By Alison Campbell 12/06/2019 7

One of the questions students often ask, when we’re discussing human evolution, is “what happened to the Neanderthals?”

After all, this was a large-brained species closely related to our own, with some fairly complex tool technologies and the ability to survive (and thrive) in harsh environmental conditions. Yet they appear to have been replaced by anatomically-modern humans (in Europe, anyway) by around 35-40,000 years ago – and with at least some level of coexistence, given the presence of Neanderthal DNA in modern non-African human populations. So why did the Neanderthals disappear?

In mulling this over, students have come up with quite a few possible answers: maybe Neanderthals just didn’t breed as fast as sapiens, so that their populations grew much more slowly; maybe they were out-competed by a species that was a better hunter; maybe they suffered from inbreeding depression; maybe it was just down to chance? A just-published paper (Shultz, Montrey & Shultz, 2019) might help to answer the question.

As the researchers point out, it can be hard to find evidence of any particular fitness advantage in the archeological record. In attempting to solve the puzzle, scientists have generated various models that hypothesise either that anatomically modern Homo sapiens had some fitness advantage over H. neandertalensis, or that neutral drift was operating. In this new study, Shultz & colleagues compare the two using computer modelling to predict the outcomes and compare their findings with the available genetic and fossil data.

They begin by listing the possible advantages that ‘fitness arguments’ propose for modern humans (students will be pleased to see some of theirs made the list): better cognitive and language ability; more advanced hunting and technological skills; a wider range of diet choices and other resources; social cooperation; and higher birth rates (and/or lower death rates. They also cite work suggesting that Neanderthals were at a genetic fitness disadvantage that would eventually have seen them disappear.

The alternative, neutral drift, invokes things like an increased rate of migration by sapiens into Neanderthal territory, or Neanderthal DNA being swamped by breeding with larger populations of modern humans. (Of course, both of these suggestions could well be fitness arguments as well). That model is discussed in this 2017 paper by Kolodny & Feldman.

In their simulations, the team looked at the effects of:

  • differential fitness: their modelling found that if one species or the other had a fitness advantage, then replacement was much faster than if simulated drift was operating;
  • differential population sizes: replacement was faster, and unaffected by population size, for the species with a fitness advantage; neutral drift was again much slower;
  • total population size: “at every level of population size, neutral drift is again much slower to achieve fixation than differential fitness is”;
  • migrations: the models saw their digital Neanderthals staying clear of sapiens territory if the modern humans had some fitness advantage;
  • the possible lifespans of bands of modern humans and Neanderthals: if drift was all that was operating, the simulations suggested that it would take “a continuous period of interaction and competition between [the two species that lasted] at least 50,000 years”.

Overall, the team concluded that it was possible that neutral drift alone could have seen moderns eventually replace Neanderthals – but the archaeological evidence tells us that modern humans also replaced Denisovans and archaic African populations. The odds that all three replacements happened solely by chance are low.

If moderns had a larger starting population, and had a higher migration rate, then they could replace neandertalensis by drift alone. But then, wouldn’t their higher numbers and rate of dispersal imply some fitness advantage? Certainly the simulations indicate that drift would be far too slow a mechanism to have resulted in replacement during the time frame demonstrated by the existing fossil record.

Similarly, the simulations predicted “an archaeological and genetic signature of Neanderthal artefacts, fossils, and DNA reaching into Africa” after Neanderthals had first encountered modern humans in the Middle East, if replacement was due to neutral drift. However, there’s no fossil or molecular evidence to support this. On the other hand, there is some evidence that sapiens went back, from Europe to the Horn of Africa, about 30,000 years ago. So the team concluded that since there was no physical or ecological barrier to such migration, the Neanderthals were at something of a fitness disadvantage that stopped them making the same journey.

I suspect that the students, like Oliver, will be left asking for more. While the results of the simulations presented in this new paper do appear to gel with existing fossil and archaeological data and support the concept of a biological  fitness advantage for our own specieswe still don’t know what that advantage might actually be. But, as the authors say, their modelling “provides a useful framework for future attempts” at working out what happened.

Perhaps some of those students will go on to contribute to finding an answer.

Featured image: Neanderthal, by Allan Henderson, Flickr.

Shultz DR, Montrey M, Shultz TR (2019) Comparing fitness and drift explanations of Neanderthal replacement. Proc.R.Soc. B 20190907.

The post what happened to the neanderthals? appeared first on BioBlog.

7 Responses to “What happened to the Neanderthals?”

  • Did Homo sapiens migration bring diseases to which H. neandertalensis had no resistance? Is this included in your definition of biological fitness advantage?

  • I’m not sure it helps that popular media call Neanderthals and modern humans ‘species’ when they were evidently not separate species. It may be more accurate to call them distinct races, hence the interbreeding. Of course, this still leaves the question of how the relative abundance of the races changed. Another caveat is the very limited archaeological evidence from the time periods.

    I thought there was evidence of Neanderthals using tools and having a culture, like other humans. Over thousands of years, one can imagine how cultures and genes gradually merged into modern Europeans. I do not understand why this may work with Neanderthals but not Denisovians and Africans? Considering the long timescales, and (I think) evidence of cultural and genetic mixing, could genetic drift and selection for individuals best adapted to European climates (pale skin absorbing vitamin D?) explain the outcome?

    • I think many (most?) palaeoanthropologists view them as distinct species (bearing in mind that the biological species concept is a rather leaky one); hybridisation between species isn’t all that rare. When the first draft Neandertal genome sequencing was published, the authors commented that the differences between that & sapiens’ genome probably justified the separate species epithet.

      Yes, there’s plenty of evidence that Neanderthals made & used tools. Their tool culture was somewhat distinct from that of sapiens in Africa (from memory) & then in Europe/the middle East there seems to have been a certain amount of interchange.

      Don’t forget that sapiens is not just Europeans. There is a small proportion of Neandertal DNA in European, Asian, American & Pacific populations, which scientists assume was acquired via interbreeding as sapiens migrated out of Africa & encountered the Neanderthals. There’s none in the deep African lineage, though. Similarly there is a small proportion of Denisovan DNA in modern human populations in Asia & the Pacific, but not in African or European lineages. Again, the assumption is that this reflects a certain amount of interbreeding as migrating sapiens moved through the regions occupied by Denisovans – current evidence places the latter in the Caucasus mountains & on the high Tibetan plateau. (Interestingly, it seems that modern Tibetans acquired the EPAS-1 allele (which allows them to cope with the low partial pressure of O2 at those altitudes) from the Tibetan Denisovans.)

    • Pretty much all the literature I’ve seen (with the exception of your link) views sapiens & neandertalensis as distinct species (on the basis of both physical & genomic differences). The jury still seems to be out on the Denisovans, which is hardly surprising given the paucity of material we have for them.
      I’m inclined to think that if interbreeding was as extensive as you’re suggesting, then there would be a higher proportion of admixture evident in modern sapiens genomes.

  • Alison, I would like to think that the dialogue around Neanderthals can move away from using the terms extinction, species and race. These concepts can be loaded with negative “us versus them” stereotypes. I would be prefer to see discussion using the terms “populations and integration”. Parts of the Neanderthal genome live on in many of us, and although we do not expect to find an extant Neanderthal population, their genetic lagacy remains.