The evolutionary history of humans is a murky one that’s full of gaps, but now researchers from Australia and South Africa claim to have plugged one of these plot holes. Combining studies of genetics, geology and climate history, the scientists say they’ve identified what could be called the “homeland” of modern humans – a vast, ancient wetland system in southern Africa.
Generally speaking, we have the broad strokes of our history worked out. Anatomically-modern humans arose in Africa between 300,000 and 200,000 years ago, before spreading into Eurasia and eventually all around the world. But exactly where and when we first appeared is a matter of debate.
Although fossil evidence remains a crucial part of how we piece the story together, it isn’t the only way. Many gaps can be filled in with DNA studies, and in particular mitochondrial DNA (mtDNA). This is the information stored in the mitochondria, the structure that generates energy for cells. Unlike regular (or nuclear) DNA, mtDNA evolves extremely slowly, meaning it’s great at preserving ancient lineages.
“Mitochondrial DNA acts like a time capsule of our ancestral mothers, accumulating changes slowly over generations,” says Vanessa Hayes, lead author of the study. “Comparing the complete DNA code, or mitogenome, from different individuals provides information on how closely they are related.”
So the researchers on this study set out to create a more comprehensive catalog of the earliest human mitogenomes. In particular, they focused on the L0 lineage, which is regarded as more or less the “trunk” of the modern human family tree.
Today, the L0 lineage is known to be most common in sub-Saharan African populations, and is much rarer in the rest of the world. So the researchers on the new study collected blood samples from 1,000 participants in Namibia and South Africa, and mapped the L0 lineage to track down our ancestral homeland.
“We merged 198 new, rare mitogenomes to the current database of modern human’s earliest known population, the L0 lineage,” says Eva Chan, first author of the study. “This allowed us to refine the evolutionary tree of our earliest ancestral branches better than ever before.”
Using this new information about the timeline of the L0 lineage, as well as how different sub-lineages were spread out, the researchers were able to trace where the earliest maternal lineage of modern Homo Sapiens emerged, about 200,000 years ago. This homeland lies in an area south of the Zambezi River, encompassing parts of what’s now known as Botswana, Namibia and Zimbabwe.
Nowadays this region doesn’t look very inviting – it’s mostly covered in desert and salt pans. But hundreds of thousands of years ago, this spot was home to Africa’s largest-ever lake system, known as Lake Makgadikgadi.
“Prior to modern human emergence, the lake had begun to drain due to shifts in underlying tectonic plates,” says Andy Moore, a geologist and author of the study. “This would have created a vast wetland, which is known to be one of the most productive ecosystems for sustaining life.”
The study shows that this oasis provided a perfect home to early humans for more than 70,000 years. After that, the genetic timeline suggests that two major migrations occurred.
“We observed significant genetic divergence in the modern humans’ earliest maternal sub-lineages, that indicates our ancestors migrated out of the homeland between 130,000 and 110,000 years ago,” says Hayes. “The first migrants ventured northeast, followed by a second wave of migrants who traveled southwest. A third population remained in the homeland until today.”
So why did our ancestors leave this lush homeland? To find out, the researchers analyzed climate simulations and geological data of southern Africa over the last 250,000 years.
“Our simulations suggest that the slow wobble of Earth’s axis changes summer solar radiation in the Southern Hemisphere, leading to periodic shifts in rainfall across southern Africa,” says Axel Timmermann, co-corresponding author of the study. “These shifts in climate would have opened green, vegetated corridors, first 130,000 years ago to the northeast, and then around 110,000 years ago to the southwest, allowing our earliest ancestors to migrate away from the homeland for the first time.”
While it’s certainly a fascinating story, and it sounds neatly self-contained, it doesn't necessarily fit into the wider context of human evolution and migration that previous studies have pieced together. Fossils attributed to modern humans have been found in Morocco from 300,000 years ago, in Greece from 210,000 years ago, and in Israel from 177,000 years ago. That suggests our ancestors were widespread across Africa, Europe and the Middle East at the time (and long before) this study claims modern humans were isolated in one wetland area in southern Africa.
Tracing mtDNA back down the oldest lineages is a useful tool to apply to the puzzle of human history, but it's just one of many – and perhaps it's not wise to draw conclusions as huge as the "homeland of all humans" from this one piece.
The research was published in the journal Nature.