There was a time when different humanities shared the planet. In addition to Neanderthals and Denisovans, Homo luzonensis , Homo erectus , Homo naledi and Homo florisiensis coincided with sapiens at different stages of Prehistory.
Neanderthals are the closest to us in body and brain size, in addition to being the last species to become extinct. For hundreds of thousands of years, they inhabited the forests and mountains of Europe, but disappeared 40,000 years ago, shortly after the first anatomically modern sapiens began to settle on the continent.
That coincidence makes the role that the Sapiens could play - if they played any - in its extinction is still subject to debate. The hypotheses range from the direct conflict between the two species to the mere occupation of the regions that the Neanderthal left empty in its decline. An investigation published this Wednesday in the journal Plos One reinforces this last idea: according to its authors, demographic factors are sufficient to explain the disappearance of Homo neanderthalensis .
Neanderthals lived in small and geographically isolated groups. Random demographic fluctuations - variations in the rate of births and deaths or in the sex ratio - could have been sufficient to cause their disappearance over a period of 10,000 years.
"That does not completely rule out that humans had some role in extinction," says Krist Vaesen, a researcher at the Eindhoven University of Technology. "But our models suggest that this role may not have been what some scientists usually claim. According to those scientists, Neanderthals were surpassed by a more intelligent and numerous species, but our results suggest a peaceful coexistence as a more plausible alternative ."
Researchers have taken data from current populations of hunter-gatherers such as the Hadza of Tanzania, the Achés of Paraguay, or the! Kung of Namibia and Bostwana. With that base they have developed statistical models to simulate the evolution of Neanderthal populations over long periods of time. In addition, they took into account the impact that inbreeding would add (common in isolated Neanderthal clans) and the so-called Allee effect: the reduction in population size lowers the reproduction rate by scarce individuals.
Rivalry
According to these estimates, if 25% or less of Neanderthal females had given birth (a common figure in current hunter-gatherers), that would have meant the disappearance of populations of up to 1,000 individuals. The authors do not rule out that the appearance of sapiens could have aggravated certain factors. "A strong competition for resources would have made Neanderthals even more vulnerable to extinction," Vaesen acknowledges, "but it is reasonable to think that competition was very weak, given the geographical distribution area of Neanderthals (from the Black Sea to the Atlantic coast) and that the population was small (70,000). "
Neanderthals were long regarded as a coarse and little evolved relative of the sapiens. Partly because of their appearance and physical strength and partly because admitting the closeness between the two species subtracted exceptionality from ours. "In many ways, Neanderthals were a human species very similar to ours, because both inherited the same qualities of common ancestors and because in evolution both species acquired similar abilities," says Antonio Rosas , a researcher at the National Museum of Natural Sciences ( CSIC).
The conclusion of the study in PLOS One links to a work published this year by Rosas and other CSIC researchers, which pointed to a combination of ecological and demographic factors as responsible for extinction . Specifically, the tests performed in the cave of El Sidrón (Asturias) revealed a high inbreeding that would have limited the viability of the clan.
That study focused on a family group that lived in the Asturian grotto (composed of 13 individuals) and revealed up to 17 congenital anomalies distributed throughout the skeleton, all of them shared by several members.
Social connections
Despite having a brain size similar to that of modern humans (although greater body mass), the Neanderthals' brain had a different structure. A significant part was dedicated to vision, to the detriment of other functions such as social connection. In contrast, the sapiens, from Africa, did not need this adaptation and instead developed frontal lobes, which are associated with better information processing.
"It is possible that subtle brain and cognitive abilities differentiated the way of thinking and reasoning of modern Neanderthals and humans," Rosas explains. "These small basic differences could determine key aspects such as group size, forecasting capacity or long-distance connections, which ultimately determined that their geographical areas of distribution were more restricted. And consequently the genetic exchange was smaller. " .
Oxford University anthropologist Robin Dunbar developed in the 90s the famous Social Brain Theory, in which he showed that brain size is the main indicator of the size of social groups in primates. The larger the neocortex (the outermost layer) of the species, the greater the number of individuals living in the community.
However, if the Neanderthal brains were essentially dedicated to vision and movement, this may mean that they had other smaller areas of the brain. Thus, they would have been cognitively limited to smaller groups, which ended up condemning them from an evolutionary point of view.
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