Human Migration: Challenging the Chronology of Our First Road Trip

Dead Things iconDead ThingsBy Gemma TarlachSep 21, 2016 9:00 PM


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An unprecedented amount of high-quality genetic data, released today in three separate studies, sharpens our understanding of when the first of our kind left Africa. Credit: Our species evolved to be great distance walkers, with big brains and plenty of curiosity. Is it any wonder we were made to wander? And today, a trio of studies in Nature provides tantalizing new data about when we started exploring our world and expanding our horizons in an intercontinental kind of way. Thanks to genetic data from previously underrepresented populations, we've got a clearer yet more complex picture of human migration out of Africa. For decades, the dominant theory about human migration out of Africa was a single wave anywhere from 40,000 to 80,000 years ago. But in recent years, a steady stream of discoveries, both archaeological and genomic, has chipped away at that idea and hinted at not only an earlier departure, but also more than one wave. Human remains in caves in the Levant, for example, are up to 115,000 years old. Many researchers have suggested those remains point to a small wave of wanderers who reached the Middle East but died out, which is certainly possible. Today's hat trick of new studies don't offer any definitive answers, but they do provide ample new clues about when humans hit the road and headed out of the homeland, possibly as early as 120,000 years ago, with some of those early explorers making it as far as Australia. I won't lie: the papers out today are complex, even for genomic studies, in part because the researchers included an unprecedented number of genomes from different populations, many of which had not been studied before. The easiest way to get a handle on the new info is to take them one at a time.

#1: Data Down Under

Aboriginal Australians have long been known as one of the oldest surviving distinct populations of our species. But, for a number of reasons, including cultural sensitivities, their genome has been seriously understudied. Led by Danish evolutionary geneticist wunderkind Eske Willerslev, researchers have, for the first time, published a comprehensive study of the genetic diversity of these native peoples. Willerslev's team sequenced genomes from 83 Aboriginal Australians and 25 highland people from nearby Papua New Guinea. They found that, among the Australians in the study, the genetic diversity was on par with that of Europeans. In other words: a lot of diversity within the population, which you'll find when a group is well-established in a region for a long time, expanding territory as it expands the amount of genetic variation among its people.

Aubrey Lynch, an elder from the Wongatha-speaking people, participated in a new genetic study of Aboriginal Australian populations. Credit: Preben Hjort, Mayday Film. The results of their analysis suggest that all non-Africans are descended from a single wave of migration out of Africa, but that those pioneers split into two branches soon after leaving the homeland. One bunch settled in Eurasia and the other continued east until finding Australasia to their liking.

#2: Bigger Is Better

In the second big paper, Harvard geneticist David Reich and colleagues looked at 300 genomes from 142 ethnic groups from around the world, many of which had not been included in large-scale studies of the past. It's the largest such dataset ever compiled, and is open-access for any researcher who wants to crunch her own numbers from it. One of the curious findings of the paper: our ancestors, the population that led to every human on the planet, started to diversify into distinct groups 200,000 years ago or more. That's earlier than many researchers figured. Another intrigue: Researchers pay close attention to the rate of mutations in any given population because it helps them establish a timeline for when different groups split from each other; this team found that the mutation rate in non-Africans accelerated after the groups split, and was about 5 percent faster than the mutation rate of Africans. While there is no definitive reason for this uptick yet, the researchers theorize it may be due to a shortened generational span (babies having babies! What! Did they not watch Teen Mom on MTV?!). Bonus: Reich and company's dataset adds a ginormous* amount of variants to the human genome sequence published at the dawn of the 21st century. (*Not actual scientific term.) Even if you don't care about the history of the human species, that new information about genetic variations specific to certain populations could affect you. It means researchers will be able to develop better tests and treatments for a range of conditions that are known to be linked to different genes in different populations. For example, a 2015 study identified particular genetic signatures for depression in Han Chinese women. But a crowdsourced study of European-descended people, released this August, found different genes linked to the mental illness.

New genetic data helped authors of one of today's Nature papers build a more accurate family tree of the genus Homo and its nearest relative, the chimpanzee. Though simplified (The Australopithecus genus and other pre-Homo groups are lumped into one bubble of hominins), the model shows how some groups interbred while others were isolated. Sidenote: Bubble of Hominins is an excellent name for a band. Credit: Swapan Mallick, Mark Lipson and David Reich.

#3: The Earliest Explorers?

The third paper out today, which added nearly 400 new genomes to an existing dataset, discovered something curious about Papuans: at least 2 percent of the modern Papuan genome comes from a population other than the one that left Africa to set up shop in Eurasia. The authors conclude that it points to an early wave of human migration out of Africa roughly 120,000 years ago which ultimately reached, genetically speaking, today's Papua New Guinea. It might sound like papers 1 and 2 are down with the single out-of-Africa expansion and paper 3 says no way, there were at least two waves. But actually, the first two papers don't completely rule out earlier waves of migration (most of which undoubtedly did not go well, resulting in a localized extinction of that particular group of rovers).

So What?

Okay, so maybe you're not a genome geek. That's fine. And even if you are, right about now you may be getting dizzy from all the new genomic data stuffed into these studies: nearly 800 brand-new, high-quality genomes from more than 270 populations. That's right, it's quality, not just quantity, that make today's trio of studies important. Sequencing a genome is getting easier and cheaper all the time, but it's still not rocket science: It's arguably tougher. To have so much more of our species' diversity represented in the scientific literature gives medical researchers more opportunities to identify population-specific genetic causes for conditions and develop tools to treat them more precisely. Having a clearer picture of today's genetic diversity will also help us identify ancient episodes of interbreeding with archaic humans such as the Neanderthals and Denisovans and possibly other, as yet unidentified, kissing cousins on the Homo family tree.

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