The role of drastically variable and changing climates in the story of human evolution is also evident. As we witness first-hand the potential for climate disaster in our own lifetimes, we can more easily imagine the catastrophic effects of some of the major climate swings of the last 150,000 years upon small bands of humans spread thinly across the seemingly unending expanses of Eurasia. One particularly crushing winter, one piece of bad luck, and things would have been extraordinarily perilous for small and isolated populations. The death of an experienced member of a group results in the death of ideas and traditions that may then have to be reinvented. In the modern world we see that creativity is more common in denser and more populated places. In fragile worlds with fewer people, ideas and knowledge are precious commodities, and can disappear in an instant.
Finally, in this light it also seems to me that luck too played a role in the story of our early human ancestors. But for a different set of biological or environmental circumstances, I think that it might easily have been a Denisovan or Neanderthal population that survived instead of us and went on to disperse into the rest of the known world like the invasive species we eventually became. The fact that Neanderthals were present on the planet for more than 250,000 years shows that they were a well-adapted and successful experiment in evolution. We have evidence now that they were capable, like us, of wide dispersals and movement, from Western Europe to the Altai Mountains. We may find that Denisovans, and others, were even more successful. But they too ultimately disappeared. The human lineage went through bottlenecks, where our genetic diversity and population fell dangerously low. We could easily have joined these others in extinction but for moments of luck.
When we talk about living and dying, succeeding and failing, however, we must always remember that these human cousins of ours are not completely lost. They live on in us in different and now fragmented parts of our own DNA. They are within and part of us. This is a quite beautiful thought. More than 20 per cent of the Neanderthal genome can be recovered from modern human populations, perhaps more.14 We will soon find out how much of the Denisovan genome can be sieved out of us and put back together, and discover more and more about what we inherited and what we did not. The stunning realization that we receive from these lost cousins minute traces of genetic coding without which we could not live at altitude in Tibet, or as well in the cold of Greenland, or resist disease in Melanesia, or have adapted so readily to new environments outside our African homeland, is surely one of the great contributions of ancient genetics to understanding our current human condition. The success of our own species and the wide and exuberant diversity of humanity are due in part to these precious genetic gifts. We are not simply human, we are the sum of all of the branches of that braided river that touched and parted on our way to today.
14. Vernot, B. and Akey, J. M. 2014. Resurrecting Surviving Neandertal Lineages from Modern Human Genomes. Science 343: 1017–21.
From The World Before Us: The New Science Behind Our Human Origins by Tom Higham. Published by Yale University Press in 2021. Reproduced with permission.
Tom Higham is professor of archaeological science at the Research Laboratory for Archaeology and the History of Art at the University of Oxford and director of the Oxford Radiocarbon Accelerator Unit.