Molecular Connections: The Genetic Evidence
With revolutionary advances in molecular biology , fascinating new avenues of research have become possible in the study of earlier hominins. It’s becoming fairly commonplace to extract, amplify, and sequence ancient DNA from contexts spanning the last 10,000 years or so. For example, researchers have analyzed DNA from the 5,000-year-old “Iceman” found in the Italian Alps as well the entire nuclear genome from a 4,000-year-old Inuit (Eskimo) from Greenland (Rasmussen et al., 2010).
It’s much harder to find usable DNA in even more ancient remains, since the organic components, often including the DNA, have been destroyed during the mineralization process. Still, in the past few years, exciting results have been announced about DNA found in more than a dozen different Neandertal fossils dated between 50,000 and 32,000 ya. These fossils come from sites in France (including La Chapelle), Germany (from the original Neander Valley locality), Belgium, Italy, Spain, Croatia, and Russia (Krings et al., 1997, 2000; Ovchinnikov et al., 2000; Schmitz et al., 2002; Serre et al., 2004; Green et al., 2006). As we previously mentioned, recently ascertained ancient DNA evidence strongly suggests that other fossils from central Asia (Uzbekistan and two caves in southern Siberia) dated at 48,000–30,000 ya are also Neandertals (Krause et al., 2007b) or even an entirely different species (Krause et al., 2010; Reich et al., 2010).
The technique most often used in studying most Neandertal fossils involves extracting mitochondrial DNA (mtDNA), amplifying it through polymerase chain reaction (PCR;) and sequencing nucleotides in parts of the molecule. Initial results from the Neandertal specimens show that these individuals are genetically more different from contemporary H. sapiens populations than modern human populations are from each other—in fact, about three times as much.
Major advances in molecular biology have allowed much more of the Neandertal genetic pattern to be determined with the ability to now sequence the entire mtDNA sequence in several individuals (Briggs et al., 2009) as well as big chunks of the nuclear DNA (which, as you may recall, contains more than 99 percent of the human genome). In fact, the most exciting breakthrough yet in ancient DNA studies was achieved in 2010 with the completion of the entire nuclear genome of European Neandertals (Green et al., 2010). Just a couple of years ago, this sort of achievement would have seemed like science fiction.
This new information has already allowed for crucial (as well as quite surprising) revisions in our understanding of Neandertal and early modern human evolution. First of all, Neandertal DNA is remarkably similar to modern human DNA, with 99.84 percent of it being identical. However, to detect those few (but possibly informative) genes that do differ, the team sequenced the entire genome of five modern individuals (two from Africa and one each from China, France, and New Guinea). To the surprise of almost everyone, the researchers found that many people today still have Neandertal genes! What’s more, these Neandertal genes are found only in nonAfricans, strongly suggesting that interbreeding occurred between Neandertals and modern H. sapiens after the latter had emigrated out of Africa. In fact, the three modern non-African individuals used for comparison in this study all had the same amount of Neandertal DNA. What makes this finding even more startling is the fact that the three individuals evaluated come from widely scattered regions (western Europe, China, and the far South Pacific). Further evidence, including complete genomes from another seven modern people from even more dispersed populations, have further confirmed these findings (Reich et al., 2010).
The best (and simplest) hypothesis for this genetic pattern is that shortly after modern H. sapiens migrants left Africa, a few of them interbred with Neandertals before these people and their descendants dispersed to other areas of the world. The best guess is that this intermixing between the two groups occurred in the Middle East, likely sometime between 80,000 and 50,000 ya. DNA data from more individuals, both within and outside of Africa, will help substantiate this hypothesis. For the moment, the degree of interbreeding appears to be small but still significant—about 1 to 4 percent of the total genome for living non-Africans.
Another quite astonishing molecular finding also came in 2010 during the analysis of the Denisovan DNA from Siberia. These ancient hominins from central Asia quite possibly represent a different branch of recent human evolution (Reich et al., 2010). They are also more closely related to just some populations of modern humans, sharing about 4 to 5 percent of genes with contemporary people from Melanesia (a region of islands in the south Pacific, including New Guinea, located north and east of Australia). All of us derive mostly from fairly recent African ancestors. But when these African migrants came into contact with premodern humans living in Eurasia, some interbreeding occurred with at least two of these premodern groups. And we can tell this by distinctive genetic “signatures” that can still be found in living people.
What’s more, we’ve already had tantalizing clues of how we differ from Neandertals in terms of specific genes. As the data are further analyzed and expanded, we will surely learn more about the evolutionary development of human anatomy and human behavior. In so doing, we’ll be able to answer far more precisely that age-old question, What does it mean to be human?
Seeing Close Human Connections: Understanding Premodern Humans
As you can see, the Middle Pleistocene hominins are a very diverse group, broadly dispersed through time and space. There is considerable variation among them, and it’s not easy to get a clear evolutionary picture. We know that regional populations were small and frequently isolated, and many of them probably died out and left no descendants. So it’s a mistake to see an “ancestor” in every fossil find. Still, as a group, these Middle Pleistocene premoderns do reveal some general trends. In many ways, for example, it seems that they were transitional between the hominins that came before them (H. erectus) and the ones that followed them (modern H. sapiens). It’s not a stretch to say that all the Middle Pleistocene premoderns derived from H. erectus forebears and that some of them, in turn, were probably ancestors of the earliest fully modern humans.
Paleoanthropologists are certainly concerned with such broad generalities as these, but they also want to focus on meaningful anatomical, environmental, and behavioral details as well as the underlying processes. So they consider the regional variability displayed by particular fossil samples as significant— but just how significant is debatable. In addition, increasingly sophisticated theoretical and technological approaches are being used to better understand the processes that shaped the evolution of later Homo at both macroevolutionary and microevolutionary levels. Scientists, like all humans, assign names or labels to phenomena, a point we addressed. Paleoanthropologists are certainly no exception. Yet, working from a common evolutionary foundation, paleoanthropologists still come to different conclusions about the most appropriate way to interpret the Middle/Late Pleistocene hominins. Consequently, a variety of species names have been proposed in recent years. Paleoanthropologists who advocate an extreme lumping approach recognize only one species for all the premodern humans discussed in this chapter. These premoderns are classified as Homo sapiens and are thus lumped together with modern humans, although they’re partly distinguished by such terminology as “archaic H. sapiens.” As we’ve noted, this degree of lumping is no longer supported by most researchers. Alternatively, a second, less extreme view postulates modest species diversity and labels the earlier premoderns as H. heidelbergensis. At the other end of the spectrum, more enthusiastic paleontological splitters have identified at least two (or more) species distinct from H. sapiens. The most important of these, H. heidelbergensis and H. neanderthalensis, have been discussed earlier. This more complex evolutionary interpretation .
we know that disparities like these can be frustrating to students who are new to paleoanthropology. The proliferation of new names is confusing, and it might seem that experts in the field are endlessly arguing about what to call the fossils. Fortunately, it’s not quite that bad. There’s actually more agreement than you might think. No one doubts that all these hominins are closely related to each other as well as to modern humans. And everyone agrees that only some of the fossil samples represent populations that left descendants. Where paleoanthropologists disagree has to do with which hominins are the most likely to be closely related to later hominins. The grouping of hominins into evolutionary clusters (clades) and assigning different names to them is a reflection of different interpretations—and, more fundamentally, of somewhat different philosophies.
But we shouldn’t emphasize these naming and classification debates too much. Most paleoanthropologists recognize that a great deal of these disagreements result from simple, practical considerations. Even the most enthusiastic splitters acknowledge that the fossil “species” are not true species as defined by the biological species concept. As prominent paleoanthropologist Robert Foley puts it, “It is unlikely they are all biological species. . . . These are probably a mixture of real biological species and evolving lineages of subspecies. In other words, they could potentially have interbred, but owing to allopatry [that is, geographical separation] were unlikely to have had the opportunity” (Foley, 2002, p. 33).
Even so, Foley, along with an increasing number of other professionals, distinguishes these different fossil samples with species names to highlight their distinct position in hominin evolution. That is, these hominin groups are more loosely defined as paleospecies rather than as fully biological species. Giving distinct hominin samples a separate (species) name makes them more easily identifiable to other researchers and makes various cladistics hypotheses more explicit—and equally important, more directly testable.
The hominins that best illustrate these issues are the Neandertals. Fortunately, they’re also the best known, represented by dozens of well-preserved individuals and also a complete genome. With all this evidence, researchers can systematically test and evaluate many of the differing hypotheses. Are Neandertals very closely related to modern H. sapiens? Certainly. Are they physically and behaviorally somewhat distinct from both ancient and fully modern humans? Yes. Does this mean that Neandertals are a fully separate biological species from modern humans and therefore theoretically incapable of fertilely interbreeding with modern people? Almost certainly not. Finally, then, should Neandertals really be placed in separate species from H. sapiens? For most purposes, it doesn’t matter, since the distinction at some point is arbitrary. Speciation is, after all, a dynamic process. Fossil groups like the Neandertals represent just one point in this process.
We can view Neandertals as a distinctive side branch of later hominin evolution. It is not unreasonable to say that Neandertals were likely an incipient species. The lesser- known “Denisovans” from Siberia probably represent another partially distinct incipient species, separate from both Neandertals and early modern humans. Given enough time and enough isolation, Neandertals and Denisovans likely would have separated completely from their modern human contemporaries. The new DNA evidence suggests that they were partly on their way, but not yet reaching full speciation from Homo sapiens. Their fate, in a sense, was decided for them as more successful competitors expanded into their habitats. These highly successful hominins were fully modern humans.