By Eric Vandenbroeck

Was Eden perhaps in Botswana?

We first referred to Botswana following a discovery in November 2006 which we took as an opportunity to present an overview of human evolution. This was followed by an overview of the Genographic project (started in 2005) including other sources available at that time thus presenting a peopling of the earth world-history.

As new discoveries have accumulated it is worthwhile to revisit this subject. For example, in a study released in the scientific journal Nature on 28, Oct. 2019 scientists once more referring to Bostwana now claim they have traced the homeland for all modern humans to a group of early humans in a region of northern Botswana, south of the Zambezi River.

The group remained in the region until a shift in the climate, driven by changes in the Earth’s tilt and orbit, brought rains to the north-east and south-west, producing lush green corridors that allowed the early humans to spread into new territories, the scientists say. And although referred to as "controversial" the findings are worth a closer look. In fact Rebecca Cann, a geneticist at the University of Hawaii at Manoa who was a reviewer of the study and has conducted pioneering work on mitochondrial DNA, argues that the new research is innovative, crossing multiple disciplines in search of answers.

Ancient DNA and the history of the Human Past

That Homo sapiens began as an African species was pretty-much proved in the 1980s by Allan Wilson of the University of California, Berkeley. He developed what has come to be known as the Mitochondrial Eve hypothesis by looking at a special type of DNA which is passed, unmixed by sexual reproduction, from a mother to her children. This so-called mitogenome is independent of a cell’s nucleus, where the rest of the genes are found. It resides in structures called mitochondria that are the descendants of once-free-living bacteria and which now act symbiotically as a cell’s power packs.

Wilson’s research showed that the family trees of present-day human mitogenomes, their branches caused by mutations over the millennia, converge in a way that makes clear that their common ancestor lived in Africa. Hence the nickname Mitochondrial Eve. This woman was by no means the first human being. But everyone now alive can claim descent from her.

What is true for Eve is also true for Adam. Part of the DNA on the y-chromosome, which is passed unmixed from father to son, can be used to draw up a similar tree that is also rooted in Africa. Where, exactly, y-chromosomal Adam resided has not yet been established. But as they describe the above referred to article in Nature, a group of researchers led by Vanessa Hayes of the Garvan Institute in Sydney, Australia, think they have found that Mitochondrial Eve, or, at least, people closely related to her, lived for tens of thousands of years in splendid isolation in northern Botswana.

Fossils carrying a varying mix of features from both modern humans and more ancient hominins seem to be scattered across Africa, from the 260,000-year-old Florisbad remains in South Africa and 195,000-year-old Omo remains in Ethiopia to the 315,000-year-old Jebel Irhoud remains in Morocco. But after baking in the African heat, the DNA from these ancient fossils seems to have largely degraded.

That northern Botswana was a habitat of early humans has been known for years. Makgadikgadi is littered with stone tools dropped there eons ago by Palaeolithic hominids. Which particular hominids, however, is not clear. Unlike later artifacts, Palaeolithic tools are not species-specific. Though they were invented about 1.8m years ago by Homo erectus, an early human that spread over Africa and Asia, they were also used by erectus’s numerous daughter species, one line of which leads eventually to Homo sapiens.

Out of Eden

The story that Dr Hayes and her colleagues are proposing is that, whoever might have been living there beforehand, by 200,000 years ago the land around Lake Makgadikgadi was indeed occupied by Homo sapiens. For the following 70,000 years these people evolved in isolation, penned into their homeland by desertlike surroundings. Then, in two bursts, one 130,000 years ago and the other 110,000 years ago, they were unleashed on the wider world.

Mitogenomic ancestral trees are made by looking for typos in the sequences of genetic “letters” in mitogenomes, places in the DNA possible to work out in what order the mutations happened. And because even random processes have measurable averages, it is also possible to estimate when a particular mutation arose.

Follow the branches of the human mitogenomic tree back through time and they converge on a group of mitogenomes known as l0. This group is largely confined to southern Africa. It is the characteristic mitogenome of the Khoesan people, who long predate the arrival in the area of both Bantu from farther north in Africa and Europeans from overseas. Dr Hayes and her colleagues, therefore, gathered all of the existing versions of l0 that they could find, and also collected 198 new ones, to bring together a total of 1,217 variants from which they sought to refine the ancestral tree.

With that information, and data about where the samples were collected, maps of how people who share l0 spread can be constructed. And that is what Dr Hayes and her colleagues did. The branches of their new tree converge in time about 200,000 years ago. In space, they converge on northern Botswana.

The tree also suggests that the l0 population lived in one place for perhaps 70,000 years before part of it moved south-west, and a further period of about 20,000 years before another part moved north-east. This suggestion of an isolated population that underwent two outward migrations is supported by work by Dr Hayes’s collaborator, Axel Timmermann of the Institute for Basic Science in Busan, South Korea. He is a climatologist and has pieced together, from paleogeographic and astronomical evidence, a history of Makgadikgadi and its surroundings. In particular, he has looked at the effects on the climate thereof the shifts in Earth’s orbit and axial spin that cause ice ages.

His conclusion is that for most of this time Lake Makgadikgadi was surrounded by desert, but that this encircling wall was twice penetrated by green corridors along which animals, people included, would have been able to migrate. The first corridor opened 130,000 years ago to the south-west. The second, 110,000 years ago to the north-east.

The mitogenomic and climatic data thus seem to match. The south-western dispersal would have carried the ancestors of today’s l0 individuals into other parts of southern Africa. In particular, it would explain the traces of habitation along South Africa’s coast that date from shortly after.

It was the north-eastern dispersal, though, that unleashed the children of Makgadikgadi on the wider world. Their descendants spread through what is now Zambia and into the rest of Africa, interbreeding with people already living there, including the descendants of y-chromosomal Adam, as they merged into the wider gene-pool of humanity. Indeed, the history of human nuclear genes resembles a web more than it does a tree, which is one reason Wilson sought the clarity of the mitogenome in the first place.

Eventually, about 50 millennia after these events, some intrepid adventurers crossed to Asia, took up residence there, and thence spread to Australia, Europe and the Americas. The DNA of these travelers was further changed by interbreeding with at least two other species of human: Neanderthals in Europe and Denisovans in Asia.

Not everyone believes Dr Hayes’s version of history. The further back the human mitogenomic tree is traced, they point out, the more uncertainty creeps into it, so further investigation would be desirable. But the mix of evidence, genetic and climatic, that she and her colleagues present does paint quite a plausible picture of the experiences of one particular branch of modern people’s ancient ancestors.

In broad strokes, the results of the new study paint a similar picture to some past work: Today’s southern African populations harbor a deep mitochondrial genetic line. But the details of what the latest analysis revealed remain unclear, says John Hawks, a paleoanthropologist at the University of Wisconsin-Madison.

It’s difficult to know whether the populations living in those regions today are the same as those hundreds of thousands of years ago, he says. As a result, it’s possible that the researchers are tracing mass migrations around southern Africa. But it’s also possible that there was instead something beneficial in the mitochondrial genetics, giving it a selective advantage that allowed the DNA to spread without massive population shifts.

“It’s giving you one part of the whole story of evolution at very high resolution, and that’s pretty cool,” Hawks says. “But you sort of want the rest of the story.”

Mitochondrial DNA makes up a minute fraction of our genomes: While it contains around 16,500 base pairs, nuclear DNA has more than three billion, explains Carina Schlebusch, an evolutionary geneticist at Uppsala University in Sweden. Untangling information in our complete genomes promises a more complex tale. Researchers have made similar trees for Y-chromosome DNA, which is genetic material present in men. While the details remain hazy, it hints at a very early branching genetic line in some modern humans living in western Africa’s Cameroon.

 

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