The fossil evidence, we believe, is now sufficient to show that populations of hominids with species-level anatomical differences replaced Homo erectus in Asia. Replacement in mainland Asia happened earlier than in insular Southeast Asia and involved a replacement of Homo erectus by Homo heidelber-gensis. The evolutionary transition in Java was a replacement of Homo erectus by Homo sapiens, a difference brought about by the relatively isolated population that had evolved without gene flow from mainland populations. This observation forces us to reject the tenets of Multiregionalism and the idea that Asian Homo erectus gradually evolved in situ, with only some gene flow from outside.
The abrupt transition to Homo sapiens that apparently occurred in Java may be an example of extinction and replacement of populations, and not an example of clinal replacement. Replacement models, like the "Out of Africa" model, differ from clinal replacement in that they hypothesize total replacement of the resident hominids. Clinal replacement posits a more gradual population transition than this (but more rapid that Multiregionalism). The fossil evidence may be too sparse to resolve this issue at present, but it is entirely possible that gene continuity may be seen even in this rapid and apparently abrupt transition to Homo sapiens in Java.
On the other hand mainland extinction of Homo erectus seems to have occurred less abruptly and more in keeping with the hypothesized mode of change in clinal replacement. Homo heidelbergensis, an Afro-European, separate species and thus a population that could not produce fertile offspring in matings with the resident population, came in and occupied the entire former range of Homo erectus, driving it to extinction. The replacement of Homo erectus that we see in the Asian fossil record was by populations of Homo heidelbergensis with substantially more genes from the west, not all at once by a separate species. Incrementally, but relatively rapidly in terms of geological time, this process continued until the geographically defined and anatomically discrete genetic constellation that was Asian Homo erectus ceased to be. This scenario is fully in keeping with the fossil evidence for replacement, and explains the continuity in traits long cited by Multi-regionalists. What further supports it, in our opinion, is the genetic evidence. Total replacement does not satisfy the expectations of population genetics. It cannot resolve the disparity between census size (the global population of hominids), which had to be large, and effective population size, which that model requires to be small. Clinal replacement does resolve this problem by its many small populations moving and replacing other populations.
The model of clinal replacement is in a sense a process of "microevolution," that is, it is a process of small-scale changes in populations. One of the greatest challenges in all of evolutionary biology is to relate phenomena at one level to observations at other, higher levels of organization. In the case of the endemic evolution of Homo erectus in Southeast Asia and its subsequent replacement, there had to be a speeding up of the microevolu-tionary changes to account for the macroevolutionary change that we see in the fossil record. To be a convincing explanation, clinal replacement has to explain this acceleration of genetic and anatomic change.
A species may evolve to become extinct, thereby passing on its genes largely intact, but altered sufficiently for its descendants to present a significantly different adaptation. Evolutionary biologists call this "anagenesis." Homo erectus over part of its range as a species became extinct because it evolved by anagenesis into a descendant species, Homo heidelbergensis.2
For the ancestors of Homo erectus to move out of Africa, there needed to be both an open geographic pathway of dispersal and a motivating impetus. We discussed these in the context of environmental change that not only opened up "savanna-like" environments but also pushed populations up and out of Africa because of spreading aridity. Paleoclimatic evidence accumulated over the past 25 years has shown that the period just before the beginning of the Pleistocene, some two million years ago, was a time of such climatic change in the Old World. It also corresponds to the dates of the appearance of the first hominids, migrating from Africa, in Asia and easternmost Europe. A "paleoclimatic pump" of encroaching zones of aridity was likely the push out of Africa that impelled the movements of populations.30
In order to test expectations of the clinal-replacement model for the extinction of Homo erectus, we must now look at the paleoclimatic record in Eurasia over the subsequent million and a half years. What were the events in the real world that might have driven the evolutionary changes that we have hypothesized?
The climatic history of China in the Pleistocene shows a repeating pattern of ups and downs. The oxygen-isotope curve from drill cores in the deep sea that records a continuous global temperature change over the past 1.5 million years shows a sawtooth pattern of fluctuations. These fluctuations come about because heavier oxygen-18 isotopes become preferentially trapped in ice, leaving the lighter oxygen-16 isotopes to become relatively more abundant in the environment. Thus, the ratio of oxygen-18 to oxygen-16 at any particular time in the past provides a record of relative amounts of global ice volume during that time. Ice volume shows a close relationship with global temperature. In China, the deposits of loess settle from the air and become consolidated by rainwater. Their thickness and grain size have been found to match the deep-sea core closely, and both increase during times of cold. Dutch researcher D. Heslop and his colleagues showed that loess thicknesses and grain sizes in northern China record the relative force of cold and dry winter monsoonal winds blowing off the Tibetan plateau during glacial periods.31 Measurements of magnetic
The top of the figure illustrates the oxygen isotope record from the deep sea for the past 1.5 million years, showing significant fluctuations in global ice volume (peaks of the graph represent periods of large ice volume in polar ice and in glaciers, primarily in the northern hemisphere). The next figure records relative strengths of the summer monsoons that dump rain on northern China, as measured by the relative amounts of magnetic activity in iron-containing minerals formed by rain-related soil building. The lower two figures record activity of the winter monsoons, which bring cool and dry continental air from the west. In the figure next to the bottom, larger windblown sediment particles (MGS or "mean grain size") indicate stronger winter monsoons (depressions in the curve), and thus colder and drier conditions in northern China. In the bottom figure, the sedimentation rate shows peaks during glacial times when the winter monsoons blow more loess into northern China and thus create thicker deposits. Abbreviations used in this figure: MIS = marine isotope stage; MPT = Mid-Pleistocene Transition. This model is based on research by D. Heslop and colleagues (2002).
iron-containing elements found in weathered loess corresponded to increased rainfall and periods of increase in force of the summer monsoons blowing in from the Pacific Ocean.
From paleoclimatic studies undertaken at Longgushan and already discussed in chapter 6, we have evidence that the large ungulates, as well as Homo erectus, disappeared from Dragon Bone Hill and went south during the harsh Pleistocene glacial periods. To understand how widespread this pattern of climate change and migration was during the Pleistocene, we can compare the period preserved at Longgushan with the overall patterns of climatic change discovered by Heslop and his colleagues. We see that the period of time during which Homo erectus is recorded in the Longgushan Locality 1 sediments—670,000 to 410,000 years ago—was relatively warm in its first half, with increasing warm summer monsoons and sustained low-force cold winter monsoons. Beginning five hundred thousand years ago the winter monsoons increased in force, but even so they did not have a major effect on the amount of windblown sediment until the very end of Longgushan deposition, just before four hundred thousand years ago. After this peak in cold, dry, glacial conditions, Homo erectus disappears from the record of mainland Asia. The species may have held on in the relatively warm regions of Southeast Asia for some time after this. The Ngandong (or Solo) fossils seem to document such a late-surviving insular population of Homo erectus?2
The global isotopic paleotemperature curve and the loess records from China demonstrate that there were major shifts between cold and warm periods roughly every one hundred thousand years after the middle Pleistocene. Between 922,000 and 641,000 years ago fluctuations from cold to warm, when they occurred, were greater than before. This turning point in climatic history has been termed the Mid-Pleistocene Transition. Each of these cycles witnessed a major change in the Asian monsoonal pattern, which changed rainfall, vegetation, and animal life. We would predict that during these periods of more significant climatic change clinal replacement would be of greater magnitude and more observable as species replacements.
Geneticist Alan Templeton recently posited an "out of Africa" migra-tional event that he estimates from molecular clock data to have occurred between 840,000 and 420,000 years ago.33 There is approximate agreement of these genetic changes with the timing of increased amplitude of climatic change in the middle Pleistocene. Perhaps even more significant from the standpoint of human evolution is that the even greater amplitude of change—colder glacials and warmer interglacials—typified the late Pleistocene and began immediately after the age of the fossil deposits at Dragon Bone Hill Locality 1.
It may have been the increasing cold at higher latitudes and the increasing aridity in the tropics as the Pleistocene progressed that caused the extinction of Homo erectus and that drove the evolution of modern human populations. But it may also have been simply that the pace of environmental change was too fast and the amplitude of change was too much for human biological evolution to handle. Homo erectus was probably the last species in our lineage to adapt to environmental change primarily by biological means, and that mode of adaptation proved inadequate for the rigors of the Pleistocene.
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