A new study shows that early Homo sapiens in Europe coexisted with Neanderthals in specific locations where there were sufficient herbivores to support both populations. The results add texture to the growing image we have of the interactions between these two species.
Herbivore carrying capacity
In ecology, there’s an idea called the “competitive exclusion principle” which states that competitive species cannot coexist over a long period of time. It's an old idea and was established by Thomas Malthus and subsequently built upon by Charles Darwin during the 18th and 19th centuries. However, more recent research complicates the idea, showing that complete competitors can coexist if there are enough resources to keep things balanced.
When it comes to our expansion out of Africa (which probably started around 60,000 years ago), the traditional assumption was that our ancestors competed with other early human species for scarce resources. Eventually, this pressure killed off large numbers of herbivores upon which the ecosystem was reliant, which also led to the extinction of species such as Neanderthals in Europe.
But again, the reality appears to be far more complicated. It seems the extent of this mass extinction did not occur all at once. In fact, there was a kind of “mosaic” of cultural and biological overlap between our two species for some time.
Ancient DNA shows that humans and Neanderthals interbred and coexisted in some places. Yet in other parts of Europe, Neanderthals died off much quicker, sometimes a few millennia before humans arrived.
At the same time that humans were first arriving in Europe, global climatic conditions were experiencing extreme oscillations between mild conditions and full glacial ones. As such, these oscillations placed even more pressure on the ecosystem and may have contributed to the extinction of various herbivores in Europe.
But yet again, the extent of these oscillations was not uniform. Some places in Europe experience far harsher conditions than others, and for longer.
In those areas that were less impacted, humans and Neanderthals competed for shared resources, such as herbivore species, but the extent to which the numbers of these prey animals impact the balance between the two species is not well known. That is what the authors of this latest study sought to address.
“This study tests whether the regional differences in the timing of the Neanderthal disappearance, the spread patterns of H. sapiens, and the temporal overlap between both human species were affected by the carrying capacity (CC) of the ecosystems that they inhabit in Europe.”
To test this, the team compiled an extensive dataset of herbivore species recovered from archaeology-paleontological levels that occurred at the same time as the Greenland ice cores 60,000 years ago. They then tested a climate simulation they created from an atmospheric general circulation model against various paleoclimatic proxies.
The team then drew up biogeographic regions of Europe based on their paleoclimatic conditions, as well as the herbivores that lived there, and the temporal trends that occurred through this period.
Finally, the team used hierarchical Bayesian age models (BAMs), and optimal linear estimation (OLEs) to create a new chronology of the Neanderthals’ disappearance and the arrival of humans in each region. This allowed them to estimate the carrying capacity of each herbivore species in each region.
The results show that the disappearance of Neanderthals occurred at different times and was impacted by the carrying capacity of herbivores in different regions, which also influenced their overlap with humans.
It seems that this coexistence was partially sustained by local spatial segregation, where Neanderthals and H. sapiens kept away from one another, which may have eased the pressure for direct competition. However, this interpretation should not be overstated.
“Consequently, the observed temporal overlap at a regional scale does not necessarily imply coexistence at the local level,” the authors write. “Nonetheless, it is worth noting that spatial segregation and low population densities do not negate the competitive exclusion hypothesis.”
According to the researchers, Europe could be clustered into three main groups based on their ecosystem productivity – regions with high constant carrying capacity, regions with high but “sharply fluctuating” carrying capacity, and regions with low productivity (low plant and herbivore species).
“While the former witnessed a temporal overlap between both human species and thus represented areas of high habitat suitability for humans, the second and third groups correspond to regions prone to experience discontinuities in populations of both human species.
Contrary to what has previously been thought, areas of high carrying capacity were not limited to southern Europe but also occurred in mid-latitude areas, such as the Danube and Rhône basins. These areas probably witnessed a greater expansion of humans, as well as a longer period of overlap between them and Neanderthals
Ultimately, this research challenges the idea that the arrival of humans necessarily caused the extinction of Neanderthals, as the regions where the latter disappeared before the arrival of the former were the ones that also experienced the lowest herbivore carrying capacity. Nevertheless, the competitive exclusion principle remains a possibility.
The study was published in Science Advances.