Traditionally, the evidence to reconstruct our evolutionary history has come from comparative anatomy and from the prehistoric evidence of artefacts and fossils. But we also have a evolutionary history within us, locked up in the genetic code of our DNA. When this is passed on from parent to child there may be copying mistakes (mutations) in the DNA, and if such changes are not too harmful, they are passed on in turn to further generations. Thus tracing similarities and differences in DNA can lead us back into the past as surely as any ancient relic.
This area of science has moved on so fast that there is now even serious debate about the recreation of a prehistoric human such as a Neanderthal, by editing a modern genome to resemble an ancient one, and then implanting this genome into an unfertilised egg, via a process called somatic cell nuclear transfer. Beyond the still formidable technical difficulties, the potential rebirth of an ancient human via a surrogate mother would surely raise huge ethical issues.
Mutations shed light on human history
For our purposes, there is autosomal DNA (in chromosomes within the nucleus of our body cells, containing the vast majority of our genome), and mitochondrial DNA (mtDNA), which is found outside the nucleus of cells and which is inherited through females only.
The first applications of DNA studies were simple ones that compared similarities and differences in the inherited products of the DNA, rather than its actual code – comparing such things as blood groups, enzymes and proteins in living groups. As techniques of investigation improved, parts of DNA coding could be compared between, say, apes and humans, and this demonstrated that chimpanzees were more closely related to humans than were gorillas and orangutans, with estimates based on accumulated mutations that the human lineage separated from that of chimpanzees about six million years ago. Studies within living people demonstrated that all human mtDNA today could be traced back to a female ancestor who lived in Africa about 150,000 years ago, so-called ‘mitochondrial Eve,’ and this provided strong support for the idea that our species Homo sapiens had evolved recently on that continent.
But until the latest technical breakthroughs, none of this research could be conducted on ancient materials such as the human fossils themselves. However, in 1997, mtDNA was recovered from the type specimen of the species Homo neanderthalensis – the skeleton found in the Neander Valley in Germany in 1856. This presaged the whole new field of palaeogenomics, which has led to the reconstruction of whole genomes from two extinct human forms: the Neanderthals, and the Denisovans. The former group were already well known from a range of fossils across Europe and western Asia, with evidence that they went physically extinct about 30,000 years ago. The latter group were completely unknown until teeth and a finger bone from Denisova Cave in southern Siberia were analysed, producing a genome that is related to, but distinct from, that of Neanderthals.
Most of the fossil and genetic data up to 2010 pointed to an origin of the modern form of Homo sapiens in Africa by 150,000 years, and then a dispersal of our species from Africa about 60,000 years ago. Our shared physical features such as a rounded braincase and chin evolved in Africa first, and then the regional or ‘racial’ differences were added in the last 60,000 years as modern humans began to spread. During this dispersal it appeared that other forms of humans outside of Africa were replaced, with little or no interbreeding between Homo sapiens and these other groups such as the Neanderthals.
But the latest genomic comparisons of fossil and modern humans suggest that populations outside of Africa contain about a 2% input of Neanderthal DNA, from at least one episode of ancient interbreeding, while populations in Australasia contain an additional component (about 4%) of Denisovan DNA, from at least one further and more localised interbreeding event.
Let our ancestors rest in peace
So what effect does this ‘hybrid’ DNA have? That question is largely unanswered, although most of it does not seem to be functional. However, there are suggestions that some differences in immune systems, comparing people within and outside of Africa today, could be due to a Neanderthal or Denisovan inheritance in non-Africans. This would not be surprising, since the more ancient populations outside of Africa would have evolved immunities to local diseases, so by interbreeding with the natives, modern humans could have rapidly acquired valuable defences against diseases that would have been completely new to them.
There may also be implications for biology and behavior, and this can be investigated by examining what the relevant region of genetic code does in humans today. Distinct mutations in the melanocortin gene of Neanderthals and Denisovans suggest that some Neanderthals had red hair and others had dark hair, while the Denisovan girl who owned the fossil finger bone had dark coloring in her skin, eyes, and hair. Other comparisons show up areas where the Neanderthals and Denisovans share the same coding as apes, while modern humans uniformly share new mutations. These include regions known to code for such things as the development of the skeleton, the way the brain works, sugar and fat metabolism, reproductive physiology, cell division in the body and brain, and personality traits associated with different levels of empathy or impulsivity.
Since high quality Neanderthal and Denisovan genomes have only been available in the last couple of years, much research remains to be done, but in the next few years we will undoubtedly learn some of the fundamentals of what made a Neanderthal a Neanderthal, what made a Denisovan a Denisovan, and what make a modern human a modern human. But personally I hope that the ultimate test of this research – cloning an ancient human – is never attempted. These ancient humans evolved and lived in a completely different world, and we should let them rest in peace.