Charting the Waves and Currents in the Evolutionary

Genes have metaphorically been referred to as existing in "pools" within populations. But for populations that spread widely over continents, as did early Homo, perhaps an image of "gene sea" is more appropriate. The forces that cause the currents, waves, and eddies of change are of interest to us because they will explain the course of change and the anomalies in the fossil record that we encounter. But how can we chart them? Deducing patterns of continuous populational change through recourse to the discontinuous data of the fossil record will always be difficult, but we do not think it impossible. Fossil sites represent discrete data points, geographically and temporally circumscribed, that must be connected one to another in a continuous matrix of time and paleoenvironmental change. As this matrix becomes better known for hominids, we may anticipate finding instances of fast gradualism and steep clinal gradients ("tachytely" to use George Gaylord Simpson's term4) and slow gradualism and low clinal gradients ("bradytely").

We and other researchers have used a method of paleoecological reconstruction of Europe to posit the changes in past climate and geography that would have isolated populations of Neandertals and then facilitated the later movement of the modern Homo sapiens that replaced them into Europe.5 Our thinking was that understanding a relatively recent transition in human evolution—roughly 150,000 to 30,000 years ago—would help us to understand the many more distant transitions that have occurred throughout the last two million years of the history of the genus Homo. We assembled all known records of past climate, including fossil pollen and animal fossils, geological records of maximum extensions of glaciers during the

Ice Age, and the locations of past bodies of water. With these data we constructed a paleo-map of Europe with vegetational zones and major physical features. Onto that template we placed all the known fossil hominid sites of the time. The composite map showed that Neandertals (considered either as a separate species, Homo neanderthalensis, or a subspecies, Homo sapiens neanderthalensis) occupied a region encompassing Europe and the Middle East (exclusive of North Africa) that was largely cut off from the rest of the world by mountains, glaciers, meltwater rivers, and seas.

Fossils discovered in Europe since this research was done have shown that the origin of the Neandertals from more archaic populations of Homo, termed Homo heidelbergensis, is still interpreted most conservatively as gradual and progressive in nature. The spectacular finds of early Neandertal-like skeletons at the Spanish site of Atapuerca near Burgos are the most dramatic examples of recent discoveries.6 Neandertals were peoples who lived in glacial Europe and surrounding areas. They were descended from populations of Homo heidelbergensis that had become relatively isolated from the global human cline, becoming a more differentiated population. Their limb proportions, their facial structure (protruding mid-facial regions with large noses), and their advanced culture all suggest a successful adaptation to a cold climate. We can understand how the Neandertals gradually diverged from their predecessors by an evolutionary process of clinal replacement.

Another suite of recent discoveries, however, this time at one of the most venerable of Neandertal sites, Mount Carmel in Israel, is more of a challenge to our clinal replacement model. The fossil humans from this site were studied by Theodore McCown of the University of California at Berkeley and British anatomist Sir Arthur Keith in a landmark monograph published in 1939.7 Two caves in Mount Carmel yielded fossils that McCown and Keith assigned to anatomically modern Homo sapiens (from the cave of Skhul) and to Neandertals (from the cave of Tabun). In the age before radiocarbon and potassium—argon dating, they speculated that Tabun was older than Skhul. Because of the anatomical similarities, they postulated an ancestor—descendant relationship of Neandertal to modern Homo sapiens. Absolute dates from fossil bone at Mount Carmel, however, showed that some Neandertals from Mount Carmel were actually later in time than anatomically modern fossils, and that some anatomically modern Homo sapiens fossils dated to earlier levels than Neandertals. Archaeologist Ofer Bar-Yosef of Harvard University and his colleagues concluded that this area had been a region of give-and-take population replacements over a period of some seventy thousand years.8 What caused these waves of human evolution lapping back and forth on the shores of ancient Israel is important to ascertain because whatever it was, it will likely be a key element to understanding the dynamics of human evolutionary change on a larger scale.

The Pleistocene Epoch, encompassing the extensions of northern hemisphere glaciation known as the "Ice Age," was a time of increasingly severe fluctuations of climate. Climates, as we have seen, did at times become colder and drier (during "glacial" times) but they also became warmer and wetter (during "interglacial" times). These fluctuations in climate may ultimately be traced to variations in the amount of sunlight hitting the earth, the so-called Milankovich cycles, driven by solar system dynamics. Closer to home, changes in Antarctic ice volume, changes in global ocean temperature, and changes in the monsoonal rain patterns in Africa and Asia have been shown to have exerted important effects on animal and plant communities and their evolution. The pattern of increasingly wide shifts in climate from cold to warm began about 2.5 million years ago, the date of onset of northern hemisphere glaciations, and became more and more noticeable as time passed. Local responses to the global pattern of temperature change may vary, however, and this is why paleoenvironmental research is so important at each individual site. As we saw in the case of northern China, local effects that were not expected or accurately predicted by the global climate pattern were importantly mediated by the winter monsoons with their cold, dry blasts of loess-laden wind. Continuing field and laboratory research is needed at all hominid sites to assess the local effects of climate change on evolutionary events.

The stark environmental challenges of the Pleistocene have long been considered the icy anvil on which the human species was forged.9 We believe that the redated sequence of Pleistocene human populations at Mount Carmel and elsewhere in Israel provides detail to the metaphor. When climates became colder, a larger habitable region was opened to the cold-adapted Neandertals, who spread into the new areas. Although some have suggested clinal gradation of Neandertals to modern Homo sapiens at some places (such as at Vindija Cave, Croatia10), or even interbreeding of anatomically modern Homo sapiens and Neandertals (at Lagar Velho, Portugal11), Mount Carmel does not seem to be such a place. When times were right, Neandertals flooded the region, displacing modern Homo sapiens to the south and east. If there was any interbreeding it is not apparent in the bones that they left behind. A few millennia later, however, when the climate had swung back to warm and the sweaty Neandertals at Mount Carmel began eyeing cool northern climes with envy, anatomically modern Homo sapiens showed up again in the Levant. The Neandertals either retreated to the north or died out locally. By forty thousand years ago they were gone for good. We believe that paleoanthropological reconstruction at this level of detail, supported by in-depth paleontological and geological data, pro vides a model on which research should be pursued at other sites. A number of such investigations will ultimately test the broader hypotheses, such as clinal replacement. Mt. Carmel is one of the anvils of the Pleistocene— a place where the hammer blows of Pleistocene climatic change formed the modern human species. In such places we might expect to see a more rapid clinal replacement, an "edge effect" between populations.

A well-documented Late Pleistocene scenario of evolutionary transition from one early human population to another in one region, such as the Levant, is informative for interpreting earlier phases of human evolution, ones for which we have less complete information. There are some important similarities between the Neandertals and East Asian Homo erectus, for example. Both can be shown to have evolved along a trajectory of gradual change from earlier widespread populations—Homo erectus ergaster in the case of Chinese Homo erectus erectus, and Homo heidelbergensis in the case of the Neandertals. Both lived in geographically circumscribed areas— Asia, south and east of the steppes of the Tibetan Plateau; the Gobi Desert; and the Himalayas, for East Asian Homo erectus, and an area bounded by the Mediterranean, northern glaciers, and the Ural and Caucasus Mountains on the east for Neandertals. Both showed regionally characteristic archaeological traits—a lack of emphasis on Acheulean choppers in sites east of the "Movius Line" for Asian Homo erectus, and technologies emphasizing large, Levallois-type, spear blades for Neandertals. Finally, both diverged and survived in their core regions while more modern populations, which were later to replace them, evolved elsewhere.

It is possible to explain these data and the respective appearances and extinctions of Homo erectus and Neandertals as species transitions, and a number of paleoanthropologists do just that.12 Homo erectus comes from Homo habilis/ergaster and is replaced by Homo sapiens in the former case; in the latter case, Homo heidelbergensis evolves to Homo neanderthalensis, which is later replaced by Homo sapiens. Such explanations are neat, but they do not explain some critical data. We believe that fossil evidence showing anatomical gradations at the edges of populations, regional clustering of traits that would seem to cross "species" boundaries, and possible evidence of interbreeding between Neandertals, as well as population genetic evidence that shows a large degree of population relatedness within the human species, are not compatible with this explanation. We accept the fossil evidence for a much more abrupt and rapid transition from Neandertals to modern Homo sapiens, but our interpretative model of clinal replacement holds this as a likely example of tachytely, and therefore of restricted temporal span and more difficult to document clearly in the fossil record.

We predict that as the fossil record of hominids improves, more and more blurring of the presumed species boundaries will occur, making much more of a "lumped" pattern of species names for Pleistocene hominids. The model of clinal replacement can account for the gradual origins of Homo erectus and Neandertals—through the process of small-scale extinctions of local populations, and replacements by related neighboring populations. It is during the extinctions of these populations at "anvil sites," where change seems to be more abrupt, that some disagreement may arise over the mode of evolutionary change. When geography and climate acted to separate populations over time, as happened in the cases of Asian Homo erectus and the Neandertals, a more apparent boundary between populations existed. If clines in such cases were of greater magnitudes, further evidence should demonstrate the existence of blurred population edges, and thus clinal, not species, boundaries.

Any one site, such as Longgushan, is not sufficient to test a large-scale hypothesis like clinal replacement completely. However, if renewed research incorporates an intensive multidisciplinary investigation of sediments from all time periods preserved at Dragon Bone Hill, and of indicators of the overall paleoenvironments in northern China during this time span, one excellent set of data can be compared with the predictions of our model.

Recent archaeological research has suggested that there were three waves of premodern human migrations of hominids out of Africa—at 1.6—1.7 million years ago, documented by large core-chopper stone tools; at circa 1.4 million years ago, documented by Acheulean bifacial hand axes; and at four hundred thousand years ago, characterized by advanced Acheulean flaked cleavers.13 The paleoenvironmental scenario that we have outlined, and the model of clinal replacement, would suggest that there were many more. Furthermore, we would expect that there would have been gene flow back into Africa. Future research will need to investigate these interesting possibilities and accurately date them in order to test our hypothesis.

Examining the Lifeways of Homo erectus

We have suggested an unusual behavioral adaptation to explain the unusual pachyostotic cranial anatomy of Homo erectus. We hypothesize that head bashing was so common and ingrained a behavior in this species that it led to multiple changes in skull form. This is a hypothesis, first advanced by Peter Brown,14 that needs much further testing. Taphonomic investigations of other Homo erectus crania should be undertaken to see if depressed fractures can be seen in their skulls also. Other anatomical researchers should contemplate and propose tests of whether some other biological function of bone that we have not thought of might have been implicated in creating such a thickheaded species. Archaeologists should investigate this model of Homo erectuss behavior to see if it is supported by their data. And there are interesting questions surrounding the intersection of our "protective pachyostosis" hypothesis and Dean Falk's "radiator brain" hypothesis. How did a thickened skull allow for cooling of an expanding brain in Homo erectus, especially since the species also evolved in the hot tropics? Did a requirement for cooling in the very enlarged brain of Homo sapiens explain the eventual loss of this suite of pachyostotic features in the skull? Major events must have occurred to make Homo erectuss skull anatomy evolve into Homo sapienss skull anatomy, but as yet we do not have a clear understanding of what they were.

If our postulated functional interpretation of Homo erectus cranial anatomy is correct, there are some profound implications for human behavior because, after all, Homo erectus is ancestral to our species through Homo heidelbergensis. We have suggested that interpersonal violence was so prevalent in this species for so long (some one and a half million years) that major skull reorganization resulted. The effect of this extended period of evolution on our modern psyche must have been significant. If our model is correct, it means that Homo erectus was much more of a "bloodthirsty killer" than ever were the australopithecines, characterized as such by their discoverer and other writers.15 If we recollect how slowly Homo erectus culture changed and how formulaic and unvarying it was for so long, it is chilling to contemplate how mindless, methodical, unquestioning, and unmerciful killing of individuals outside of one's own small group would have been. Is it only coincidence that the same terms spring to mind to describe any of our modern-day genocides, from Bosnia to Rwanda to East Timor? We predict that future Homo erectus sites will hold evidence, even more direct than the healed depressed fractures of the Longgushan skulls, of past intraspecies conflict and aggression. Aggression between groups was likely a consequence of the constant expansion and contraction of population ranges and territories, mediated by Pleistocene climate change, that created the ecological setting in which these behaviors evolved in Homo erectus. It remains for modern Homo sapiens to recognize this ancient behavioral trait as a remnant of a successful, if brutal, Pleistocene heritage, and to modify it as it occurs in the modern world by exercising those other traits that most set our species apart—culture and intelligence.

Homo erectus was not only a killer, however. The evidence from ER 1808, the Homo erectus from Kenya with presumed hypervitaminosis A, though sparse, is direct evidence that Homo erectus people cared for their own. Even without this evidence, however, we could reasonably infer a high degree of solidarity in Homo erectus groups simply because of the environmental challenges that they encountered and must have overcome to survive. Again, if our conception of Homo erectus culture is correct, we might characterize their group solidarity as unswervingly loyal, total, and lifelong. There must have been innumerable cases of extreme sacrifice and altruism in which Homo erectus individuals died for the group, their genes passed on through their children and relatives who survived and reproduced. The nonverbalized altruism that motivates the most noble of Homo sapiens activities—from rescuing a drowning stranger to global philanthropy—probably derive from our Homo erectus heritage. But so do the unquestioning self-oblations of the kamikaze pilot or the suicide bomber. We have inherited from our Pleistocene ancestors powerful feelings, many of which we struggle to put into words because they evolved before our brains were wired for language. In a changed world of global interdependence, modern Homo sapiens has the cultural choice of extending and applying these ancient, beneficent emotions to the whole species, or of maintaining the primitive status quo of Homo erectus, an unreasoning, noninclusive, and xenophobic collective state of mind formerly termed "savagery" and "barbarism," or alternatively, "patriotism," "nationalism," or "tribalism."

Much of modern human behavior is learned, and that learning occurs within culture and during a relatively long childhood. Homo erectus is hypothesized to have been the first of our ancestors to show an increased duration of childhood, with accelerated growth rates during infancy and adolescence to compensate for the intervening period of slow down in physical growth. Analysis of the growth rates of tooth enamel in fossil hominids has been the primary evidence for this important deduction, and this conclusion needs to be tested and verified with other tests and more data. The sequence and rate of cultural change in Homo erectus are still poorly understood, and a goal of future research should be to relate these to physical growth changes in evolution.

In order to investigate cultural and biological adaptive changes in Homo erectus and how they subsequently evolved, hypotheses must be built on a firm substrate of population ecology. These parameters are the basic tenets of any species' adaptation—how it makes its living, where it spends its time on a daily basis, what it eats, what eats it, and how it reproduces. Dragon Bone Hill provided some of the first forays into these aspects of paleoanthropology.

Dragon Bone Hill Diet, Disease, and Ecology

The first archaeological sites in Europe, investigated as far back as the eighteenth century, were interpreted within the context of big game hunt ing. Teilhard de Chardin and Henri Breuil, the architects of archaeological interpretation at Zhoukoudian, were steeped within this tradition, and the first interpretations of Longgushan Homo erectus were along these lines. Indeed, it made sense to extend clear archaeological evidence of hunting of reindeer and other large mammals from geologically recent sites in Europe to earlier sites such as Longgushan in the absence of data to the contrary. As we have seen, in the 1930s Chinese researcher W. C. Pei first showed evidence to the contrary by disagreeing with the eminent Breuil over the presumed bone weapons and other tools found at Dragon Bone Hill. Many years later Lewis Binford's and our own research have confirmed that the bones that Breuil thought were hominid tools were in fact bone refuse from hyena meals. And although stone tools were found in abundance in the cave, none of them can be said to be sufficient or even adequate for killing large mammals on the hoof. Cut marks document that Homo erectus cut meat from large mammal carcasses but not that they hunted the animals down.

Our scenario for Homo erectus food getting supports those who have hypothesized that early hominids were scavengers,16 but many questions still surround this fascinating and, to modern gustatory sensibilities, disgusting dietary adaptation. Primary among the questions that remain is whether fire, which we think was in the Homo erectus behavioral repertoire, was regularly used to cook meat or other foods, or whether it was used only as an ecological tool to displace other species and to burn away vegetation.

In their groundbreaking research on the use of fire at Longgushan, Steve Weiner and his colleagues determined conclusively that only already fossilized bone, which turned colors, had been burned in the fires that had ravaged the cave. Carbonized bone that Davidson Black had hypothesized to be evidence of charring were found to be coverings of finely laminated organic deposits laid down under water, probably in pools within the cave. The bone fragmentation thought by Lewis Binford to be due to burning or roasting horse heads remains the best evidence for any intentional cooking at Dragon Bone Hill. The exact relationships of Homo erectus, fire, and animal remains at the site are outstanding questions that need to be probed in much greater depth in the future.

General arguments about Homo erectus's adaptation make a strong case for meat-eating in the species. The species' increase in body size correlates to an increased need for calories, which was required for covering more ground in its search for food, presumed because of theoretical expectations that larger animals occupy larger home ranges. Added to this, Leslie Aiello and her colleagues have made the convincing argument that as the brain enlarged in Homo erectus, the gut (stomach and intestines) decreased in size.17 Known as the "expensive tissue hypothesis," this idea seems to be supported by anatomical changes in the Homo erectus skeleton that indicate that the abdominal area was smaller as were the chewing teeth and masticatory muscles. Because of the decreased food-handling and -processing anatomy, Homo erectus must have eaten substantially more high-quality food, that is, high-protein and high-calorie foods. Cut marks on bone that can only have been made by hominids confirm that meat, organs, and animal fat were a major component of this new, high-quality diet. But the range of meat, what species it came from, and when in the year it was eaten remain a mystery. Basic questions, such as how much meat was eaten, and by whom in the group, remain for future research. Was bone marrow eaten? How did Longgushan Homo erectus avoid getting too much vitamin A from eating carnivore liver? If these hominids ate too much meat, including purine-rich organs, did they ever get gout? Much needs to be explained about the meat component of the Homo erectus diet alone.

Longgushan was the first paleoanthropological site at which presumed evidence of Homo erectus plant foods was discovered. Ralph Chaney, a paleontologist and botanist from the University of California at Berkeley, investigated fossilized and burned seeds at Longgushan in the 1930s. He determined that the seeds known colloquially as hackberries, and widely eaten around the world, were from the Celtis tree. Chaney hypothesized that hackberries formed part of the diet of Homo erectus. As enticing as this idea may be, it is equally probable that the location of the hackberries in the site, which we have now determined was near the southern cave opening in the upper part of the deposit, is explicable by a Celtis tree's growing near the opening and dropping its berries into the cave. The burning of the berries, which Chaney had argued connected them with hominid activity, remains, but is attended with the same problem as the burned fossilized bone in the site. These problems await further research.

Two recent studies have emphasized the potential importance of tubers as food sources for Homo erectus. When we realize that there are no fossil remains of tubers or even indirect archaeological evidence that early homi-nids ever ate tubers (termed "USOs" or "underground storage organs" by specialists), we might question whether or not these ideas are to be taken seriously. There are some cogent arguments, however, that make the case for focused research even if they do not prove the case that tubers formed a major part of the diet of early Homo. Archaeologist James O'Connell of the University of Utah and his colleagues use data from modern African foragers (the Hadza of Tanzania) to argue that women's gathering of plant foods, particularly the //ekwa root,18 is critical to sur vival in the often marginal habitats in which the Hadza live.19 These authors relate this possible dietary adaptation specifically to Homo erectus and its spread out of Africa, terming it the "grandmother hypothesis." They suggest that post-menopausal women in the group could help feed children with the tubers they collected, and thus promote reproduction of their daughters and younger female relatives. The other study, headed by primatologist Richard Wrangham of Harvard University, starts from the observation that chimpanzee males are now known to hunt and then share meat, but these males do not provision nuclear families who live in "home bases," as the traditional-hunting hypothesis for Homo erectus maintained.20 Chimpanzees do not share with Homo erectus many of the anatomical and physiological adaptations that allow the latter to eat and digest meat, so the analogy is somewhat strained. Chimps, for example, engage in coprophagy—picking partially digested meat out of their own feces to reingest it. Nevertheless, both models relate their hypotheses to the spread of open, savanna-like conditions, increased home-range sizes of Homo erectus, and the spread of hominids out of Africa. They both put a valid emphasis on the non-meat component of the Homo erectus diet, and, indeed, this must be better investigated.

Tests of dietary hypotheses are possible using isotopes of chemical elements, such as strontium, carbon-13, oxygen-18, and nitrogen-15 obtained from fossil teeth or bones. Michael Richards of Oxford University recently led one study that showed that Neandertals in Croatia regularly ate as much animal protein as large mammalian carnivores. This study analyzed a bone protein, collagen, which unfortunately is rarely, if ever, preserved in bone as old as Homo erectus. Other isotopes, however, that reside within the crystalline apatite structure of bones and teeth can be used to assess diet,21 and should be employed to assess Homo erectus diet. Another promising area of dietary research is coprolite analysis. Coprolites are fossilized feces,22 and there are hundreds, if not thousands, in the collections of the Institute of Vertebrate Paleontology and Paleoanthropology in Zhoukoudian and Beijing. The vast majority are presumed from their shape to be from hyenas, but analysis of even one Homo erectus coprolite that might be in the collection would provide a wealth of dietary information unknowable from any other source.

Diet and ecology naturally lead to questions about disease in Homo erectus. We have discussed a vitamin overload disease (hypervitaminosis A) and a parasitic disease (tapeworms) in Homo erectus. Archaeologists Ofer Bar-Yosef and Ann Belfer-Cohen have focused on Homo erectus disease in the context of the dispersion of earliest Homo erectus out of Africa and into Eurasia.23 Citing Africa as a "garden of germs," these authors think that hominids could have escaped the parasite-infested and disease-laden tropics of Africa to expand into a much healthier environment in the colder north. They mention parasites, sleeping sickness, malaria, and elephantiasis as afflictions endemic to Africa that may have prompted population movements.

The late Pliocene time period, around two million years ago, was one in which several diseases may have become more prevalent in Africa, lending some credence to Bar-Yosef and Cohen's idea. At about this time, or slightly earlier, the lake snail Bulinus first appeared in the fossil record of eastern and central Africa.24 This snail is one of the primary vectors of the human parasitic disease, schistosomiasis (also known as bilharzia), which may date from this time. Recent research indicates that malaria is a much more recent disease, with a protective genetic mutation (causing glucose-6-phosphate dehydrogenase deficiency) dating to only a few thousand years ago,25 so at least this defense against the malarial parasite is not likely involved in this proposed mechanism. We have already seen that tapeworms seem to have diverged evolutionarily about 1.7 million years ago, but emigrating hominids did not escape them by going to Eurasia—the tapeworms rode along out of Africa within the hominids' and carnivores' guts. Sleeping sickness (trypansomiasis), caused by the bite of the tsetse fly, and elephantiasis, caused by the filarial worm (Wucheria bancrofti) are undated as to their probable earliest occurrence. Unmentioned by Bar-Yosef and Cohen are viruses, of which there are many deadly varieties in Africa. The presence of viruses affecting humans in Africa, especially in the forests, is probably ancient because of long coevolution with our ancestors and relatives there. HIV, and Ebola, Bunya, Semliki, West Nile, and Rift Valley Fever viruses are some of the more well known. Escape from endemic African viruses might well have been one advantage of the move out of Africa, particularly when population densities of early hominids were such that viral transmission from one population to another was limited. However, when Homo erectus left the African tropics, it soon spread to the Asian tropics. Here an abundant and diverse group of potentially infective parasitic, bacteriological, and viral diseases residing in primate populations from monkeys to apes exists, kept alive and well by warm, wet forests. One may well ask, if escape from disease was important to Homo erectus, why go from the African frying pan into the Asian fire? In addition, a whole new spate of diseases that tends to afflict European and Asian populations preferentially, such as cystic fibrosis, arose that would have counterbalanced the beneficial effects of leaving Africa. There are many questions still to be addressed in this interesting new arena of research, and molecular approaches may prove the most effective in investigating them.

Top: The Upper Cave at Longgushan, shown here, was discovered to the south of the main excavation of Locality 1. It was excavated between 1931 and 1933, and yielded a late Pleistocene fauna, abundant stone tools, and an example of modern Homo sapiens. Bottom: The most complete skull of Homo sapiens (no. 101) recovered from Upper Cave. This skull is fully modern in appearance. Unlike Homo erectus, Upper Cave Homo sapiens may have lived in the cave, documenting the remarkable persistence of hominids at Dragon Bone Hill for over half a million years.

Testing Theories on the Extinction of Homo erectus

After being found in the uppermost strata of Dragon Bone Hill, dated at roughly 410,000 years ago, Homo erectus is no longer reliably recorded in mainland Asia. The next fossil hominids from China are skulls of Homo heidelbergensis from the sites of Jinniushan, approximately 280,000 years old; Dali, approximately 200,000 years old; and Mapa, approximately 132,000 years old. What transpired between the end of Dragon Bone Hill's time and the appearance of the earliest Homo heidelbergensis in China—a period of approximately 130,000 years—is a blank. Homo heidelbergensis in China, however, shares a number of erectus-like cranial features. Mainstream paleoanthropological opinion has held that these populations moved into China from western Eurasia or even Africa, replacing or living alongside Homo erectus. Actual temporal overlap between latest Homo erectus and earliest Homo heidelbergensis in China has never been demonstrated, however. Chinese paleoanthropological opinion tends to favor an in situ evolution of Homo sapiens from Homo erectus.26 Recent dating of a modern Homo sapiens at the site of Liujiang in China—at least 68,000 years ago, and possibly older—has reasserted this position.27 It is claimed that this specimen may be as old as anatomically modern Homo sapiens from Africa and the Levant, thereby lending support to an in situ evolution in China. Alternatively, Liujiang and its early date may be considered support for our clinal-replacement model, in which we would expect more or less synchronous appearances of new species worldwide.

The anatomically modern Homo sapiens discovered in the Upper Cave at Dragon Bone Hill is later in time than the Liujiang find, but it is anatomically similar and probably belonged to a closely related population. That population may have migrated to Dragon Bone Hill from Africa, as "Out of Africa" theorists maintain; it may have evolved in place, as Multi-regionalist theorists maintain; or it may have replaced another slightly more primitive population as the last relay in a wave of replacement, as our clinal-replacement model suggests. The evolutionary continuity that seemed so obvious to Davidson Black and Franz Weidenreich between Sinanthro-pus and the Upper Cave Homo sapiens is far from agreed upon. Only more research will answer the question.

We moderns are wont to look at the past not only with a backward gaze, but down our noses. It is all too easy to regard Homo erectus, our parahuman, beetle-browed ancestor of long ago, as ultimately destined for extinction. After all, the species had subhuman intelligence and a cultural adaptation that changed more slowly than the glaciers that periodically descended on them from the north. Such a view misses the length of time that Homo erectus existed—1.5 million years, or 15 times as long as modern Homo sapiens has been around—and it ignores the vast advances the species made. The largest member of the genus Homo to have evolved up to its time, it also became the most widespread primate species in the world other than its own, later descendants. Despite the uniqueness of Homo erectus cranial anatomy—their skulls were bony carapaces buttressed by huge brow-ridges and massive tori that protected their brains in aggressive encounters— and the inferences that they could not speak and did not have modern human levels of manual dexterity, the species nevertheless invented the stone biface, tamed fire, and learned to survive the fluctuations of the northern hemisphere's Ice Age. The unusual mix of capabilities that Homo erectus possessed signaled zoological success for the species. We inherited many of those anatomical and behavioral traits from Homo erectus, for good or ill, and we must bear in mind that the species became extinct by eventually evolving into ourselves.

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