Within the last decade there have been a number of truly significant discoveries relating to the evolution of humans and their ancestors. Most recent have been the discovery and publication of the late Miocene fossil specimen from Chad allocated to Sahelanthropus and the mid-Pliocene fossils from Kenya allocated to Kenyanthropus. Ongoing discoveries of more recent human remains, especially from the Pleistocene of Africa, Europe, and Australia are also forcing us to reassess our views of modern human origins. Discoveries by archaeologists over the last decade have not only pushed back the earliest dates for stone tool manufacture, but are also challenging our current view of past human behavior. New methods of collecting, analyzing, and interpreting molecular evidence have also had considerable impact on the way we interpret the evolution of our species. Molecular biology has enabled us to identify the likely period when proto-chimpanzees and proto-humans last shared a common ancestor (around 6 million years ago), and the most recent contribution from this field to the study of human evolution has been the extraction and analysis of Neanderthal mtDNA. All of this evidence supports the idea that human evolution over the last few million years is a complex story, defined by considerable species diversity.
It is becoming increasingly clear to both authors that the "Out of Africa" model for recent human origins is supported by the available fossil, archaeological and molecular evidence, though, as we will also argue, there was more than one "Out of Africa," and in some cases there were dispersals into Africa during the early Pleistocene by some human species. That is not to say that we both agree on the details of human evolution over the last 5 million years or so. As the reader will see, we agree to disagree, which is shown most markedly in our differing taxonomies of the hominids, both of which suggest distinct relationships within the more recent members of our own family, the Hominidae.
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Is the evolution of modern humans an African genesis followed by prehistoric worldwide genocide of earlier pre-sapiens, or is it a slow progression from pre-sapiens to modern humans? Theories concerned with modern human evolution have been polarized by these extreme views. These two basic positions have been referred to, respectively, as the "Out of Africa" and the "Multiregional" hypotheses. Does the paleontological, archaeological, and molecular evidence support the mass extinction of earlier humans, the last of all being the Neanderthals, or did these diverse pre-sapiens interbreed with the more "successful," modern H. sapiens, thus being swamped genetically and physically? Indeed, are Neanderthals just an extreme version of the one species H. sapiens — is there still a little Neanderthal left in us all?
Any understanding of human evolution, undoubtedly, must be based on an interpretation of human physical (bone) and cultural (stone) remains. This is particularly true of the remains that predate the origins of our own species, H. sapiens, whose earliest representatives appear around 250,000150,000 years ago (Bräuer, 1984, 1989; Rightmire, 1984, 1993; Stringer & Andrews, 1988; Groves, 1989a; F.H. Smith et al., 1989; Stringer, 1989, 2003; Stringer & McKie, 1996; F.H. Smith, 2002; T.D. White et al., 2003). With the late emergence of our own species, we are also able to invoke molecular evidence from preserved human mitochondrial DNA (mtDNA). The molecular evidence, if assessed cautiously, provides a date for the origins of our own species, which is independent of other "hard" evidence, such as bones and stones, and also suggests likely evolutionary relationships between different human groups. Unlike bones and stones, however, the molecular evidence does not provide a picture of what our ancestors looked like or how they adapted physically and behaviorally to their seasonally fluctuating environments.
The overall tempo and mode of evolution best fits in with long periods of morphological stasis followed by rapid speciation. While this was suggested by Haldane as long ago as 1932 (and even earlier by Huxley in correspondence to Charles Darwin), it was Eldredge and Gould (1972) who first popularized this theory of evolution, most commonly referred to as punctuated equilibrium (Figure 1.1) (see also Gould & Eldredge, 1977;
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