Rethinking the Neanderthal – A Cycle of Population Expansion and Contraction

Our picture of the ancient world co-inhabited by Homo sapiens, Homo neanderthalensis, and Homo denisova is constantly changing as new information emerges from scattered scientific studies. The origins of modern humanity were once thought to be very clear: supposedly our ancestors arose somewhere in Africa and migrated outward to other parts of the world, displacing “older” groups of human cousins.

Now we’re more convinced that very ancient ancestral groups — probably belonging to Homo heidelbergensis — spread outward from Africa at least 500,000 years ago, perhaps longer. We believe that the Neanderthals and Denisovans probably evolved from those ancient populations due to isolation from (lack of interaction with) their relatives in Africa. Hence, Homo sapiens most likely evolved at approximately the same time as Homo neanderthalensis and Homo denisova, rather than after they did.

We can speculate that there may have been various hybrid populations of less evolved clans living alongside more evolved clans across the globe — but the differences in evolved states may have been very minor. That is, regional populations most likely did not divide into “evolved” and “unevolved” clans, but the rates of change and adaptation among each regional group of clans could have varied enough that one region might have experienced more evolutionary adaptations than those around it. In fact, new research even suggests that Neanderthals had as much a gift for language as our direct ancestors.

This regional differentiation hypothesis may explain why our genetic diversity is so rich. Populations would rise and fall according to the changes in climate around them, and larger groups would either push aside, exterminate, or absorb smaller groups. Hence, our concept of three main groups is based on what survives of common traits across larger, geographically diverse populations. A new study suggests that Homo sapiens outlasted Homo neanderthalensis simply by surviving in a better refuge during times of drastic climate change. This Refugium Hypothesis may explain variations in population among the three Homo groups.

For the time being researchers believe that Denisovans had a smaller population than Neanderthals. And though estimates of Neanderthal and Homo Sapiens populations vary, we are certain that Neanderthals ranged over a broad region including the Middle East, Europe, and Asia for thousands of years before Homo sapiens began expanding outward from Africa. It may therefore be reasonable to argue that Homo neanderthalensis represented the dominant species of human for an extended period of time.

The archaeological record for the Neanderthals is quite rich, much more detailed in some respects than for Homo sapiens at equivalent times. One reason why we may have better archaeological evidence for the Neanderthals is most likely climate-related — organic material lasts longer in the colder latitudes than in the warmer latitudes closer to the equator. But we know that Neanderthals had a sophisticated social structure and that they made clothing, tools and weapons, and ate a variety of land and sea-based foods that were comparable to the inventions of Homo sapiens.

And yet genetic studies all suggest that Homo sapiens eventually conquered the world. Only a small percent of our DNA can now be traced back to either Neanderthals or Denisovans. Denisovan DNA has now been found in Native American and Southeast Asian Islander populations; but what may be more remarkable is that the Neanderthal DNA in these populations is greater than the Denisovan DNA. One possible explanation for these facts is that Neanderthals retreated eastward before the advent of Homo sapiens, displacing and/or merging with smaller Denisovan groups, before finally being displaced or absorbed by Homo sapiens.

But what if the retreating groups were responsible for the mingling? Let us suppose that a band of Neanderthals was pressured into migrating east by advancing Homo sapiens. Unprepared for such a migration they may have lost too many children and women of child-bearing age, so their males turned to capturing mates from among the invaders. Never quite able to rebuild their numbers to a point where they could withstand the Homo sapiens these retreating bands gradually settled on islands and crossed the Bering Bridge, entering North America.

Research published earlier this year suggests that Neanderthals may have experienced a population “bottleneck” similar to that of Homo sapiens. The Homo sapiens genetic bottleneck is attributed to the Toba Event, the explosion of a massive super volcano in Indonesia about 73,000 years ago. Neanderthals may have been less directly affected by the climactic changes resulting from the Toba event because they were in some ways more evolved than their African cousins. Humans in Africa had access to more food sources than Humans living in Europe and Asia.

Scientists have long believed that Neanderthals and modern Humans coexisted for thousands of years in the Middle East. We don’t have a full geological timeline for their co-habitation, so it is possible that successive waves of migration repopulated the area after the Toba Event. That is, Neanderthals might have expanded for a few thousand years into southern lands, never quite reaching Africa. But as Homo sapiens reorganized both culturally and intellectually, their clans began to differentiate and migrate away from their common ancestral lands in southern Africa.

There is evidence that male aggression among modern Humans emerging from Africa forced many males outward, thus driving expansion into new territories. There is no doubt that Homo sapiens women also migrated outward but there seem to have been more males on the move than females. The quest for mates could have led to direct competition (or trade) with older populations of humans in the Middle East, and from there Homo sapiens began the push northward that would eventually divide the Neanderthal world into eastern and western spheres.

As the Neanderthals retreated before Homo sapiens their numbers declined. The competition for mates could have led to more than one genetic bottleneck. What we know for certain is that Neanderthal populations declined until they lost their genetic distinctiveness. The only modern candidates (based purely on speculation) for strong connections with Neanderthals might be the Basque people, whose language and genetic groupings predate the arrival of IndoEuropeans. However, no DNA studies published to date have revealed any greater percentage of Neanderthal DNA among Basques than among other Europeans.

The pattern of decline, retreat, and loss of Neanderthal clans suggest that they suffered a massive systemic shock from which they never recovered. The genetic bottleneck believed to have happened around 48,000 years ago may represent a turning point in Neanderthal cultural experience. Climactic studies suggest that the northern latitudes experienced severe shifts in temperatures over several thousand years around this time. The region became colder and then warmed, colder and then warmed, etc. The cycle may have proved too severe for the Neanderthals to adapt to.

But should Homo sapiens have been any better at adapting to these changes in climate? Is it possible that, instead of adapting, they were simply better able to replace dying fringe populations through migrations from warmer lands? The Neanderthals by this time had only one warm refuge left to them in Iberia (modern Spain and Portugal). The Neanderthal population rose again after the bottleneck but it preserved less genetic diversity than the pre-bottleneck population. The regional model for population expansion and contraction would explain this quite easily.

Hence, as Homo sapiens entered Europe the climate changed drastically, probably killing off groups of Neanderthals and Sapiens indiscriminantly. But as the Sapiens had more regions to expand from they were better positioned to reclaim the opened lands of eastern Europe and western Asia. Using this model as a guide, we can argue that the Neanderthals and Denisovans may not have been fighting with Homo sapiens so much as surviving in fewer and fewer refuges. Homo sapiens may not have been competing directly with Neanderthals and Denisovans so much as expanding in to recently deoccupied regions and assuming their ecological niches.

This drama of expansion and contraction must have played out over the course of at least 100,000 years, starting well before the Toba Event. Homo sapiens expanded from southern Africa to the Middle East and southern Mediterranean region by about 100,000 BCE. But then something happened, a shift in climate or a shock to the ecosystem, and they retreated through loss of habitat. By the time of the Toba Event Homo sapiens was vulnerable enough that their population was almost completely wiped out.

But the Toba Event must have affected Neanderthals and Denisovans as well. Neanderthals, however, were already better adapted to living in colder climates. They were able to preserve and replenish their population by expanding into warmer lands ceded by the Homo sapiens groups. But when another climactic change occurred 20,000 years later it was Homo sapiens that expanded more rapidly, claiming the lost lands. Thus, Neanderthals (and Denisovans) simply failed to recover their numbers fast enough.

The intermingling could have resulted from conflict, absorption of lone individuals, and trade between relatively isolated groups. Over periods of thousands of years there would have been fewer and fewer “pure” Neanderthals and Denisovans to absorb into survivor groups. The pendulum could have swung the other way at any time, but it did not. So maybe random chance and mathematical laws of distribution and percolation had more to do with the decline of Neanderthals than anything else. They were equipped with the tools to survive alongside Homo sapiens but nature devastated too many of their regional populations while sparing a greater number of Homo sapiens‘ regional populations.