A provocative study claims every living human has an ancestral homeland in what’s now Botswana, and that our early ancestors dispersed from that area due to climate change. Dig into the details, however, and there are a few hefty caveats about the researchers’ methods.
There’s a reason Ancestry.com and other genealogy sites are all the rage: As a species, we possess an insatiable curiosity to know where we come from. Our yearning to uncover our roots also fuels many a debate in paleoanthropology, home to multiple and often contradictory models of human evolution and dispersal.
“I think it’s something that fascinates all humans,” said Australian geneticist Vanessa Hayes, speaking at an Oct. 24 press briefing. “If we really want to understand our future, we need to understand our past and who we are.”
Hayes is the senior author of the paper, published in Nature, with the bold title “Human origins in a southern African paleo-wetland and first migrations.” And yes, they went there.
Home Sweet Home
Hayes’ team analyzed more than a thousand mitogenomes — maternally-inherited genetic material — from people living in Southern Africa today, including isolated and rarely sampled populations. Building on their previous research, they created a timeline of when and where one specific maternal lineage, L0, emerged and then split. (That’s L-zero, by the way, not the word “Lo”, in case the typeface is ambiguous.)
“We overlaid that with geographical data, linguistic data, the culture and history of people across Southern Africa,” Hayes said. “Using that, we could pinpoint what we believe is our human homeland.”
As if that wasn’t ambitious enough, the multidisciplinary team’s effort also “describes the first human explorations,” said Hayes. The researchers matched splits, or divergences, in the L0 lineage with periods of climate change that would have encouraged at least a portion of the early human population to spread across the southern expanse of the continent.
Stay At Home Moms
According to the study, the L0 lineage — which Hayes and her co-authors describe as the “oldest maternal human lineage” — emerged about 200,000 years ago in what’s now Botswana.
At the time, the general region, formally known as Makgadikgadi-Okavango, was home to an enormous lake that had started to dry up and fragment into several smaller lakes, creating an expanse of paleo-wetlands.
According to the team’s analysis, these earliest members of the L0 lineage stayed put for about 70,000 years, presumably making the most of a lush and resource-rich environment.
About 130,000 years ago, however, things changed. Using divergences in the L0 lineage seen in modern populations, it appears that a portion of this “founding population,” as Hayes described it, set out to the northeast. About 20,000 years after that, a second group dispersed to the southwest, eventually fanning out along much of the Southern African coast.
These dispersals from the “homeland” region are supported, according to the study, by archaeological and linguistic research that shows how technology and culture moved through the region.
Both dispersals appear to coincide with climate shifts identified by the authors. Specifically, increased humidity led to “green corridors” opening up, first to the northeast and then to the southwest. There’s genetic evidence that animals such as giraffes and zebras used these corridors to move into new territory, so it makes sense that early humans would as well.
All About Eve
There’s no question that the study is significant, and that it has built appreciably on Hayes’ previous work in the area of comparative human genomics, particularly in Africa, which is too often overlooked.
There are, however, caveats that dull its shine.
Let’s start with the team’s decision to look only at mitogenomes. While this maternally-inherited genetic material is valuable, it provides considerably less information than nuclear DNA, which includes the genetic stories of both parents.
“Every Eve has an Adam. This is only half of the story,” says John Kappelman, a paleoanthropologist at the University of Texas at Austin, who was not involved in the new research.
The study did not include ancient DNA (aDNA), which can reveal otherwise hidden details. Recent aDNA research on leprosy, for example, completely revised what we know about the origin of that disease.
The team also focused on L0, but it’s just one of several very early human lineages. While the authors call L0 the oldest of these surviving lines, there is uncertainty that geneticists have fully mapped out all of the ancient lineages. There’s also the question of whether splits within a lineage represent two groups dividing geographically, or if they could possibly mean genetic drift within a single population.
Then there’s the team’s basic assumption, expressed in the opening words of the paper’s abstract: “Anatomically modern humans originated in Africa around 200,000 years ago.”
Well, maybe not.
In and Out of Africa
Consider the fossils out there, some considerably older than 200,000 years, that suggest anatomically modern humans were not only around before the study’s “ancestral homeland” but were also thousands of miles away from this ostensible ground zero of our species.
In 2017, for example, researchers announced the discovery of a trove of fossils, from at least five individuals, in Jebel Irhoud, Morocco, that are about 300,000 years old. The material is considered to be the oldest evidence of our species, though some researchers argue the individuals are pre-anatomically modern humans.
Hayes and her team do not mention the Jebel Irhoud finds in the paper, and, during the press briefing, she appeared dismissive of their relevance.
“The skull in Morocco is Homo sapiens, but its relation to people living today? We don’t know,” she said in response to a reporter’s question.
Hayes also pushed back when asked about other, more recent finds, including the Misliya-1 jawbone from Israel, which may be 194,000 years old — that’s more than 60,000 years older than the first dispersal from the ancestral homeland Hayes’ team identified — and a skull fragment from Greece that may be 215,000 years old. The geneticist suggested it would be “speculation” to attempt to reconcile these and other fossil finds with the timeline her team developed.
And, while most paleoanthropologists hold that anatomically modern humans evolved in Africa, a growing minority advocates for a multiregional model, with archaic members of the genus Homo mixing and mingling from Southern Africa to East Asia for hundreds of thousands of years. It’s another wrinkle in the human origin story ignored in the new paper.
Making It Rain
There’s also an issue with the study’s other major claim: That these early ancestors left the area due to climate change opening up attractive green corridors.
That idea rests on a reconstruction of the specific region’s paleoenvironment at various periods. The data that reconstruction is based on, however, comes from general patterns recorded across the continent.
“What was actually going on in this area, instead of broadly around the area?” Kappelman wonders. “We just don’t have a lot of good climate data from the sites themselves.”
He adds: “If it were me, I’d drill a deep core right in that lake.”
Similar paleoenvironmental reconstruction projects typically take cores of sediment from the actual site. These long, layered cylinders can be read almost like diaries, recording both significant single events and highly localized climate patterns.
Even if the paleo-wetlands dried out and green corridors opened, Kappelman adds, he doesn’t see early hunter-gatherers necessarily leaving home. His own research in East Africa has shown that humans then, as now, were exceptionally good at adapting.
“Anatomically modern humans at this site could have handled any climate conditions,” he says.
A Good Start
While skepticism remains around the new study’s conclusions, the data collected and models produced are in themselves valuable.
“It does represent a capture of evolution in action in these groups,” says Kappelman, adding that the paleoenvironmental reconstruction is also significant. “It’s a testable hypothesis, which is a good thing. It’s a great jumping-off point.”
So basically, if you’re one of those people into genealogy tours, great — but don’t pack your bags for Botswana just yet.