by Dr Himla Soodyall
MRC/SAIMR/WITS Human Genomic Diversity and Research Unit, Department of Human Genetics, School of Pathology, The South African Institute for Medical Research and University of the Witwatersrand, Johannesburg.
Humans have pondered their origins for as long as they have existed. The most direct account of our past is inferred from the fossil record. Skeletal remains have been instrumental in establishing the evolution of human ancestors in Africa, and they have also provided important information about the evolution of modern Homo sapiens. However, the fossil record is fragmentary and many critical gaps remain. The genetic variation found among living peoples offers another way of studying human evolution. Genes are blueprints/biochemical instructions that determine inherited traits; they consist of sequences of building blocks of deoxyribonucleic acid (DNA). Genes are collected together to make up the larger chromosomes of which humans have 23 pairs. We inherit half the complement of chromosomes from our mothers and the other half from our fathers. Chromosomal DNA is found in the nucleus of the cell and is referred to as nuclear DNA. In addition to nuclear DNA, the mitochondria, the energy-producing organelles in the cytoplasm of all cells, also contains DNA and is referred to as mitochondrial DNA (mtDNA). MtDNA is inherited only from our mothers and only females can pass it on to their children .
The Y chromosome which is found in the nucleus, is also uniparentally inherited, and is transmitted exclusively from father to son. Together, nuclear DNA and mtDNA carry the information needed to synthesise all 40 000 or so proteins in our bodies. The code is determined by the order of nucleotide bases, many thousands of which go to make up a single gene. The total genetic complement, or genome, of humans contains some 3 billion of these bases in different combinations controlling the development of the organisms and producing the genetic variation amongst humans. As soon as it became possible to study genetic variation in humans, biologists began to use the data to assess the affinities and origins of the various populations that go to make up humankind. Major demographic events (population migrations, bottlenecks and expansions) leave imprints, in the form of altered gene frequencies, on the collective human genome.
Because these imprints are transmitted to succeeding generations, the modern human genome contains an indelible record of our evolutionary past. As more genetic loci are examined in populations throughout the world, our evolutionary history should be largely decipherable. In 1987, Rebecca Cann, Mark Stoneking and the late Alan Wilson from Berkeley fuelled the debate concerning modern human origins by suggesting that mtDNA found in all living humans could be traced to a single ancestor who lived in Africa about 200 000 years ago.
Their findings advanced the "Out of Africa" theory concerning modern human origins which posits that modern humans evolved in a single location (Africa) from where they expanded and replaced the existing hominid populations around the world. In contrast, the multiregional model claims no single location for the origin of modern humans; instead, it posits that over the last 1-2 million years humans have evolved gradually from their Homo erectus ancestors throughout the Old World, and there was gene admixture between the archaic and modern humans. One recent genetical breakthrough involved the recovery and sequencing of a fragment of Neanderthal mtDNA by Svante Pääbo and colleagues working in Germany. These researchers found that the inferred mtDNA sequence from the Neanderthal type specimen (about 40 000 years old) was very different from homologous sequences found in living humans.
This observation has been taken as evidence for the claim that modern humans and Neanderthals did not interbreed, and that Neanderthals left no descendants among extant humans. Moreover, these researchers estimated that modern humans and Neanderthals diverged from a common ancestor about 650 000 years before present. However, mtDNA data from the remains of ten ancient Australians, including the gracile Lake Mungo 3 specimen dated to about 60 000 years before present, has been used to claim "that anatomically modern humans were present in Australia before the complete fixation of the mtDNA lineage now found in all living people" (Adcock et al., Proc. Natl, Acad. Scie, USA, Vol 98, 2001, 537). In other words, Alan Thorne and colleagues interpreted these data to suggest that the oldest mtDNA types found in modern humans evolved in Australia instead of Africa, and that there was gene admixture between the archaic and modern humans, providing additional support for the multiregional theory of human evolution.
It would be useful to the scientific community to have these samples tested by other laboratories specialising in work on ancient DNA to have these findings verified so that these data could be used to challenge current theories concerning modern human origins. Nonetheless, the technological advances of the past decade have given researchers more tools to exploit the human genome and as more evidence is gleaned from ancient specimens, we would have more data to understand our past evolutionary history.
However, there is still overwhelming evidence from genetic data to support the theory that modern humans evolved in Africa. By tracing the patterns of mtDNA and Y chromosome DNA in people throughout the world, there are at least eighteen mtDNA lineages and ten Y chromosome lineages from which all other mtDNA and Y chromosome DNA patterns could be derived. The oldest patterns using both mtDNA and Y chromosome DNA are found in Africa, dating to between 80 000 to 120 000 years before present. Moreover, all other lineages can be traced to one of the ancestral types found in Africa. Our own research has shown that some of the oldest mtDNA lineages and Y chromosome haplotypes found in living humans are found in Khoisan populations. Thus, there is stronger evidence from genetic data to claim that the origins of our species lie here in Southern Africa, and not East Africa, as is usually claimed.