By now, the big picture of humanity’s origins is quite clear. Modern humans evolved in Africa more than 100,000 years ago, but it took them tens of thousands of years to leave the continent. Once they did, they quickly spread across Asia and Australia, mating with some of the pre-modern (read: archaic) humans along the way.
But that big picture, often referred to as “Out of Africa,” is missing some details. First, our understanding of genetic diversity in Africa is surprisingly poor — so poor that we’ve missed a completely distinct African Y-chromosome lineage for decades. And there have been countless debates about the number of times humanity has left Africa. Was it one big migration or did we leave in waves?
A new batch of articles has significantly narrowed the scope of the controversy. While there may have been more than one push out of Africa, in the end only one really mattered.
The archaeological evidence for “Out of Africa” is pretty spotty. Modern humans first appear in the Middle East about 100,000 years ago, and there are a few intriguing skeletons and some advanced tools found even further into Asia. But clear evidence of occupation of the rest of Asia does not appear until tens of thousands of years later, at which time humanity invaded everywhere from Europe to Australia.
There has been much debate as to whether this evidence points to a gradual expansion followed by a major push or simply an immature migration that was held up near Africa, awaiting the technological or genetic changes that enabled expansion.
To sort out these possibilities, we can either wait for archaeological finds that may never come, or try to understand what happened using the genetic legacy these events left in modern populations. To do the latter, we need a better understanding of what exactly that legacy is. That’s what the series of newspapers is in Nature come in, because they give a much better picture of the modern population, and use that information to deduce what happened in the past.
One of these articles takes a detailed look at the indigenous people of Australia and New Guinea. The ancestors of these populations appear to have migrated along the southern coasts of Asia, arriving in the area more than 40,000 years ago. This migration led to the suggestion that the ancestors represented a distinct migration outside of Africa.
But the new data spans more than 100 genomes from the modern lineages of these populations, and it indicates that the lineage is clearly part of the same group that expanded into Eurasia. However, the group diverged from Indo-Asians more than 50,000 years ago, shortly after migration from Africa began. It may be that those following the southern coastal route remained isolated from the rest of the expansion into Asia. As the authors of one paper put it, “the data suggest an early divergence of Australo-Papuans from the ancestry of all non-Africans, consistent with two waves of colonization in Asia.”
Is there evidence of an earlier wave of modern humans? The article described above seemed to find one in its initial analysis. But when the authors took into account the presence of Neanderthal and Denisovan DNA, this hint disappeared. A second paper, based on fewer Aborigines and more genomes from populations across Eurasia, found an indication that as many as two percent of Aboriginal genomes could come from a modern human lineage that split from the African population about 100,000 years ago . This discovery may be evidence of an earlier migration to Asia, but we’ll probably have to wait for more focused analysis to sort out the data.
Meanwhile, another new paper looks at Africa itself. Here, some of the most genetically diverse (and thus ancient) human populations seem to have separated from other African populations and non-Africans more than 80,000 years ago. Another major split within the African population seems to have occurred just before the out-of-Africa migrations. So while the main focus of the study was on non-African populations, just as much (if not more) seems to have happened in Africa.
It is important to note that this information does not exclude previous migrations from Africa. It is possible that earlier genera also spread through Eurasia. The data does indicate that there was one migration that really mattered, about 50,000 years ago. The older populations, if they existed at all, died out of their own accord or were driven out by these new arrivals.
Relevant to that matter, one of the articles contains no genomes at all; instead, it relies on climate models. Two researchers from the University of Hawaii at Manoa have linked a model of human migrations to climate models that provide measures of things like water availability and plant life in past climates. They started these models with conditions from 125,000 years ago, placing a human population in East Africa.
The results indicate that humans were trapped in Africa for at least 25,000 years, but that climatic conditions would allow for multiple migration pulses from about 100,000 years ago. The problem is that these simulations also show that modern humans populated southern Europe 80,000 years ago, and that they could have been present in India and Southeast Asia as early as 90,000 years ago. There is no evidence, either archaeological or genetic, that any of these things happened.
Still, the models match some of the earliest signs of the presence of modern humans in the Middle East and Asia. But the genetic data now indicates that the legacy of such populations is slight or nonexistent. When it comes to leaving Africa, we are all one big family.
Nature2016. DOI: 10.1038/nature19792, 10.1038/nature18299, 10.1038/nature18964, 10.1038/nature19365 (About DOIs).