Masters of the Planet (28 page)

Just
as members of our species do, Neanderthals varied a bit in appearance from individual to individual, from place to place, and from time to time. Still, again like us, they all shared a distinctive common physical aspect. While Neanderthal braincases were capacious, they were also long and low, bulging at the sides and protruding at the back. (In contrast, our skulls are lightly built and globular, with a tiny face tucked underneath the front of a high, balloon-like braincase.) Neanderthal faces, hafted somewhat in front of the cranial vault, bore large noses (within which were some very unusual bony structures), and their cheekbones retreated rapidly at the sides. Below the neck the contrasts were equally striking. Compared to us the Neanderthals were heavily built, with thick-walled long bones bearing large, clunky joint surfaces at each end. Where our torsos are formed rather like barrels, tapering inward at top and bottom, theirs were funnel-shaped, tapering outward and down from a narrow top to match a broad, flaring pelvis below. This evidence of the skeleton thus joins other details in favoring the notion that Neanderthal gait was different from ours, stiffer and featuring greater rotation of the hips during striding. Beyond this, the general robustness of the Neanderthal skeleton also suggests great strength, and perhaps also high metabolic demands. Altogether, we are looking at a hominid that, although a fairly close relative, was anatomically distinct from
Homo sapiens
in numerous important details—although it seems to be us, not them, who have departed from the general hominid pattern in acquiring our unusual slender and gracile build. As far as we can tell from a less-than-perfect postcranial fossil record (though one that, significantly, contains the wonderful Sima sample) a broad pelvis and robust bone structure seem to have been characteristic of the entire Neanderthal lineage, and probably of all early
Homo
species.

We also differ from Neanderthals—and, as far as we know, from all other hominids—in the way in which we achieve our adult body form. We saw earlier that the Turkana Boy and other individuals of the
Homo ergaster/erectus
grade appeared to have developed much faster than
Homo sapiens
does, resulting in much shorter periods of both dependence and learning. And, despite its large brain,
Homo neanderthalensis
was no exception to this pattern. A recent study of Neanderthal dental development, using ultra-high-resolution techniques, has revealed that
while
the Neanderthal developmental period was indeed extended relative to earlier hominids, it was nonetheless shorter than our own. For example, the upper wisdom teeth (third molars) of one Neanderthal began developing at under six years of age, which is between three and four years earlier than in modern human children. Similarly, the first molars erupted substantially earlier in Neanderthals than in us. Translated into the overall developmental schedule, such data imply strongly that Neanderthals had a significantly shorter period of dependence on their parents than we do, and followed a faster path to sexual maturity. This conclusion coincides with analyses of the Neanderthal genome, which reveal that genes relating both to bodily and cognitive development differ from their equivalents in our own genomes.

Neanderthals also attained their characteristic cranial form through developmental trajectories that were not only faster than ours, but distinctly different. Sophisticated imaging and modeling techniques have shown that many of the characteristics that differentiate our faces from those of Neanderthals not only follow distinctive pathways of development postnatally, but also are well established at the time of birth. We cannot regard those many differences as superficial. Yet the actual shape of the brain is not among those features that are distinctive early on. Like Neanderthals, humans are born with longish skulls, which turns out to be a requisite of getting the neonate successfully through the birth canal; and we achieve our globular braincases in the first year of life, in the very rapid developmental spurt that propels the brain toward its unique external form. This dramatic early alteration in external form of the modern human brain and braincase is very unusual; and it is only possible once the constraints of the birth process are relaxed. The scientists who discovered it speculate that it may in some way be related to the internal reorganization of the brain that makes symbolic cognition possible.

NEANDERTHAL GENES

In 1997
Homo neanderthalensis
became the first extinct hominid species to have its DNA characterized. In that year, a German team ingeniously extracted a length of mitochondrial DNA (mtDNA) from
the
original Neanderthal specimen that had been found in Germany's Neander Valley in 1856. Mitochondrial DNA is a short ring of DNA that resides in tiny organelles that supply energy to each of our cells. These contain their DNA independently of the much greater quantities of the stuff in the cells' nuclei—and this is a huge advantage for scientists trying to compare the mutations that have accumulated over evolutionary time. The advantage arises from the fact that mtDNA is inherited uniquely through the mother and, unlike nuclear DNA, doesn't get jumbled up in each generation as the egg and sperm of the parents combine. The historical message it contains is thus much simpler to sort out. Among modern humans, mtDNA has turned out to be an amazingly useful marker for characterizing various populations and tracing their spread; and the Neanderthal mtDNA turned out to fall well outside the envelope of variation that describes all human populations today. To be precise, while the German researchers found an average of eight differences in the relevant part of the mitochondrial genome between pairs of modern human populations, and about 55 between humans and chimpanzees, the number for Neanderthals was 26. What's more, the Neanderthals lay equidistant from all the modern human populations tested.

Since 1997 mtDNA has been obtained from numerous Neanderthal specimens originating in all parts of the species' range, always with the same result. As expected, the Neanderthals differed somewhat among themselves, though a relatively low diversity has suggested to researchers that Neanderthal populations were typically small, something that archaeologists had also guessed from the low relative abundance of the occupation sites they left behind. All the Neanderthals still clustered together, in contradistinction to
Homo sapiens,
and in numerous studies researchers have been unable to detect any Neanderthal contribution to the DNA of an extensive sample of modern Europeans.

These findings reinforced the notion derived from anatomical studies that
Homo neanderthalensis
was its own species, an effectively individuated entity with its own history and its own fate. However, nature is an untidy place, and species can be leaky vessels—especially where they are very closely related as well as actors in a fast-moving evolutionary drama, as hominids during the Pleistocene most assuredly
were.
In 2010, the German group announced another first—a draft version of a complete Neanderthal nuclear genome (taken from three samples of bone from the Croatian cave of Vindija, dated to about 40 thousand years ago). These samples provided a vast data base. There are more than three billion individual “nucleotides”—basically, data points—in a human genome; and interpreting this Neanderthal genome meant massaging all of those data points through some very hefty computer algorithms. But after all the necessary manipulations (with which not everyone is entirely happy), the researchers reported that “Neandertals shared more genetic variants with present-day humans in Eurasia than with present-day humans in sub-Saharan Africa, suggesting that gene flows from Neandertals into the ancestors of non-Africans occurred before the divergence of Eurasian groups from each other.” Actually, on closer examination the apparent gene flow (i.e., gene transfer due to interbreeding) turned out to be in the order of 1 to 4 percent: hardly vast and, oddly, only one way: from Neanderthals into modern humans.

Even odder is a result reported by the same group shortly thereafter. These industrious researchers had already found that a morphologically uncategorizable finger bone from the southern Siberian cave of Denisova, only some 30 thousand years old, yielded a DNA fingerprint that distinguished it from both modern humans and Neanderthals, although it seemed to be somehow related to the latter. A complete genome was then obtained from this specimen, and is said to share a small proportion of its genes with modern-day Melanesians (and nobody else), suggesting—if true—that the ancestral Melanesians might have picked up these genetic variants on their way out of Africa and across Asia to the Pacific. A molar from Denisova produced basically the same genetic signature; but this tooth is both extremely large and morphologically dissimilar to any other hominid teeth known from so late in time, emphasizing that morphological and genetic evidence may sometimes be apparently at odds. However these findings are eventually interpreted, they suggest that events in later hominid evolution may have been very complex, and that the historically and functionally individuated entities that we recognize as hominid species may nonetheless have occasionally exchanged genetic material.

Perhaps
such an exchange has even been an important source of genetic innovation in the human past. Not long ago, a group of molecular biologists in Chicago reported that a rapidly spreading variant of the microcephalin gene, important in regulating brain size, appeared to have been imported into the
Homo sapiens
genome only some 37 thousand years before the present. Their calculations suggested that it might have been introduced into our species from a relative that had separated from our lineage a little over a million years ago; and the Neanderthals seemed to them to fit the bill, though in fact any other hominid “donor” species might have been involved. At this point it's probably too early to know quite what to make of observations such as these (and 37 thousand years ago is too late to have made a material difference in the emergence of our own species); but it is not out of the question that minor gene exchange among closely related hominid species at an earlier time may have had a significant role in furnishing the ancestors of
Homo sapiens
with new genetic material.

This in itself is nothing remarkable. It has long been known that genes are occasionally exchanged between well-differentiated mammals. Indeed, there is a pair of ligers—huge hybrid beasts with lion and tiger parents—resident in an animal park in South Carolina right now. These are fearsome creatures indeed; and especially in view of their vigor you might be surprised to learn that lions and tigers are not even each other's recently diverged closest relatives. Lions are actually more closely related to jaguars, and tigers to snow leopards; and the last common ancestor of lions and tigers lived around four million years ago. But in spite of these impressive hybrids, nobody is out there arguing that lions and tigers are not fully individuated entities, each one with its own independent history and evolutionary trajectory. Despite that little genetic romp, there is no reasonable likelihood whatever that the two big cats will ever merge into a blended unit combining the characteristics of both parental populations. Closer to hominid home, the same thing seems to go for closely related primates that intermix. In Ethiopia, hybridization regularly occurs in a specific zone between hamadryas and gelada baboons, two closely related monkeys that are strikingly different to the eye. But even there, we see no indication that either broader parental species is losing its distinctive physical identity.

To
put all this in context, the difference in skull structure between
Homo neanderthalensis
and
Homo sapiens
is far greater than what we see between hamadryas and geladas—and also greater than the one between lions and tigers. And whether or not acts of mating may occasionally have occurred between members of the two hominid species, the probability is negligible that there was any evolutionarily significant genetic interchange between them. In other words, nothing seems to have occurred that might have influenced the future fate of either, and the populations never integrated to any significant extent. Claimed “hybrids” such as the very late skeleton discovered at the Abrigo do Lagar Velho in Portugal, or the odd early
Homo sapiens
skull from the Pe
tera cu Oase in Romania, turn out on closer inspection to be somewhat unusual modern humans. What's more, and very significantly, the archaeological record is in parallel sending us a more or less identical signal of inconsequential or nonexistent cultural intermixing. From every line of evidence we have, it seems that
Homo sapiens
and
Homo neanderthalensis
were differentiated entities, each with its own history and way of doing business. Even if the odds may be reasonable that there was occasionally a bit of Pleistocene hanky-panky, swapping the odd stretch of DNA didn't change that functional reality.