What happened to the Neanderthals?

The draft Neanderthal genome, published in 2010 from bones in Croatia, showed that people outside Africa share more variants with Neanderthals than African populations do, which is what interbreeding after the migration out of Africa would produce.

The proportion is small. The draft genome put it at one to four per cent of non-African genomes and later work narrowed it to about two, and the segments differ from person to person, so the total surviving Neanderthal genome is much larger than any individual’s share: one study recovered about a fifth of it from 665 people.

A 2014 study dated forty sites from Spain to Russia with improved radiocarbon methods. Contamination that older treatments of bone had failed to remove made many earlier dates too young, and most claims of a late Neanderthal survival rested on them.

The result places the end of the Mousterian, the stone-tool tradition taken as the mark of Neanderthals, between about 41,000 and 39,000 years ago, reached at different times in different regions. One team still argues that Neanderthals held on south of the Ebro in Iberia until about 37,000 years ago; radiocarbon specialists, including members of the 2014 team, found the southern dates behind earlier claims of this kind unreliable.

It also puts modern humans in Europe alongside them for between about 2,600 and 5,400 years, though not necessarily in the same places: a 2022 estimate for France and northern Spain puts the overlap there at about 1,400 to 2,900 years.

Genomes have since narrowed the other end of it. Sequencing the earliest modern humans in Europe traced the Neanderthal ancestry shared by all non-Africans to a single admixture event, roughly 45,000 to 49,000 years ago; a study of more than 300 genomes spanning 50,000 years puts it in one extended period between about 50,500 and 43,500 years ago. Some early Europeans had a Neanderthal ancestor within the previous ten to twenty generations, so encounters went on after that, but the mixing people alive today carry came almost entirely before the disappearance rather than through it.

Candidate explanations include climate change, disease, competition in which modern humans held some advantage (a broader diet, a more efficient way of making a living, greater cognitive capacity), absorption through interbreeding, and demographic accident: small, thinly spread populations are vulnerable to inbreeding and ordinary bad luck over long periods.

The last of these needs no superiority on anyone’s part. Two models of a population the size genetic estimates give Neanderthals find that inbreeding, the difficulty of finding mates at low density and chance might have been enough to end it, with no competitor at all. A review of the decade of evidence to 2016 holds that the genetic and archaeological data “falsify inferences to the effect that” Neanderthals were behavioural outliers beside their African contemporaries, which weakens accounts built on modern human superiority. Its authors still see some form of competition at the end: modern humans outbreeding and absorbing a sparse population.

A model with no selective advantage in it anywhere, only repeated migration out of Africa and neutral drift, predicts the replacement on its own and cannot be rejected over a wide range of parameters. Its authors offer it as a null hypothesis rather than as an answer, which is the right way to hold it. Critics answer that demography cannot stand alone, because a population’s size is tangled up with its competitors and its environment.

Some inherited variants are associated with traits of skin and hair, and others with the risk of diseases including lupus, Crohn’s disease and type 2 diabetes. Mapping the ancestry across the genome found stretches of millions of base pairs carrying almost none of it, and much less than elsewhere on the X chromosome and in genes expressed in the testes, which suggests selection removed much of it.

Interpreting individual associations is difficult and the popular versions run far ahead of the evidence.