list positions in the genome where the Neanderthal looked like the chimpanzee and other apes while all humans, no matter where they lived on the planet, differed from the Neanderthals and the apes. However, in 2009 there were many limitations to how complete and correct such a catalog could be. First of all, we had sequenced only about 60 percent of the Neanderthal genome so the catalog could only be 60 percent complete. Second, even if we saw a difference from the human reference genome at a position where the Neanderthal genome looked like the chimpanzee genome, this did not necessarily mean that all humans today looked like the human reference genome. In fact, most such positions would vary among humans, but our knowledge about genetic variation among humans was too incomplete to differentiate real finds and false positives. Fortunately, there were several large projects under way aimed at describing the extent of genetic variation among humans, including the 1,000 Genomes Project, the goal of which was detecting all variants in the human genome present in 1 percent or more of humans. But that project was just starting. A third apparent limitation was that our genome was a composite of sequences from only three Neanderthals, and for most positions, we had only the sequence of a single Neanderthal individual. However, I didn’t view this as overly problematic. As long as one single Neanderthal had the ape-like, ancestral version at a given position, it didn’t matter if other Neanderthals that we hadn’t sequenced carried the derived, new version that we saw in humans today. The knowledge that the ancestral variant was in at least one Neanderthal told us that it had still been around when Neanderthals and modern humans parted ways, perhaps 400,000 years ago. This made it a potential candidate for defining what might be universally modern human.
Janet and Martin compared the human reference genome with the chimpanzee, orangutan, and macaque genomes and identified all positions where they differed. They then compared all four genomes to our Neanderthal DNA sequences, being careful to compare only those Neanderthal DNA sequences for which we had complete certainty as to where they came from in the genome. They found that we had Neanderthal sequence coverage for 3,202,190 positions where nucleotide changes had occurred on the human lineage. For the vast majority of these positions, the Neanderthals looked like us, which was not surprising, given that we are much more closely related to Neanderthals than to apes. But for 12.1 percent of these positions, the Neanderthal looked like the apes. They then checked whether the ancestral variants seen in apes and Neanderthals were still present in some humans today; in most cases they found both the ancestral and the new variants in present-day humans. This was not surprising because the mutations responsible happened quite recently. But some of these new variants were, as far as we could tell, present in all humans today. These were the positions that we found particularly interesting.
Most tantalizing were those changes that might have functional consequences. First and foremost among these were the ones that change amino acids in proteins. Proteins, of course, are encoded by stretches of DNA sequence in the genome called “genes.” Proteins are made up of strings of twenty different amino acids and perform many jobs in our bodies, such as regulating the activity of genes, building up tissues, and controlling our metabolism. As a result, changes in proteins are more likely to have consequences for an organism than a mutation randomly chosen from the set of all the mutations we identified. Such potentially meaningful mutations—which result in one amino acid in a protein being replaced by another one, or change how long a protein is—occur much less often during evolution than nucleotide substitutions that do not cause such dramatic alterations. Ultimately, Martin showed me a list of 78 amino-acid-altering nucleotide positions where, as far as we knew, all humans today were similar to one another but different from the Neanderthal genome and the apes. We expected to both add and subtract mutations from this list as both the Neanderthal genome and the 1,000 Genomes Project neared completion. So an educated guess might be that the total number of amino-acid changes that had spread to all modern humans since we separated from Neanderthals would be less than 200.
In the future, when we’ll have a much fuller understanding of how each protein influences our bodies and minds, biologists will often be able to affix a function to a particular amino acid in a protein and to identify whether it functioned the same way in Neanderthals. Unfortunately, such a comprehensive knowledge of our genome and biology will likely be achieved only long after I have joined the Neanderthals in death. However, I take some solace in the thought that the Neanderthal genome (and the improved versions of it that we and others will achieve in the future) will be a crucial contribution to this endeavor.
For the moment, though, the 78 amino-acid positions provided us with very few and only the very crudest of insights. Just looking at what the changes were gave us very little idea about what might have changed in the biology of the first individual to carry the new variant. However, one thing we did notice was that there were five proteins that each carried not just one but two amino-acid differences. This was very unlikely to have occurred by chance if a total of 78 mutations were to be randomly scattered among the 20,000 proteins encoded by the genome. These five proteins may therefore have altered their functions recently in human history. It is even possible that they lost their function or importance so that they were now free to accumulate changes unhindered by any constraints imposed upon them by their function. Either way, we knew we had to take a closer look at these five proteins.
The first protein with two changes was involved in sperm motility. I was not very surprised by this. Among human and nonhuman primates alike, genes involved in male reproduction and sperm motility have been known to frequently change, probably due to direct competition between sperm cells from different males when females copulated with multiple partners. This overt competition means that any genetic change that makes a sperm cell more likely to fertilize the egg than its competitors, perhaps by swimming faster, will spread in the population. Such a change is considered to be under positive selection, because it increases the chance the individual with the mutation will leave progeny in the next generation. In fact, the more direct competition there is between sperm cells from different males in a single female (head-to-head, so to speak), the more positive selection can act. So there is a correlation between the level of promiscuity in a species and the extent to which positive selection can be detected in genes that have to do with male reproduction. Among chimpanzees, where a female in estrus tends to copulate with all males that happen to be available to her, there is more evidence for positive selection on such genes than among gorillas, where one dominant, silverback male tends to monopolize all females in his group. The sperm of a patriarchal gorilla silverback have all the time they need to fertilize the egg, since the sperm of younger and subordinate males cannot enter into the race. Or rather, the competition has already taken place at an earlier stage on the social level, when the hierarchy in the group was established. Amazingly, even crude measures such as the size of the testicles relative to the body reflect this difference in male competition for fertilizations. Whereas chimpanzees have large testicles, and the even more promiscuous but smaller bonobos carry around even more impressive sperm factories, the intimidatingly huge silverback gorillas have puny little testicles. Humans, as measured both by testicle size and evidence for positive selection on genes relevant for male reproduction, seem to be somewhere between the extremes of chimpanzee promiscuity and gorilla monogamy, suggesting that our ancestors may have been not so unlike us, vacillating between emotionally rewarding fidelity to a partner and sexually alluring alternatives.
