Genetic changes allowed important human developments in 40,000 BC that hadn't been possible in 100,000 BC. Moreover, other genetic changes may have been necessary precursors to later cultural changes. Here we shall argue that the dramatic cultural changes that took place in the Upper Paleolithic, whichhave been referred to as the "human revolution," the "cultural explosion," or (our favorite), the "big bang," occurred largely because of underlying biological change.
We are not the first to suggest this. Richard Klein has said that some mutation must have been responsible for this dramatic increase in cultural complexity.4 We wholly agree with the spirit of his suggestion, but we believe that such dramatic change probably involved a number of genes, and thus some mechanism that could cause unusually rapid genetic change. As it turns out, we know of such a mechanism, and the necessary circumstances for that mechanism turn out to have arrived just in time for the human revolution.
NEW AND IMPROVED
So what exactly were the innovations of the Upper Paleolithic that have drawn attention to this period as a time of revolutionary change? For one thing, we see new tools, made from new materials—tools made using careful, multistep preparation. Modern humans still used stone (although their methods of preparation had grown more elaborate and efficient), but they often used bone and ivory as well, in sharp contrast with Neanderthals. They also used particular types of high-quality stone from distant sources, sometimes from as far as hundreds of miles away, a pattern that suggests trade. New types of light, high- velocity weapons appeared, such as javelins, atlatl darts, and eventually the bow and arrow. These weapons, which could be used at a distance, must have made bringing down big game far safer than it had been among hunters using thrusting spears. The skeletons of modern humans in this period, unlike those of
Neanderthals, are not thoroughly beat up. The weapons were no doubt used for warfare and defense, but they were primarily for hunting, and their benefits included broadening the modern humans' diet.
Moderns hunted small game and fish, in addition to the large game favored by their predecessors. This more varied diet (and perhaps safer hunting methods) led to higher population density—archaeological sites for modern humans during this period become several times more common than they had been among the Neanderthals. The moderns were able to catch fish using newly devised tools such as fishhooks, nets, and multi- barbed harpoon points. Those nets are a manifestation of another technological innovation, the use of plant fibers to make baskets, textiles, and rope as well as nets and snares.
Moderns developed new methods of preserving food, such as using drying racks and pits dug in the permafrost, which acted as natural refrigerators. They employed fire more efficiently than their ancestors had, developed hearths that had draft channels for better air flow, and began to use warming stones for cooking. Fire was used in other specialized ways as well—in lamps, for example, and to make pottery figures.
Burial—deliberate burial with clear-cut evidence of ritual— also becomes much more common in the Upper Paleolithic. The remains are often accompanied by grave goods such as tools, shells, personal items of jewelry, and red ochre. In some cases, production of those grave goods took tremendous effort. In Sungir, near Moscow, individuals were buried in clothes decorated with thousands of ivory beads whose manufacture required several man-years of effort. These findings suggests a hierarchically differentiated society, with chiefs as well as Indians.
These elaborate burials are in sharp contrast to Neanderthal burials, which show no sign of ceremony. We don't find weapons or decorative objects associated with those graves. It may be that for the Neanderthals, burial was more a way of disposing of unpleasant remains than a ritual occasion, something like flushing a goldfish down the toilet.
Modern humans began to build much more substantial protective structures. At Dolni Vestonice, located in what is now the Czech Republic, archaeologists have found the remains of five structures marked by mammoth bones, blocks of limestone, and postholes, the largest covering more than 1,000 square feet. In Russia and Ukraine, where natural shelters such as limestone caves were scarce, we see dwellings that use many mammoth bones. Building them must have involved serious effort: One such house contained some 23 tons of the bones of these large mammals.
The most striking change of the Upper Paleolithic, to modern eyes, is the birth of art. The most spectacular examples are the cave paintings, found primarily in France and Spain. Typical subjects are large animals such as bison, deer, and aurochs, but sometimes predators such as lions, bears, and hyenas are depicted. Made with carbon black or ochre, these paintings usually depict animals naturalistically. Humans, which show up rarely, often look quite strange.
The first real sculptures also appeared during this time. The most famous, the Venus figurines, such as the famous Venus of Willendorf (see page 38), may have been portable pornography. At Dolni Vestonice, researchers found ceramic figures made about 29,000 years ago, long before the invention of pottery in other parts of the world.
Lascaux cave painting, =14,000 BC
The art of the Upper Paleolithic was qualitatively different from the first symbolic objects seen in Africa before the expansion of modern humans: Compare the incised piece of ochre from Blombos Cave in South Africa dated about 75,000 BC5— representative of the most sophisticated symbolic objects discovered in pre-expansion Africa—with the lion-headed sculpture carved from mammoth ivory, found in Germany and dated about 30,000 BC (see pages 38 and 39).
FUSION
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 thatgenerated 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 multi- regional 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 whetherthere 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.
Interfertility
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.
Bestiality?
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
Blombos ochre, one of the oldest known symbolic objects, =70,000 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 Neanderthalswould 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 ofthat 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 twicethe 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 oftime, 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.
HOW DID IT HAPPEN?
If there really was interbreeding between Neanderthals and anatomically modern humans, how and where might it have happened?
There certainly might have been some gene flow among hominid species in earlier times. After all, Homo heidelbergensis (the common ancestor of Neanderthals and anatomically modern humans) somehow managed to settle both Europe and Africa about half a million years ago, so communication must have been possible, at least occasionally. It may have been impossible most of the time because of the Sahara Desert, which is a potent barrier today, just as it was during the ice ages. The Sinai Peninsula, also often a desert in its history, may also have been an important barrier, since it was the only land connection between Africa and Eurasia. More than that, Neanderthal alleles that were advantageous outside of Africa may not have been so in Africa and thus might not have spread to anatomically modern humans there.
The 10,000 Year Explosion Page 4