“The question of where we come from is one that has fascinated humans for centuries,” said Dr. Trevor Cousins, a Cambridge University researcher. Such thousands of years of curiosity have now been answered with a mind-boggling find: modern humans are not children of a single line of lineage but products of intermixing genes between two groups in the distant past. The two groups split about 1.5 million years ago and reunited about 300,000 years ago to create the gene building blocks of Homo sapiens.
volutionary genetics had previously assumed axiomatically that the human species evolved in Africa approximately 200,000 to 300,000 years ago from a single line. Fresh evidence discovered in Nature Genetics turns the hypothesis around and shows much more intricate history of evolution. One of the researchers, Professor Richard Durbin, explained, “Our research shows clear signs that our evolutionary origins are more complex, involving different groups that developed separately for over a million years and then came back together to form the modern human species.”
Scientists used advanced computer methods to analyze human DNA today instead of working with fossilized remains. Scientists used data compiled by the 1000 Genomes Project, an international research initiative and had sequenced DNA from individuals of African, Asian, European, and American populations. Scientists next used a computer program, cobraa, to simulate the splitting apart and then unification of such old human populations. Scientists could now deduce the existence of such populations which did not have a material trace left behind in the fossil record.
The most astounding discovery was the asymmetrical origin of these two groups in our genome. Approximately 80% of the DNA of present-day humans comes from one of these groups, which suffered a catastrophic bottleneck quite early after branching off for the first time. The bottleneck reduced the population to an infinitesimally small proportion before it expanded over a million years. This group is considered to be the ancestral source that gave rise to later Neanderthals and Denisovans. The second group contributes 20% of human genes found in us today. Their contribution is toward brain function and neural processing genes. As described by Cousins, “Some of the genes from the population which contributed a minority of our genetic material may have played a crucial role in human evolution.”
In contrast to Neanderthal DNA, which makes up approximately 2% of the current human genome in non-Africans, ancient admixture was as much as 10 times higher and is present in all populations. This establishes the central role that genetic admixture has played in the evolution of our species. The scientists also found that second-population genes were found to be marked as occurring far from regions of the genome that are linked to gene functions. This is typical of a process known as purifying selection, where natural selection gradually eliminates deleterious mutations in an effort to ensure genetic compatibility in the long term.
The outcomes of this study have implications beyond human history. The cobraa algorithm was even tried on other animals’ genetic data like bats, dolphins, chimpanzees, and gorillas, and some but not all of them had remnants of ancestral population structure. “What’s becoming clear is that the idea of species evolving in clean, distinct lineages is too simplistic,” Cousins said. This is in accordance with the function of interbreeding and gene exchange towards the development of new species in the animal world.
Original names for such groups are yet to be determined. The fossil record leads us in the direction of groups like Homo erectus and Homo heidelbergensis, of Africa and the rest of the world, who at the time in question were most likely the candidates. But more data are required to set up confirmatory correlations between genetic information and fossil records. As paleogeneticist Carles Lalueza-Fox described it, “A derivative of the study would be to look with new eyes at the African fossil record of the last million years to see if it is possible to identify these two ancestral populations or species.”
For the future, the researchers hope to expand their cobraa model to include less sudden migrations of genes between populations rather than immediate mergers and splits. They also hope to integrate their findings with anthropological data, such as fossils indicating early humans to be far more diverse than has been believed. As beautifully phrased by Professor Aylwyn Scally, “The fact that we can reconstruct events from hundreds of thousands or millions of years ago just by looking at DNA today is astonishing.”
Not only does this research rewrite the history of human evolution, but it opens the door to further excursions in genetic heritage throughout species. It is an affirmation of the power of modern computer techniques to strip away secrets from the past and a reminder of the intertwined network of oneness that is the hallmark of life on this earth.
