Searching for the homeland of the modern human species has been a fundamental quest in science and culture, particularly because we all carry genetic echoes from our original fathers and mothers.
Travel back in time far enough by poring over fossils and peering at genetic ancestry and the signs say all modern humans roaming the Earth came from Africa.
But Africa is an enormous continent featuring an overwhelming scope of geographical and cultural diversity. Today, anywhere between 1,000 to 2,000 languages stretch from the Saharan desert across the mountain rainforests to the savannah grasslands.
So, where exactly did our modern ancestors come from, long before they spread to the farthest reaches of the world? Northeastern Botswana.
This region of the African nation is the answer that the journal Nature revealed Monday in a study that traced the origins of the deepest maternal lineage known to humans.
The project, conducted by a dozen scientists from three continents, claims that the mother of all modern humans living today — from New Zealand to New York — originated in this region of Africa 200,000 years ago.
“We have known for a long time that modern humans originated in Africa,” Vanessa Hayes, a geneticist at the Garvan Institute of Medical Research who led the study, said at a press briefing last Thursday. “What we hadn’t known until this study was where exactly this homeland was.”
The team combined genetic ancestry tracing with climate modeling to paint a picture of how our ancient mothers must have lived. This northern portion of Botswana — known as the Makgadikgadi basin — now contains arid salt flats, but the study says it was once a lush wetland, filled with enough animals and edible vegetation to sustain our hunter-gatherer ancestors.
This legacy carries on today in the way the Khoisan live. They are a group of hunter-gatherers who still inhabit this region and whose voluntary DNA contributions made the study’s discovery possible.
But before booking a flight to visit this motherland of all motherlands, it’s important to know that not all anthropologists and archaeologists believe this is the cradle of life. The debate has arisen because genetics appear to indicate one thing about human history, but fossils and artifacts point to something completely different.
For nearly a decade, scientists believed our human species — Homo sapiens — came from in Africa because the oldest fossils bearing our resemblance date back 200,000 years. But those specimens — known as the Omo remains — were discovered in Ethiopia, thousands of miles north of the newly proposed homeland in Botswana.
Moreover, the Omo fossils lost their mantle as the oldest known sapiens when a 300,000-year-old skull found even farther away — Morocco — was unveiled in 2017. That’s 100,000 years older than the date scientists have proposed for the Botswana origins.
So how can our DNA paint one picture of our origins, while bones paint another? Well, discovering the origin of modern humans involves more than tracing your maternal heritage.
What the study did
People often speak about “DNA” as though it is a singular entity, but human cells actually contain two types of this genetic material. “Nuclear” DNA, found in a cell’s nucleus, contains the information that defines most of human existence — eye color, hair color, height, mental capacity.
But a small fraction of DNA is housed in another part of your cells, the mitochondria. Mitochondrial DNA allows mitochondria to perform their key task of making energy.
Mitochondrial DNA is also special because you only inherit it from your mother, unlike nuclear DNA which is a mixture from both parents. Mitochondrial DNA can also serve as an ancestry stopwatch because it mutates 10 times faster than nuclear DNA, about once every 8,000 years.
Let’s say our moms are sisters, and yours develops a random mutation in her mitochondrial, while mine doesn’t. Your family’s mitochondrial blueprint will be forever different than mine. That single mutation serves as a fork in the road of our ancestry — one that can now be traced backwards through time (and space if our families move away from each other).
Since the late-1980s, geneticists have known that if you collect enough human samples from enough places in the world, then mitochondrial DNA can be used to trace maternal heritage from today back to the time when we diverged from our most recent common ancestor, which some refer to as Mitochondrial Eve.
Over the years, as more genetic information was collected from more people, this Mitochondrial Eve– whose mitochondrial DNA profile is dubbed the “L0 haplogroup” — has been traced closer and closer to southern Africa, landing squarely among the click-speaking Khoisan people.
But Khoisan remains and archaeological sites stretch across the region — from below the Zambezi river and Okavango Delta in northern Botswana to the southernmost tip of Africa. Over hundreds of years, this area has been influenced by migrants, such as non-click speaking Bantu cultures from central Africa — making the genetic picture murky.
“The first effort [of the study] was to seek out people who do not identify as Khoisan, but could potentially be carrying the oldest L0 haplogroup,” said Eva Chan, a bioinformatics scientist in the Human Comparative and Prostate Cancer Genomics Lab at The Garvan Institute of Medical Research, who co-authored the study.
So, Chan, Hayes and their colleagues collected 198 new DNA samples from southern Africans and compared them with about 1,000 previously collected genetic profiles from the region.
What the study found
Their results show that mitochondrial Eve — which statistically speaking is not one woman, but a group of closely related women — emerged approximately 200,000 years ago in the Makgadikgadi basin.
“There was once a very large lake there… It stretched all the way from Namibia, across Botswana and into Zimbabwe,” Hayes said, adding that it was bigger than Lake Victoria — Africa’s largest lake.
The team proposes that by the time Mitochondrial Eve and her modern human offspring appeared in this region 200,000 years ago, Lake Makgadikgadi would have started breaking up into smaller bodies of water, creating a wetland.
Based on their genetic data, the first families stayed in this region for 70,000 years.
“Then suddenly around 130,000 years ago, we see divergence happening, or splitting up the lineages,” Hayes said. One group moved northeast, based on genetic analysis. A second branch of the original L0 families didn’t move south until 110,000 years ago.
To better explain why these migrations occurred, the research team created climate models, based partly on nearby ocean sediment cores that contain clues about ancient vegetation and rainfall.
“By comparing the climatic data with timelines of the genetic divergences, we found a striking pattern,” said Axel Timmermann, director of the Center for Climate Physics at Pusan National University in South Korea. “More rainfall around 130,000 years ago, northeast of the Makgadikgadi homeland created a green corridor — a vegetated corridor — for migration for the first group.”
Before this green corridor opened, the Makgadikgadi basin appeared to be surrounded by arid, inhospitable land with little wildlife, based on ancestral records for lions, giraffe and zebra. A second corridor opened 110,000 years ago to the southwest, allowing the later migration. Unlike the climate change seen today, which is largely human-made, those ancient patterns were due to a shift in the Earth’s axis and orbit that happens every few thousand years.
“In particular, the Southern Hemisphere summer is moving closer to the sun, so it receives more sunlight,” Timmermann said, speaking about the researchers’ climate model simulations. “It’s warming up and is also having more moisture.”
Why this matters
Anyone familiar with human genetics and fossil records knows that this new study does not close the case on our ancestral homeland.
Remember, Mitochondrial DNA makes up a tiny fraction — 0.0005 percent — of the human genetic code.
“The deepest roots of the mitochondrial DNA doesn’t really tell us the location of the deepest root of the thousands of other genes in the nucleus that are also unique to people today,” said Rick Potts, head of the Smithsonian Institution’s Human Origins Program.
Take, for example, a gene called EPAS1. Certain mutations in this gene allow Tibetans to live at high altitude, but they inherited this super-athlete trait from Denisovans. Denisovans are like Neanderthals, an extinct group of beings who are distinct from Homo sapiens but could mate with them all the same.
Here’s the thing, our common ancestor with Denisovans and Neanderthals split from our sapien forebearers and left Africa, along with other humanlike hominid species, hundreds of thousands of years before Mitochondrial Eve sprouted in the Makgadikgadi basin.
Pott said this new research does a tremendous job of tracing the maternal origins of all modern humans walking the Earth today — the mothers of all mothers.
“We all have that as part of our heritage — a strictly African story,” Potts said. “When people from Europe or North America use 23andMe or Ancestry.com, they will get results that are not L0 mitochondrial DNA, but they’re derived from L0… originally derived from African history.”
But he said the study’s main takeaway overlooks all other contributions to heritage, including some from ancient Africa.
If, for instance, a mother had only sons, then her DNA would be passed on in later humans, but that would not “register in the mitochondria,” Potts said. “So there may have been quite a number of mothers who contributed from all over Africa to modern day human genetic diversity, the genomes of modern humans and to what it means to be human.”
Potts also questions the “green corridor” requirement for these early migrations, given modern humans can live in hellishly difficult places– from the frigid poles to the scorching desert. Even the modern Khoisan survive in the less-than-hospitable terrain of the Kalahari desert.
“A number of us who study paleoenvironment and human evolution are moving away from the idea that humans had to stick with some idyllic corridor environment in order to move around,” Potts said. And also recalled that the fossil record, which includes older, anatomically sapien bones from Morocco and Ethiopia, do not square with this genetically determined Makgadikgadi homeland.
Hayes pushed back this rationale, saying DNA still needs to be extracted from the Morocco skull to figure out where it fits into the story of modern humans.
“We really wanted to define the founder population,” said Hayes, who also heads the Garvan Institute’s unit for human comparative and prostate cancer genomics. “Until we get DNA from these skeletons and find more skeletons…it’s very hard for us to speculate.”
But those fossils may no longer exist, given Africa has witnessed a constant stream of climatic, geological and architectural change.
Known fossilized bones support the idea that all humans, whether sapien, Neanderthal, Denisovan or otherwise — came from Africa, but they suggest that the emergence of our modern species happened all over the continent and in different ways. For example, his lab reported last year the oldest known evidence in Kenya of long distance trading, precise stone tools and primitive crayons, dates back 305,000 years.
Potts wonders if the genetic picture — once more ancient DNA is uncovered — will say the same thing as the artifact record. Until then, he compares the Makgadikgadi study to the parable of blind men trying to comprehend an elephant.
“That’s one of the great things about a paper like this: It contributes one part of the elephant. But it doesn’t give us the whole thing,” Potts said.