Scientists Now Have Better Idea of When Modern Humans Met Neanderthals

Somewhere around 69-77,000 years ago a supervolcano centered on what is now Lake Toba in northern Sumatra, Indonesia (just to the south of Thailand in Southeast Asia) exploded, clouding the Earth with a large amount of volcanic ash. We call this the Toba Event. Science journalist Ann Gibbons proposed in 1993 that this massive volcanic eruption may have triggered a Volcanic Winter that lasted for a number of years, reducing the global human population. Subsequent research by scientists supported Gibbons’ hypothesis that the Toba Event may explain a mysterious DNA bottleneck in modern human history, when (scientists believe) as few as 10,000 people may have survived to pass on their genetic material to the world’s current 7 billion+ inhabitants.

We now know, based on yet further investigations, that populations of hominids survived in southern Africa (Homo sapiens, aka modern humans), Europe (Neanderthals), Asia (Homo denisova, aka “Denisovans”, and Homo neanderthalensis, aka Neanderthals), southern India (possibly modern Humans), and Homo floresiensis (aka “Flores man”, or “hobbits”) on the island of Flores in Indonesia. Although early supporters of the hypothesis believed that the Toba Event may have denuded much of the Earth of flora and fauna, recent estimates suggest that severe deforestation occurred in southern Asia and that the seas near Indonesia were covered with a thick layer of ash but more distant regions probably did not suffer as much.

The last glacial period (usually identified as an “ice age” by most people, although the true ice age continues even today) began around 110,000 years years ago and lasted until about 10,500 years ago. The Toba Event did not (as some people have suggested) initiate the glacial period but it must have certainly exacerbated it. What we can be certain of is that the Toba Event initiated a significant environmental change across much of the Earth. And we also know that environmental changes usually lead to new population movements among hominids and other animals.

The close proximity of the Toba Event to what many scientists call the “last Out of Africa migration”, when the ancestors of all modern non-African humans left southern Africa and spread across the globe, has been used to support the idea of a genetic bottleneck; however, some people continue to question whether such a bottleneck really occurred, or alternatively argue that it may not have been as severe as proposed. But what makes this all so very interesting (again) is the recent discovery that modern Humans may have interbred with Neanderthals about 50,000 years ago in Asia.

A new DNA study has enabled scientists to define a more precise comingling point in the timeline by using recently acquired data from the remains of a modern human who lived about 45,000 years ago. We already know that many modern humans descended from Europeans and Asians carry Neanderthal DNA. Based on past DNA studies scientists could only estimate that humans and Neanderthals intermingled somewhere between 37,000 and 86,000 years ago (a period spanning about 50,000 years). Now they have narrowed the time frame (for modern humans and Neanderthals in Asia) to about 10,000 years lasting from around 60,000 years ago to 50,000 years ago. That is within 9-to-27,000 years of the Toba Event.

This new study raises some questions about the Toba hypothesis. For example, did modern humans survive farther north? And when did modern humans first leave Africa? And in just how many places did modern humans intermingle with Neanderthals and other long-lost human cousins? Denisovan DNA has been found in Melanasian (Pacific Islander) population in New Guinea and in Native Americans. The Melanesians left the mainland around 45,000 years ago; Native Americans began arriving in North America 20-25,000 years ago. The only known Denisovan DNA has been dated to about 40,000 years ago (in a tooth from a girl who died between ages 5 and 7).

Current thinking holds that all three known major groups of near-modern hominids (Homo denisova, Homo neanderthalensis, and Homo sapiens) all descended from a common predecessor whom we call Homo heidelbergensis (aka “Heidelberg Man”). We believe that one or more groups of Homo heidelbergensis left Africa about 500,000 years ago. By some estimates the Homo heidelbergensis remaining in Africa had become Home sapiens by 130-240,000 years ago. However, some controversial estimates push modern humans back to about 400,000 years ago.

400,000 years ago is an interesting point in time because research suggests that sea levels then were much higher (about 70 feet, or 21 meters) than they are today. Today’s Earth climate is warming up gradually but the Earth was much, warmer back then. More importantly, it would have been more difficult for hominid groups to move around the planet because fewer land bridges would have existed. This period of intense warmth may have created sufficient isolation for at least three groups of Homo heidelbergensis to evolve into their more modern descendants. And there is no reason to rule out the possibility that other “cousin” hominids may have evolved during that time, though their lines have failed to survive to modern times.

The emergence of three major groups of hominids implies not only genetic changes but possibly also cultural changes. If hominids possessed the ability to speak at this time they may have had substantially different languages, too. We now know that chimpanzees and gorillas possess culture (and chimps wage war on each other), which we can describe as shared behavorial learning. We believe humans’ ancestors broke away from chimpanzees (our nearest relatives among the great apes) about 6-7 million years ago, although new research suggests the split may go back 13 million years. Our definitions of culture and “instinctive behavior” may have to be thoroughly revised in short order because human-like culture (including territoriality and organized warfare) extends back millions of years.

Based on these bits and pieces gleaned from archaeology, genetic studies, and geology we can conclude that there probably remain significant gaps in our knowledge of the hominid timeline. Hence, we don’t know how many times and places modern humans mingled with either Denisovans or Neanderthals. Furthermore, we don’t know how widely dispersed Neanderthals were; we are sure there were at least two major groups, but we may expand that tree further depending on what turns up next in the fossil and DNA records.

For now we have begun to form a picture of a post-Toba Earth that is not quite as dire as originally believed, but that 10-25,000 year time period may have witnessed significant social upheavals and migrations among various hominid groups. We cannot speak of tribes, kingdoms, and empires but we can absolutely envision warfare among those primitive groups. One scenario has Neanderthals spreading east from Europe and modern humans spreading northeast from Africa. The Denisovans may have been squeezed out by the two expanding populations. And then the Toba Event occurred, sending more modern humans north into central Asia.

We now believe that modern humans lived in China 100,000 years ago and that modern humans probably left Africa 130,000 years ago, possibly even venturing into Europe at that time. We also believe there were violent clashes between modern humans and Neanderthals up to 75,000 years ago (although we don’t know why). So either we are missing evidence that shows modern humans are descended from those more ancient migrations, or something very drastic happened after Toba, leading to a near-complete replacement of other human populations by the post-Toba migration.

Such an expansion had to be cultural, not merely genetic. In fact, we have previously considered the possibility that the post-Toba African male descendants of “Adam” (the last common male ancestor of all modern humans, who lived around 60,000 years ago) may have replaced about 90% of all the male lines of modern human populations between 80,000 and 60,000 years ago. The 60,000-year mark continually recurs in all these different analyses; either we are unnecessarily biased toward it (possibly through too little information) or there is a world-wide record hidden in scattered traces of a massive cultural upheaval that led to the decimation of ancient populations.

The emergence of a highly aggressive warrior culture in Africa that expanded outward from Africa could explain many things, such as why human civilization arose only after the last glacial period. If previous, more ancient cultures were not as warlike as the post-Toba Africans they may have been able to co-exist for longer periods of time, and such relative peaceful co-existence may have been based on isolation rather than interaction. But if there is one thing we have learned from 5,000 years of human warfare and history it is that once the wars begin populations and cultures mix at a very rapid clip. The post-Toba modern humans may have gradually outcompeted or overwhelmed every predecessor population they encountered, killing off the males and taking the females.

And so by the time they began interacting with Neanderthals they would have continued that practice: killing the males and taking the females as breeding partners. There are several possible scenarios that lend themselves to this kind of result. For example, if older, more powerful males controlled access to females the younger men would have been forced to seek elsewhere for mates. They would have gradually spread through all human population centers, generation upon generation. Another possible scenario is that the male-to-female birth ratio could have favored males (or infanticide could have led to a significant reduction in girls). Whether due to polygyny, female infanticide, or an unnaturally high male birthrate, there is plentiful evidence to support an expansion of post-Toba modern humans from Africa through warfare by young males seeking mates. And we have strong evidence for this from recent European prehistory, where the males from agricultural families that entered Europe about 6,000 years ago supplanted the males from hunter-gatherer societies.

The expansion-by-conquest model only works if the expanding male lines enjoy a technological advantage over the older populations, either in weapon-making or food acquisition. Agriculturalists have an obvious advantage, so we must look for innovations in weapon-making. But animal husbandry almost certainly plays a role in this as well. For example, the introduction of horses into warfare brought about strategic advantages for successive human cultures. Looking back 50,000 years instead of 5,000 years we can still envision modern humans seizing the advantage over more ancient populations that were not expecting invasions. The expansions would have been both generational and exponential, unless the core populations ceased to experience surpluses of males.

These were hunter-gatherer cultures, to be sure. Their territoriality would have meant they needed to retain some excess male populations for both hunting and defense. It may be that Dunbar’s Number helps us set a practical limit on rates of expansion. If the local hunter-gatherer groups could stay together until there were about 150 people living in a hunting zone they may have been able to go for 2-3 generations before needing to split up. If those who departed were mostly males they could have merged with males from other nearby groups to form temporary warbands. Hence, the arrival of 30-40 young men in an older clan’s territory could have led to a brief, intense conflict in which some of the invading males died and the survivors took the eligible women from the local group as mates.

A practice that may have been born of necessity while the world was colder and harsher (due to the Toba explosion) could have become a culturally embedded tradition. Early survivor groups may have decided to kill off daughters because boys could grow up to be hunters; or it may be that more girl-children died due to hardship than boy-children. The boys became more competitive and aggressive and eventually had to be sent away to start their own clans. Populations would eventually have stabilized as the environment improved and as animal husbandry and agriculture were adopted.

But it could be that an emerging warrior culture drove exactly the kind of innovation that was needed to produce civilization. The slow rate of innovation from 60,000 BCE to 10,000 BCE may be explainable by the fact that the glacial period made it too cold for agriculture and animal husbandry to develop. Hunter-gatherers may have been forced to migrate just to find new food sources. But the 1200-year-cycle of climactic change may also have regulated expansion of the post-Toba hunter-gatherer groups. They passed on their warrior culture to future generations but only sent out colonists when they could no longer support large groups of males. Times of dearth when the animals left traditional hunting grounds would be natural trigger events.

From 50,000 BCE to 10,000 BCE there may have been about 33 climactic upheavals. If during each upheaval the strongest clans subdivided and sent out groups of young men in primitive Volkswanderung events they could have spread across Europe and Asia at about the pace that DNA and archaeology suggest. Evidence for the 1200-year-cycle also exists in mitochondrial DNA (the female DNA line), according to this report from 1997. The suggestion that mtDNA may evolve rapidly every 1200 years may only be coincidental; after all, if the 1200 years is on average and does not coincide with climate change then it’s not helpful for the climate-driven hypothesis. Geneticists compute a Y-chromosome mutation occurs on average every 125 years but as you expand the number of male lines of descent the rate of appearances of mutations within a population becomes more frequent.

Some of the questions geneticists have about how the common male and female ancestral lines don’t match up very well lead to speculation about polygamy (specifically polygyny). But there is more recent evidence that not all modern males are descended from the so-called “Adam” who lived about 60,000 years ago. The survival of a non-Adamic male line in African-Americans suggests that the Adamic line was not as completely successful at displacing other male lines in Africa as we have supposed. And thus we have to consider whether the aggressive culture postulated above was really necessary for the loss of other male-lines in Europe and Asia (especially among Denisovans and Neanderthals).

Without warfare to explain how Adam’s descendants were so evolutionarily successful we must look for other possible explanations, including disease, starvation, and other climactic disasters. But even if we assume that random populations vanished due to flood, fire, and famine to the extent that Adamic groups were able to move in unopposed we still have evidence of warfare. If the expansion was not predatory it was at least opportunistic. Hence, when modern humans met Neanderthals (and Denisovans) the odds were stacked against the older “native” populations, even though we can’t really explain why.