The Dawn of Human Culture (21 page)
might have been accelerated by sexual selection—the tendency for people to seek mates based on arbitrary, locally defined beauty standards.
The Neanderthal body is easier to explain. Both sexes were heavily muscled, and there is no mystery as to how they got that way—
they exercised a lot, and they probably had to, if only to obtain food under challenging circumstances. Despite the great thickness of their 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 178
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bones, Neanderthals often broke them, and anthropologists Thomas Berger and Erik Trinkaus have shown that they suffered head-and-neck trauma about as often as modern rodeo riders. Neanderthals obviously did not ride bucking broncos or Brahma bulls, but they probably found hunting the wild equivalents about equally traumatic, particularly if their weaponry was as limited as we suggest further on.
Climatic adaptation probably explains why the Neanderthals had such broad, barrel-like chests and short limbs. Over the 400,000
years or so when the Neanderthals were evolving in Europe, global climate periodically alternated between cold glacial and warm interglacial periods. On average, glacial episodes were much longer, and times when temperatures approximated historic ones were especially rare and short. This means that the Neanderthals existed mainly under cold-to-very-cold conditions, and we know that living humans who live in cold climates tend to have much larger trunks and shorter limbs than people who live in hot, tropical climates. One need only compare the stocky appearance of an Inuit person with the slender build of a Nilotic African. We explored the reason for the difference when we explained the lanky build of the Turkana Boy and other early true humans. The essential point is that as trunk volume increases, skin area increases much more slowly, and a larger trunk is thus better at conserving heat. Short limbs similarly reduce heat loss. Near the Equator, the problem is to keep cool, and slender trunks and long limbs help to dissipate heat. The bottom line is that Neanderthal body proportions were predictable from the cold conditions under which they evolved.
The story does not end there, however, because the Neanderthals had even broader trunks and shorter limbs than the Inuit, yet even 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 179
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during glacial periods, mid-latitude Europe, where the Neanderthals evolved, was milder than the high arctic where the Inuit lived historically. The Inuit of course adapted more by culture than by body form, and they are famous for their ingenious, well-heated homes and their carefully tailored fur clothing. Archeology reveals neither trait until the appearance of the fully modern humans who succeeded the Neanderthals in Europe. These people arrived with long, linear, tropical body proportions, as if to signal their recent Equatorial origin, and they never developed an “arctic” body shape, even though they soon faced peak glacial cold. They also managed to colonize the harshest, most continental parts of northeastern Europe and northern Asia, where no one, Neanderthals included, had lived before. Their success illustrates what a difference a little culture can make, and their advanced cultural capabilities help to explain how they were able to replace the Neanderthals so quickly and completely.
There remains the need to account for the Neanderthals’ large brain. In part, the explanation must be the generic one, in which a larger brain promoted new, highly adaptive behaviors, including an unsurpassed ability to flake stone. However, in living humans, average brain size tends to be greatest in populations that are heavily muscled or that inhabit especially cold environments, and the Inuit top the list, with an average brain size that approaches or equals that of the Neanderthals. The earliest fully modern Europeans had even larger brains, and they were also heavily muscled and surrounded by glacial cold. In short, if we assume that Neanderthals obeyed the same basic physiological principles as living humans, their brains were probably large in part for reasons that had nothing to do with intelligence or behavioral potential. If we consider encephalization—the ratio of brain 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 180
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mass to body mass—the Neanderthals were actually somewhat less encephalized than modern humans. This includes all living humans, none of whom equaled the Neanderthals in body mass, even though some, like the Inuit, approached them in brain size. By itself, a lower degree of encephalization need not mean that Neanderthals were less intelligent than modern humans, but it certainly suggests they could have been. This is especially true because the archeological record suggests that they were behaviorally far less innovative.
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This idea is appealing, but it is almost surely wrong. To begin with, we have skulls and other bones of very young Neanderthals, including infants, and these already exhibit classic Neanderthal facial and skull specializations. Since the very youngest individuals had probably never used tools, their Neanderthal features must have been hardwired. Second, and even more compelling, we now have Neanderthal genes, and they confirm that Neanderthals diverged genetically from living humans long before living human groups diverged from each other.
Until recently, the recovery of genetic material from Neanderthal bones seemed like the biological equivalent of squeezing blood from a stone. The problem is that after an organism dies, its DNA immediately begins to degrade from exposure to microorganisms and to the elements. Bones offer some shelter, but even thick bones will not protect DNA indefinitely. Experts put the upper time limit at about 100,000 years, and to reach that, a relatively cool burial environment is probably required. Among sites that could have provided a suitable context, the Feldhofer Grotto looked likely, and in the early 1990s, a team led by Svante Pääbo and Matthias Krings, now at the Max Planck Institute for Evolutionary Anthropology in Leipzig, began a search for surviving DNA in a 3.5-gram chunk from the right upper arm bone of the original Feldhofer Neanderthal.
The bones of living creatures are rich in proteins, which are composed of amino acids. As a first step in their analysis, Pääbo’s team sought to determine whether the Feldhofer bone retained different amino acids in the same proportions in which they occur in proteins and whether their physical state had been strongly altered during burial. When both indicators suggested a promising degree of protein 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 182
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survival, the investigators concentrated on retrieving mitochondrial DNA, routinely abbreviated as mtDNA. Unlike nuclear DNA, which by definition is confined to the nucleus of each cell, mtDNA resides outside the nucleus, in hundreds of organelles or mitochondria that supply the cell with energy. The sheer abundance of mtDNA (versus nuclear DNA) copies in a live person increased the likelihood that some would survive in the Feldhofer bone. Compared to nuclear DNA, mtDNA has two strong additional advantages for reconstructing evolutionary history: it evolves about ten times faster, and it is inherited entirely through females. The faster rate of change (mutation) means that mtDNA is much more likely to reveal recent population splits.
Inheritance only through females facilitates the tracing of individual evolutionary lineages. Nuclear DNA lines are harder to trace backwards, because nuclear DNA comes half from the female parent and half from the male, and it is reshuffled at conception, blurring its specific parental origin. For a rough understanding of the problem this presents and why mtDNA offers an advantage, consider how much more difficult it would be to reconstruct a person’s genealogy if children could arbitrarily mix portions of their father’s and mother’s pre-marital last names to formulate their own.
In 1987, University of California geneticists Rebecca Cann, Mark Stoneking, and Alan Wilson introduced many paleoanthropologists to mtDNA when they published a landmark study of mtDNA variation in living humans. They showed that mtDNA diversity is greater in Africa than anywhere else, that diversity elsewhere is essentially a subset of diversity in Africa, and that the oldest (deepest) mtDNA lineages reside in Africa. From the way the diversity was patterned, they reasoned that the last shared mtDNA ancestor of living humans must 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 183
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have lived in Africa, and from the presumed rate of mtDNA divergence, they suggested that she—and by definition it had to be a she—existed there within the last 200,000 years. In a mix of scientific and biblical metaphors, this “one lucky mother” soon became known popularly as
“African (or mitochondrial) Eve.” Subsequent studies of mtDNA variation in living humans, including an especially thorough analysis that Pääbo’s team published in December 2000, have repeatedly corrobo-rated the original University of California result. The bottom line is that even as Pääbo’s group began their quest for Neanderthal mtDNA, there was already good reason to suppose that no ancient Eurasian group—
neither the Neanderthals nor east Asian
Homo erectus
—could have contributed many genes to living human populations. Studies of nuclear DNA, cleverly designed to circumvent the problem of biparental inheritance and recombination at fertilization, support the same conclusion, and recent analyses of the Y chromosome confirm it even more strongly. Loosely speaking, the Y chromosome is the male equivalent of mtDNA, since it is inherited only through males. Its pattern of diversity in living humans reveals that mitochondrial Eve had a male counterpart—“African Adam”—who existed in Africa sometime between 200,000 and perhaps 50,000 years ago.
Structurally, DNA comprises strings of four chemical building blocks called nucleotides (or bases, individually abbreviated as A, T, C, and G), and to reconstruct evolutionary history, geneticists now routinely compare nucleotide sequences. If two individuals share similar sequences, they are presumed to share a relatively recent ancestor; if their sequences are more divergent, the individuals are assumed to be more distantly related. The mtDNA genome in living humans comprises about 16,500 nucleotides, but Pääbo and his team never expected to 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 184
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find a complete sequence in the Feldhofer Neanderthal, and they were delighted when the arm bone provided small fragments. They amplified the fragments using the now famous polymerase chain reaction (PCR), which lies at the heart of most modern molecular genetic research. Their first task was to determine whether the fragments might originate from shed skin cells or ill-timed sneezes by some of the people who had handled the Feldhofer bones since their discovery 140 years earlier. Ten percent of the fragments exhibited sequences that suggested they were modern contaminants, but the remaining ninety percent were readily distinguishable from their counterparts in living humans, and it was on these that Pääbo’s team concentrated.
They sequenced a reconstructed fragment 379 nucleotides long from the so-called mitochondrial control region, and they compared the result to sequences at the same position in the control regions of 994 living humans drawn from all over the globe. On average, the modern sequences differed from each other at eight nucleotide positions, while the Neanderthal sequence differed from the modern ones at twenty-seven positions. Using a rate of sequence divergence inferred from a chimpanzee/human split 4 to 5 million years ago, Pääbo and his colleagues estimated that the last shared mtDNA ancestor of Neanderthals and modern humans lived between 690,000 and 550,000
years ago. When they applied the same procedure to the modern human sequences in their analysis, they estimated that the last shared mtDNA ancestor of living people existed much later, between 150,000
and 120,000 years ago. Since the actual time when the Neanderthal and modern human lines split must postdate the age of their last shared mtDNA ancestor, the estimated age for the ancestor is completely compatible with a population split following the spread of 06 Neanderthals.r.qxd 1/29/02 5:05 PM Page 185
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Homo heidelbergensis
and late Acheulean artifacts from Africa to Europe about 500,000 years ago.
To provide maximum credibility for their finding, Pääbo’s group sent a sample of the Feldhofer arm bone to the Anthropological Genetics Laboratory at Pennsylvania State University, and when the second lab independently extracted mtDNA with the same sequence, the two labs published the result jointly. Their report appeared in the July 1997 issue of the journal
Cell,
accompanied by a commentary that called it “a tour-de-force investigation of ancient DNA.”
Pääbo’s team subsequently sequenced a somewhat longer fragment of mtDNA from the same Feldhofer bone, and this confirmed that Neanderthal and living human mtDNA differed from each other in about three times as many positions as modern human sequences differ from each other. There was still the problem that the Feldhofer Neanderthal comprised a sample of one, but in March 2000, a University of Glasgow team led by William Goodwin published a very similar result from the rib of a Neanderthal child excavated at Mezmaiskaya Cave in southern Russia, and in October 2000, Pääbo’s team reported a third, confirmatory sequence from a piece of Neanderthal bone recovered at Vindija Cave in Croatia. There could now be no doubt that even Neanderthals widely dispersed throughout Europe were much more closely related to each other than they were to any living humans, European or otherwise. In the words of Pääbo’s team, the fossil DNA sequences demonstrated that “Neanderthals did not end up contributing mtDNA to the contemporary [that is, historic] human gene pool.”
