Box African

Did she ever exist? Ever since she was proposed by Cann etal. (1987), African Eve has gone in and out of favour. DNA evidence suggested that (1) all modern races are closely related and shared a common ancestor no more than 200,000 years ago, and (2) that common ancestor was an African. This has been called the 'Out of Africa' theory.

Cann et al. (1987) analysed the mitochondrial DNA (mtDNA) of 147 people from different parts of the world. They found that there was only 0.3-0.4% variation among the mtDNAs of these individuals, regardless of their racial origin, and this low level of variation calibrates to the figure of 200,000 years for the origin of modern Homo sapiens.

MtDNA occurs in the mitochondria of cells, and hence this DNA is passed down only in the female line. Nuclear DNA, of course, is transmitted through the egg and the sperm to any offpsring, but sperm do not transfer mitochondria. Studies of mtDNA necessarily concern only the female line of descent, which is why the common ancestor is called African Eve. When the mtDNA sequences were used to reconstruct a cladogram, there was a fundamental split into two clades, one restricted to sub-saharan Africa, and the other included some African people, as well as all tested individuals from Asia, Europe and Australasia. The split then happened in Africa, and since the split the first clade has remained in Africa and the other has spread from Africa to all other parts of the world.

The methods used by Cann et al. (1987) were heavily criticized, but more extensive reanalyses using much larger data sets and a variety of tree-making techniques (Ingman et al., 2000) confirmed the original results. Wider studies, incorporating mtDNA and nuclear DNA, suggest that there were two episodes during which Homo sapiens moved out of Africa: 0.8-0.4 Myr ago and 150,000-80,000 years ago (Templeton, 2002). The first episode is indicated by nuclear genes, although this has been disputed. The second, uncontroversial, exodus, identified by Cann et al. (1987), is shown by mtDNA (evolution through the female line) and nuclear DNA of the Y chromosome (evolution through the male line).

This study lent strong support to the single origin model of human evolution (Stringer and Andrews, 1988; Cavalli-Sforza and Feldman, 2003), that all existing races of Homo sapiens arose from a single, relatively recent, ancestor. This model directly opposes an alternative viewpoint, the multiregional model, which proposed that the modern races of Homo sapiens arose independently from dispersed subspecies of Homo erectus and that all Pleistocene Homo are members of H. sapiens (Wolpoff et al., 2001). The contrast between the two models could not be greater, nor the implications more profound: either all living humans diversified in less than 200,000 years and the races have had a short independent history, or the major races can be traced back for more than1 Myr and hence they might rank as separate subspecies or species. The single origin model has been confirmed again and again, and the multiregional model for modern human origins has to be rejected.

in the last 40,000 years or so. This would imply that the modern human races have differentiated in this very short time. Confirming evidence has come from molecular studies, which find that there are only minute interracial genetic differences. Several studies of human DNA have also suggested an African origin for all human races 200,000-100,000 years ago (see Box 11.5).

The record of human evolution seems to show an ever quickening pace of change. Major innovations have occurred ever more rapidly: bipedalism (10-5 Myr ago),enlarged brain (3-2Myr ago),stone tools (2.6Myr ago), wide geographical distribution (2-1.5Myr ago), fire (1.5 Myr ago), art (35,000 years ago), agriculture and the beginning of global population increase (10,000 years ago). The rate of population increase was about 0.1% per annum at that time, rising to 0.3% per annum in the eighteenth century and about 2.0% per annum today. In other words, the total global human population will more than double during the lifetime of individuals born today. In numerical terms at least, Homo sapiens have been spectacularly successful!

Was this article helpful?

0 0

Post a comment