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A single strand of DNA is a string of nucleotides encoding certain proteins

Figure 1.4 ► From person to gene. Adapted from Kingdon (2003).

of deoxyribose nucleic acid (DNA for short). DNA itself consists of two long spiral strands, which form the chromosomes. Each of these strands is made up of four types of small molecules (coded A, G, C, and T). The sequence in these strands forms a code, which carries all of the genetic information transmitted from parents to offspring. The chromosomes are present in the nucleus of every cell; the DNA they contain is called nuclear DNA (nDNA). It is important to realize that genes actually make up only a very small part of nDNA; the rest does not code for anything and is (rightly or wrongly) often referred to as "junk DNA." There are pseudo-genes (segments of DNA that used to be genes in the distant, evolutionary past but that have been "switched off" over time); introns (meaningless segments inserted in the middle of genes); and repetitive DNA (varying from long sequences repeated thousands of times to short sequences repeated hundreds of thousands of times, called microsatellites). Between them, these "junk" bits make up 90% or more of the complement of nDNA (Pilbeam, 1996; Dover, 1999; Relethford, 2001).

Outside the cell nucleus, in the body of the cell itself (the cytoplasm), are thousands of tiny bodies called mitochondria, which provide the energy on which the body's metabolism runs. The mitochondria have their own DNA, mitochondrial DNA (mtDNA). Because mtDNA mutates without any of the "correction" mechanisms operating in nDNA, it changes much faster, and so its variation is an important source of information with regard to the timing of a speciation event among species, as well as identifying likely evolutionary relationships within and between groups. Importantly, mtDNA is inherited, to all intents and purposes, solely from our mothers, for the contribution from the sperm is minute compared to that from the ovum; so mtDNA traces the path of genetic development for our female ancestors in the evolutionary past. If we want to trace where male ancestors went, we have to look at the nDNA of the chromosome that is unique to males: the Y chromosome (Sykes, 2001; Relethford, 2001).

For mtDNA, as for much of DNA, a constant rate of mutation has been assumed. Whether this assumption is always justified is another matter. Certainly mtDNA includes some genes that provide energy for the cell. But because of the way in which the genetic code operates, most mutations do not seem to affect the functioning of the organism, so the assumption of a constant rate of change is, overall, quite reasonable. Accepting that the mutation rates are constant, we can examine the number of shared and unique bases along any strand of mtDNA within a given population and then calculate the molecular distance between populations. The molecular distance between species, therefore, should also be proportional to their separation in time, that is, the time when they last shared a common ancestor. (It may not be exactly the same: The DNA has to become differentiated before the populations do.)

Among the apes, the greatest distance in mtDNA is between the gibbon and the others (orangutan, gorilla, chimpanzee, and human), with a difference of around 5%, and this suggests that the earliest divergence date is between gibbons and the other apes. Next is the orangutan, which differs in mtDNA by 3.6% from the gorilla, chimpanzee, and human; and then the gorilla, at 2.3% difference from chimpanzee and human. The two chimpanzee species (the common and pygmy chimpanzees) differ in only 0.7% of their mtDNA. Chimpanzees and humans are relatively close and differ in only 1.6% of their mtDNA (Ruvolo, 1994, 1997; Pilbeam, 1996, 1997; Stringer & McKie, 1996). Because our own mtDNA differs from that of the chimpanzee by 1.6% (which is about half the distance of the orangutan from the chimpanzee), and because we know, or think we know, that the orangutan split from the other apes 12-16 million years ago (based on fossil evidence), we can use simple mathematics to calculate that the proto-chimpanzees and proto-humans diverged 4.2-6.2 million years ago, the gorilla lineage split around 6.2-8.4 million years ago, while the gibbons were the first to diverge, around 18 million years ago (Chen & Li, 2001).

It was the German paleoanthropologist Franz Weidenreich who originally argued, in the 1930s and 1940s, for a theory of regional continuity. He suggested that the Chinese Homo erectus (or what we would call Homo pekinensis) fossils, commonly referred to as "Peking Man," gave rise to the modern Chinese, while Homo erectus from Java was the ancestor to the original Australians, and Neanderthals gave rise to modern Europeans (Weidenreich, 1946, 1949). The problem with this original scheme was this: How did individual and isolated human groups manage to evolve in the same direction at around the same time through similar successive stages? Weidenreich skirted this question and never successfully addressed the contradiction. Weidenreich (1943:88-89) merely stated that the fact remains that the Paleolithic population of western France already showed a considerable variety of types. Of no less importance is the fact that these types lived close together in a relatively small area and that there are no signs of a strict separation by geographical barriers. All the facts available indicate that racial characters made their appearance as individual variations . . .

and, furthermore, that they started with a great range of variations in a relatively small population. The kind of isolation mechanism which prevented the breakdown of the gene system remains to be studied. It cannot differ much from that which causes the persistence and stability of the nongeographical differentiations of modern mankind. However, this is a problem, not for physical anthropologists alone, but also for geneticists and sociologists.

The Multiregional hypothesis (Figure 1.5) was later revised to emphasize gene flow between groups to help explain a similar rate and "direction" within the evolution of all modern humans (Wolpoff et al., 1984, 2001; Wolpoff, 1989; Wolpoff & Caspari, 1997). It should be noted, however, that while Weidenreich's theory also invoked gene flow, the revised version of Weidenreich's scheme used gene flow between groups at their overlapping peripheries as its central platform to help explain how human groups evolved through similar successive stages. The multiregionalists have proposed that there was sexual contact between different human groups, at least along the fringes of certain regional communities, that enabled traits to be spread by a sequential process of passing and receiving genetic information. Some anatomical features are said to have developed in a particular region as a result of the need to cope with new and unique environmental conditions encountered within that region and to have been maintained through time (to the present day) within those regions.

Australoid Mongoloid Caucasoid Negroid

Ngandong

Dali

Neanderthal Omo

Sangiran Zhoukoudian Petralona Kabwe

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