The fossil record is meagre and until recently there were no primate fossil finds from the period four to seven million years ago, when the hominines diverged from the lineage that was to lead to the chimpanzees of today. So other methodologies, such as molecular dating, have been used to fill the missing pieces. While hominid fossils are rare and stumbled upon through luck, the 40 000 genes found within living humans are a vital database on our species’past.
Much evolutionary change is random, especially in the parts of a species’ genome that do not influence its survival rate – including a quantity of ‘junk DNA’ and the DNA contained in the mitochondria of all human cells. Opportunely for the researcher, random processes of this type are subject to statistical laws. As David Christian observes: ‘If you take a new deck of cards arranged by suit and number and shuffle it a few times, a statistician can estimate roughly how often it has been shuffled by determining how much of the pack differs from its original condition. The larger the number of cards and shuffles, the more precise and reliable such estimates can be.’ Researchers have argued that if we take two modern species and determine the variations between their DNA sequences we can ascertain when their two lineages divided from a mutual ancestor.
Mitochondrial DNA (mtDNA) comes from the mother alone and records a pristine family history from one mother to the next. It is particularly useful to researchers (even though it comprises a mere 0.0006% of human nuclear DNA) because it has a high rate of mutation and is easy to obtain for analysis. The only way variation in mtDNA can be introduced is through mutation. The number of mutations separating two mtDNA types is a way of measuring the length of time since they shared a common ancestor. Thus mtDNA provides a genetic clock.
Preliminary studies of human mtDNA divergence led to the ‘African Eve’ hypothesis that all mtDNA types found in our global human population of today can be tracked back to a single ancestor who lived in Africa about 200 000 to 150 000 years ago. The idea of one single common ancestor of all humankind sounds a lot more dramatic than it is. Simply put, we all share an ancestor that was not the first or only female alive. She was just a member of a group – but the mtDNA of her contemporaries went extinct because they or their descendants left no progeny at all, or left only male offspring. This ‘African Eve’ had ancestors, of course, who were also our ancestors but she is simply the most recent common ancestor.
All studies of global human mtDNA divergence have shown that African populations have the biggest mtDNA divergence (followed by Asian and European populations). The fact that Africans have the greatest mtDNA sequence variation proves they have amassed the most mtDNA mutations. This provides compelling evidence for the idea of African origin of humankind because the population with the most diversity is almost certainly the ‘ancestral population’. All other population groups seem to be subsets of the diversity found in Africa, which further buttresses the argument for an African origin.
Major sites where prehistoric artefacts were found
