Neanderthals were very similar to us, making their extinction a mystery. What advantage did we have over them that allowed us to survive? However, new research suggests that we might not have had edge over the Neanderthals. Instead the Neanderthals may have driven themselves extinct with cannibalism.
There is plenty of evidence showing that our sister species would kill and eat each other. Perhaps during times of environmental stress. Although this might have allowed Neanderthal groups to survive on their own, it would be an obvious disadvantage when they had to compete with a non-cannibalistic species. Like we are (for the most part).
Neanderthal cannibalism
Evidence for Neanderthal cannibalism comes from several archaeological sites throughout Europe. This evidence is so abundant researchers have known about it for more than 100 years.
One of the largest cannibal sites is Krapina, Croatia, which contains more than 100 Neanderthal bones (possibly representing as many as 14 individuals) with cutmarks on them. These cutmarks were suspiciously similar to those found on the animals Neanderthals butchered and ate.
Several sites in France also tell a similar story. Moula-Guercy, in the Ardeche, contains half a dozen Neanderthal remains that appear to have been ripped apart and butchered by other Neanderthals.
Curiously, at many of these sites the "victims" were treated with a degree of respect. At Krapina and other nearby locales, the bones seem to have been deliberately buried after they had been butchered. This stands in stark contrast to how Neanderthals normally treated their prey. Reindeer bones were often left scattered around the floor of the camp, for example.
Cannibalism wasn't unique to Neanderthals either. The (pre)history of this morbid behaviour stretches back perhaps a million years. In fact, some of the earliest Europeans were already engaging in cannibalism. Some of the hominin bones from Atapuerca, Spain, contain the tell-tale cutmarks of butchery. This early case of cannibalism tells a rather chilling tale. It appears that infants were the primary victim. This is similar to the pattern seen when one chimpanzee group invades and attacks another.
The model
Cannibalism doesn't seem like a particularly smart move if your species is trying to survive. So a group of researchers wondered: what would be the benefits of this behaviour that might drive it's evolution. And could the negative side-effects put them at a disadvantage when competing with modern humans?
To answer these questions they came up with fancy agent-based model. It placed virtual Neanderthals in an environment with varying levels of resources and organised them into small communities. Each of these had a preference for cannibalism, which could increase or decrease through natural selection.
After letting these virtual Neanderthals run around for a while (and sometimes eat each other) they identified circumstances where cannibalism would become more common. In particular, cannibalism provided an evolutionary benefit when the Neanderthals were living in low quality environments. In these cases, cannibalism was used to supplement the food supply (and decrease competition). It eventually became universal as non-cannibals just couldn't compete.
However, when a non-cannibalistic group entered the fray things changed. Cannibalism increases individual fitness. However, this constant infighting keeps the number of cannibals small. Thus when virtual modern humans (who didn't cannibalise) were placed in the computer, they could outcompete the Neanderthals through sheer numbers. Eventually they were able to reach the carrying capacity of the simulation, something the cannibalistic Neanderthals couldn't have done.
In other words, in a harsh environment full of cannibals it's good to be the best cannibal out there. But when another group enters the picture this in-fighting proves destructive. Thus the Neanderthal extinction could have been self-inflicted.
The problems
This model presents a valid way in which cannibalism could have at first helped the Neanderthals, but ultimately led to their downfall.
But that's assuming the premises of the model are valid, which they might not be. For example, under this model cannibalism is something done when the environment is pretty rubbish. Yet the cannibalism at Moula-Guercy took place during Marine Isotope Stage (MIS) 5c. This was a relatively warm and bountiful period of European prehistory.
The Krapina cannibalism does appear to have taken place during a cooler, harsher MIS. So it seems like there's not much correlation between cannibalism and local environment, unlike what the model suggests.
This doesn't necessarily invalidate this model. It could be the case that cannibalism became common much earlier and these Neanderthals inherited it. Thus they continued to eat each other regardless of the environments. But hopefully it does show that more evidence is needed to validate that the premises of this model are actually true.
For example, the simulation also indicates that cannibalism would have been very common by the end of it. However, cannibalised remains form a small minority of all Neanderthal fossils.
Again, it's clear more data is needed to test if this model works in the real world. Or at least, worked in the Neanderthal world to help drive them extinct.
tl;dr
A simulation indicates that if environments were tough Neanderthals would have resorted to cannibalism. This would have put them at a disadvantage when modern humans turned up. However, whether this simulation is accurate remains to be seen.
References
Agustí, J. and Rubio-Campillo, X., 2016. Were Neanderthals responsible for their own extinction?. Quaternary International.
Defleur, A., White, T., Valensi, P., Slimak, L. and Crégut-Bonnoure, E., 1999. Neanderthal cannibalism at Moula-Guercy, Ardeche, France. Science, 286(5437), pp.128-131.
Russell, M.D., 1987. Mortuary practices at the Krapina Neandertal site. American Journal of Physical Anthropology, 72(3), pp.381-397.
Saladie, P., Huguet, R., Rodriguez-Hidalgo, A., Caceres, I., Esteban-Nadal, M., Arsuaga, J.L., de Castro, J.M.B. and Carbonell, E., 2012. Intergroup cannibalism in the European Early Pleistocene: The range expansion and imbalance of power hypotheses. Journal of human evolution, 63(5), pp.682-695.
