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 Yet they appear to have been replaced by anatomically-modern humans (in Europe, anyway) by around 35-40,000 years ago – and with at least some level of coexistence, given the presence of Neanderthal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 advantanges 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 (&/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 3 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 species, we 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. http://dx.doi.org/10.1098/rspb.2019.0907