If some of our ancestors interbred with Neanderthals, are we really modern?
What is it exactly that makes modern humans modern? We are subject to jet lag? We can walk and text at the same time - some of us, kind of? Our minds and bodies bear the brunt of all the junk we’ve been spewing into the earth’s air, water, and soil since the Industrial Revolution?
Chris Stringer, a research leader in human origins at the Natural History Museum London, recently wrote a Comment in Nature speculating on what the precise features might be that define us as modern. Recent deoxyribonucleic acid - DNA evidence has shown that after modern humans left Africa around 60000 years ago, they interbred with different groups of archaic humans - Neanderthals and Denisovans. As a result, different populations walking around today have varying amounts of this archaic DNA in their genomes.
Stringer is aware that this information could lead to the very dangerous assertion that all humans are modern, but some are more modern than others. So he writes, “All living humans are members of the extant species H. sapiens and, by definition, all must equally be modern humans.”
There is currently a big debate raging among paleontologists about the origin of modern humans. Stringer is in the camp that thinks that we all derive from humans that evolved in Africa by 100000 years ago and then left from there. This explains the common physical traits we share. In this view, regional or racial differences that exist between modern humans were grafted onto these common features through processes like natural selection, founder effects, and genetic drift as groups of humans became geographically separate in the last 60000 years.
Some of the regional difference may also be explained by interbreeding with Neanderthals and Denisovans.
The multiregional hypothesis, in contrast, contends that humans left Africa much earlier - closer to two million years ago - and that regional variations can be explained by parallel evolutionary paths in different locations around the globe. The debate between these two models has gone on for decades, and Stringer is definitely a proponent of the out-of-Africa view.
This isn't the only area of controversy when it comes to human origins. Stringer's Comment is accompanied by two Features: one on the academic debate about why, when, and how humans got from Africa to Asia; the other on how and when humans first got to the Americas. In both cases, some claim that the evidence indicates an arrival over land by foot, and others read it as demonstrating an arrival by sea in boats.
The presence of archaic DNA in modern genomes is certainly intriguing, regardless of which evolutionary scenario you think it suggests. But Stringer concludes by noting that whatever our genetic makeup, wherever we came from, and no matter what new discoveries lay in store, “we will have learnt nothing in the past 50 years if we let small segments of distinct DNA govern the way we regard regional variation today.”