Why Homo Sapiens Took the World Away from Denisovans and Neanderthals
New research from several fields of study may finally have pinpointed the main reasons for why you and I are Homo sapiens and not Homo denisova or Homo neanderthalensis. We now know that modern humans in populations outside of Africa have inherited DNA from both other hominin groups, but we don’t yet know why there were other hominin groups and why they did not survive. But new clues have emerged that tell an intriguing story about human evolution. Let’s take a look at what we can say with certainty.
Homo erectus was the earliest “human” species
Also known as Homo ergaster, these earliest of fully human ancestors first appear in the fossil record about 1.9 million years ago. Their fossils have been dated as recently as 70,000 years ago and they are believed to be the direct ancestors of Homo floresiensis (the “hobbits” of Indonesia), who may have died out around 50-40,000 years ago (a revised estimate from the earlier proposed 12,000 years ago).
Homo erectus was the first human species to migrate out of Africa. We have found evidence of their long-term habitation of Asia. So far there is no evidence that they moved into Europe.
Around 700,000 years ago new human species had diverged from the Homo erectus population in Africa. Homo heidelbergensis appears to have colonized Europe and Asia by about 600,000 years ago, thus living side-by-side in some regions with Homo erectus and possibly other as-yet unknown descendants of the older species.
About 550,000 years ago the last joint populations of the ancestors of both Homo sapiens and Homo neanderthalensis separated in Africa. The latter group migrated north to settle in the Middle East, Europe, and some parts of Asia. Current theory holds that the Denisovans were probably also descendants of this group.
Homo sapiens is believed to have emerged by about 400,000 years ago, although most sources still say 200,000 years ago. It is notable that DNA studies say that all male lineages converge in a single Homo sapiens male who lived about 190,000 years ago.
Everything Changed Around 100,000 Years Ago
Homo sapiens left Africa twice. The first groups of modern humans reached southern Europe and the Middle East around 100,000 years ago. For reasons we don’t know yet (and may never know), these Homo sapiens did not survive into modern human populations. Speculation suggests they may not have been equipped to compete with other hominin groups they encountered.
By 100,000 years ago, however, the Neanderthals had become subdivided into several groups. There was a group in Europe (further subdivided), a group in the Middle East and Caucasus, and an Asian group. All three major groups probably consisted of smaller populations that were more closely related to each other than to the other groups.
Homo neanderthalensis became a distinct species about the same time that Homo sapiens did, between 400,000 and 200,000 years ago. Speciation is identified by changes in anatomy and culture but DNA analysis provides more detailed explanations of how the various major groups diverged from each other.
We can only be sure that by 100,000 years ago there were five types of hominin sharing the planet: isolated groups of Homo erectus, Homo floresiensis, Homo denisova, Homo neanderthalensis, and Homo sapiens. The modern humans living outside of Africa either retreated to Africa or died out.
By this time, however, Homo neanderthalensis and Homo denisova had met and intermingled at least a few times in Asia and the Middle East. Both groups had adapted to harsher climates than the Homo sapiens of Africa were accustomed to. At least some Homo denisovan populations had adapted to particularly high altitudes.
We can say, figuratively, that these various populations had achieved type of equilibrium. There was a balance in their distribution across the landscapes.
The Toba Event Destroyed the Balance
Current theory holds that around 69,000 to 77,000 years ago a massive volcanic eruption occurred in Sumatra, Indonesia beside Lake Toba. This volcanic event is believed to have killed off large swaths of life, including most modern humans. One popular estimate suggests that modern humans in Africa may have been reduced to about 10,000 survivors from whom all modern humans are descended.
It may be no coincidence that some studies suggest the populations of Neanderthals, Denisovans, Homo erectus, and Homo floresiensis all began to dwindle or died out around this time. If the Toba Event had the ecological impact that researchers suggest it did, then a huge ecological gap appeared across the Eurasian and African continents.
Modern humans began to migrate out of Africa within a few thousand years of the Toba Event. Studies suggest that modern humans began meeting (and perhaps interbreeding) with Neanderthals and Denisovans by 60-50,000 years ago. Both hominin groups appear to have vanished by about 40,000 years ago.
New Evidence Suggests What Happened
Around 55,000 years ago, DNA researchers say, male populations in modern humans “exploded”, leading to massive migrations and possibly changes in technologies. We don’t know why there were suddenly so many males but they would have gone looking for mates. Apparently they found those mates among Neanderthals and Denisovans. Subsequent “explosions” in the male population appear to line up with periods of migration or, in historical times, massive warfare and upheaval.
Another study suggests that no male Neanderthal lineages have survived into modern human populations. This is not definitive but the scientists theorize that Homo sapiens women may have been genetically predisposed to reject Neanderthal DNA. This is a common situation when two closely related species attempt to interbreed. Apparently, Homo neanderthalensis women were more genetically compatible with Homo sapiens men; or else there were fewer opportunities for the Neanderthal males to breed with Sapiens women.
One other factor may have influenced the way hominin interbreeding played out. Homo neanderthalensis had larger hips and chests, and it could be that Homo sapiens women could not carry children of Neanderthal fathers to term, or died giving birth.
Denisovans lived throughout eastern Asia but their DNA has survived most in southeast Asia and Melanesia, primarily Papua New Guinea. Tibetans also appear to have inherited Denisovan DNA that helps them live in high altitudes.
The Neanderthals are also believed to have declined around the time that the large mammals they hunted declined. If mega fauna were also harmed by the Toba Event, then it makes sense that Homo sapiens would be able to push other hominin groups aside as their normal food resources dwindled.
All of modern civilization might exist only because modern humans were able to expand into territories previously held by other species of humans, and through that expansion create complex cultural interactions that eventually led to the rise of cities. And so we may have enough evidence to explain why civilization arose in the Holocene epoch rather than the Eemian epoch. The right ecological conditions had to occur to allow rapid human expansion across the globe. That expansion may have set the pace for population growth needed to incubate the innovations that would eventually lead to agriculture, animal husbandry, and large communities.