The tremendous changes in tools, in weaponry and hunting methods, and in art, along with the social and cultural changes they imply, could not have simply come out of the blue. The Upper Paleolithic advances point to some underlying mechanism that generated rapid genetic changes that conferred new capabilities. That mechanism, we believe, was introgression—that is, the transfer of alleles from another species, in this case Neanderthals. There is no faster way of acquiring new and useful genes.
Before we go further, we must acknowledge that this idea has not been much considered by paleontologists and anthropologists, mainly because they are not familiar with the arguments derived from population genetics that show that such introgression is highly likely. In addition, members of the general public who hear it for the first time may well be put off by the idea, since Neanderthals are usually considered backward, even apelike.
Many object to the notion of humans and Neanderthals mating and having offspring. Their first impulse is to suggest that anatomically modern humans and Neanderthals must have been too different, so that matings would not have produced fertile offspring. They say that humans would never have done such a disgusting thing. And they say that even if it happened, it was almost certainly rare, and thus biologically insignificant. None of these claims are correct: We will address them all.
The issue of whether or not there was mating between modern humans and Neanderthals is central to the debate that has raged for several decades about multiregional evolution versus a single African origin of our species. The strong multiregional position held that Neanderthals were directly ancestral to humans,6 while the strong single-Africa-origin model held that modern humans simply replaced the Neanderthals.7 It quickly became apparent in the face of genetic data that a dramatic out-of-Africa dispersal of modern humans did occur, but the extent of genetic exchange between the old and new humans was not resolved. Much debate occurred about whether there were anatomical continuities between Neanderthals and contemporary Europeans, the underlying assumption being that some sort of anatomical blending would have occurred. Our perspective on the issue, elaborated below, is quite different.
The first point made by critics is that modern humans and Neanderthals could not have been interfertile. However, we believe that they almost certainly were, since the two species had separated fairly recently, roughly half a million years earlier. No primates are known to have established reproductive isolation in so short a time.8 Bonobos, for example, branched off from common chimpanzees some 800,000 years ago, but the two species can have fertile offspring.9 Most mammalian sister species retain the ability to interbreed for far longer periods.10 Sometimes zookeepers are surprised by this, as when a dolphin and a false killer whale produce viable offspring.11 There are rumors about successful matings between primate lineages that separated as long as 5 million or 6 million years ago, but those are currently unsubstantiated. Nevertheless, there is no reason to think that during the Upper Paleolithic Neanderthals and anatomically modern humans could not have mated and had children who lived to also reproduce.
As for the idea that people just wouldn't have wanted to mate with creatures that were so different, we can only say that humans are known to have had sexual congress with vacuum cleaners, inflatable dolls, horses, and the Indus river dolphin. Any port in a storm, as it were. Jared Diamond recounted how a physician friend, treating a pneumonia patient with a limited
Venus of Dolni Vestonice, oldest Venus of Willendorf, «23,000 BC known ceramic, «27,000 BC
Lion Man of Hohlenstein, oldest known animal sculpture, «30,000 BC
command of English, had the patient's wife ask him if he'd had any sexual experiences that could have caused the infection. After the man recovered consciousness (his wife had knocked him cold as he began to answer), he admitted to repeated intercourse with sheep on the family farm.
The key point, which we will show in more detail later on, is that even rare interbreeding can be very important. If someone wanted to show that interbreeding between Neanderthals and modern humans was biologically insignificant, he would have to show that it never happened—and that is most unlikely, considering the human track record. If it happened at all, then intro-gression could have had a huge impact on human development.
A number of researchers have suggested that matings between Neanderthals and modern humans were rare and therefore biologically unimportant.12 But this objection is definitely incorrect: It is based on a misunderstanding of the genetics of natural selection. Some anthropologists who study anatomical details of Neanderthals and modern humans see evidence of Neanderthal features in some of the earliest modern humans in Europe,13 but others dispute the matter.
Imagine that humans occasionally mated with Neanderthals, and that at least some of their offspring were incorporated into human populations. That process would have introduced new gene variants, new alleles, into the human population. Many, probably most, of those alleles would have done almost exactly the same thing as their equivalents in modern out-of-Africa humans; they would have been neither better nor worse than those equivalents—in other words, they would have been selectively neutral. Those neutral alleles from Neanderthals would have been rare, and they would probably have disappeared, the typical fate of rare neutral alleles.
The reason is simply chance. When the bearer of a rare neutral allele has a child, that child has a 50 percent chance of carrying that allele. With two children (the average number in a stable population), there's a 25 percent chance that neither child will have a copy, and in that case, the imported allele disappears right then and there. More generally, the number of copies of a neutral allele fluctuates randomly with time, and any time the number hits zero, the story ends. If the original number of copies is low, this is fairly likely. Even if, by sheer luck, one or two neutral Neanderthal alleles had eventually become common in modern humans, there would have been no real consequences, since neutral alleles are boring by definition. Neanderthal alleles with negative consequences (in humans) would have disappeared even more rapidly. But some gene variants provide biological advantages and are adaptive. For those advantageous alleles, the story is entirely different.
The key property of an advantageous allele is that its frequency tends to increase with time, usually because it aids the bearer in some way. In a stable population, this means that the number of copies in the next generation is (on average) larger than the number in the current generation. If the average number of copies in the next generation were one and a quarter times larger than in the first, we would say that the allele had a selective advantage of 25 percent. As favorable alleles go, 25 percent is a very large advantage, although not unprecedented.
A single copy of an advantageous allele can still disappear, and probably will. With a 10 percent fitness advantage, a carrier in an otherwise stable population would average 2.2 offspring instead of 2, and there would still be a 23.75 percent chance of that allele disappearing in the first generation. But there is a way in which copies of this allele can survive: If luck holds out long enough, they will become more common—eventually, so common as to be effectively immune to chance. From that point on they steadily increase in numbers.
J. B. S. Haldane, the great British geneticist (1892-1964), found a systematic way of adding up all these probabilities, and his method yields a surprisingly simple answer. If the allele confers an advantage s, its chance of going all the way is 2s. In a stable population, a single copy of an allele with a 10 percent fitness advantage has a 20 percent chance of eventually becoming universal.
The fate of one copy of a favorable allele is very much like that of a gambler who starts out with one chip and a roulette system—a way of beating the odds—that really works. If he can pick the correct color (red or black) 55 percent of the time and bet one chip at a time, he'll usually go broke—but there's an 18 percent chance that he'll break the bank at Monte Carlo. And that's starting with one chip. With twenty chips, our friend (and who wouldn't want to have a friend like this?) would have a 98 percent shot at victory.
What this means is that one copy of an advantageous allele is much more likely to reach high frequencies than a single copy of a neutral allele—so much so that even a few dozen half-Neanderthal babies over thousands of years would have allowed modern humans to acquire most of the Neanderthals' genetic strengths.
Let's sketch an example. A neutral allele's chance of drifting to 100 percent (a state called "fixation") is the inverse of the number of gene copies in the population—one divided by twice the number of breeding individuals in the population, since each individual carries two copies of that gene. In other words, a neutral copy has exactly the same chance of reaching high frequency as every other neutral copy of that gene. For a population of any size, that chance is very small—for example, a chance of 1 in 20,000 for a human population with an effective size of 10,000. Such drift is also a very slow process, usually taking tens of thousands of generations.
Now consider an advantageous allele—a single copy of a new and improved version of a gene involving the immune system, one that made the bearer immune to some common and dangerous disease that normally killed off 10 percent of the population in childhood. That new allele would have a selective advantage of 10 percent. It might vanish; in fact it probably would, if, for example, the bearer managed to be stepped on by a mammoth or if none of his or her kids happened to carry that gene. But barring such accidents, the number of copies of that gene would tend to increase. Once the number of copies reached 50 or 100, the gene would be very unlikely to disappear by chance. From that point on there would be a fairly steady increase. It turns out that a single copy of that gene would have a 20 percent chance of making it big—going from one individual to, eventually, a significant fraction of the human race over the course of a few thousand years—assuming that the advantage persisted. That is, it would be 4,000 times more likely than a single neutral allele to reach fixation, and the process would be much faster.
If this advantageous allele was introduced by hybridizing with another species, rather than as a new mutation, it would likely be introduced repeatedly over a relatively short period of time, since there would probably be a number of such matings. If ten copies were introduced, the odds would be high that at least one of those copies would become a big success.
This reasoning goes against our intuition. Generally, we think that ancestry is something like mixing colors of paint: If you pour in equal amounts of blue and yellow, you'll get green— and the paint will remain green. If a population were 90 percent Norwegian and 10 percent Nigerian, intuition says that nine-to-one mix will remain the case indefinitely. But intuition is wrong: If you placed that mixed population in Africa, certain alleles that were common in Nigerians—alleles that protected against malaria, or that made skin dark and resistant to skin cancer—would become more and more common over many generations. Eventually almost everyone in that population would carry the Nigerian version of those genes.
In just this way, a tiny bit of Neanderthal ancestry thrown into the mix tens of thousands of years ago could have resulted in many people today, possibly even all modern humans, carrying the advantageous Neanderthal version of some genes.
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