A group of scientists have reported their new theory that human life was first seen in northern Botswana, south of the Zambezi river in Africa, according to their article published in the journal Nature. The scientists even revealed the exact spot – the ancient wetland of Makgadikgadi–Okavango. But opposing voices have not been slow to arise, causing deep controversy over the actual implications of their findings and the methods they used to arrive at this conclusion.
Over 30 years there have been at least 10 discoveries of supposed human ancestors, out of the total of 20. However, archaeologists were just as far from knowing the place where the human race first began as ever.
Okavango delta (Okavango Grassland) is one of the Seven Natural Wonders of Africa (view from the airplane) - Botswana, South-Western Africa. - Image Credit: Vadim Petrakov / Shutterstock
What they did
In the current study, the scientists took DNA samples from a people group in the southern part of Africa called the Khoe-San. These people have the ‘earliest’ maternal DNA in human history, in their mitochondria. They also collected samples from another group who has the same lineage of maternal DNA. Altogether, they had over 1,200 mitochondrial DNA strands to analyze.
Mitochondrial DNA is inherited from the maternal side of the family and this does not, therefore, show much change over the generations. The researchers built a kind of family tree to show the points at which later populations emerged from the basic lineage. This led them to the determination of the oldest branch of mitochondrial DNA, called L0, which is, in their view, the origin of modern humans.
Collaborating with geological and climate specialists, they worked out a possible picture of the land topography, climate and geologic characteristics of the time period from which they think L0 dates. This served as the foundation of their hypothesis that there was a sizable number of L0 mitochondrial genomes settled around the Zambezi river 200,000 years ago. They also found that most humans at that time could have belonged to one or the other of many Khoe-San subgroups.
The region of interest to the current study was formerly a large lake called Makgadikgadi, and it extended from northern Namibia to Zimbabwe, across northern Botswana. The lake exceeded all modern African lakes in size, and remained intact for about 200 million years.
Finally, tectonic action is supposed to have caused it to break up, when it became a wetland instead. The hot humid region formed as a result of this transformation encouraged stretches of rich plant and animal life to develop in response to changing rain patterns. As a result of this, after about 70,000 years, these humans started to move across the land in two distinct migrations, to the northeast and the southwest, about 110,000 to 130,000 years ago.
Why mitochondrial DNA?
The question arises, why did the researchers choose mitochondrial genomes? The reason is that mitochondrial genomes allow a more accurate timeline determination, in the absence of whole genome data, and thus permits an analysis of how a lineage first comes on the scene.
As expected, many of their peers consider the paper just another poorly-researched attempt to narrow down the origin of Homo sapiens.
They point out that mitochondrial DNA makes up a tiny fraction of the entire human genome, that too only the part inherited directly from the mother’s side. At the point the current study fixes as the origin of modern man, about 200,000 years ago, other scientists argue that population-level data is completely absent.
This leaves out a lot of other potential ancestors. Evolutionist Carina Schlebusch says, “Genetic variation can only be captured by the rest of our chromosomes.” Another theory suggests that west Africa is the birthplace, from studies on the Y chromosome that is passed down only through the male line.
Interbreeding populations from many sites, not a single origin
Another expert, Eleanor Scerri, says that there were other ancestors, not represented in the mitochondrial lineages, in the same period of African settlement. Their DNA may not have left traces behind. She points out in picturesque language, “Reconstructing deep ancestry from mitochondrial DNA is like trying to reconstruct a language from a handful of words, whereas using whole genome or nuclear DNA is like trying to reconstruct a dead language after hearing it being spoken for a day.”
One of the researchers behind the present paper, Eva Chan, concurs that it is not an easy thing to pin down the origin of the modern human race, and that more theories are bound to spring up as new data comes to light. However, she insists, “All our evidence points to this paleo-wetland as the birthplace of all humans today.”
Scerri pooh-poohs this line of argument, pointing out that reliable evidence must come from more than just genetic lineages. There is the fossil record, as well as evidence from archeology, that shows modern humans could have emerged far earlier. Others agree with her, pointing to some theories that date the origin of modern humans to 300,000 years ago.
The most the current findings can safely claim to do is to point out that a robust population group lived in the region of interest long ago, and that their DNA still lingers today in living people. There will be more discoveries of ancient lineages, says archeologist James Cole.
In fact, the current paper could, paradoxically, focus more interest on the competing theory that multiple groups sprang up almost simultaneously in many places in Africa, judging from fossils found in the eastern and southern parts of Africa. They contend therefore that the continent itself is our birthplace rather than any single site. Scerri has already published her hypothesis that many genetic characteristics came into being slowly and in a mix, across a broad geographic region in Africa.
Another serious objection is that the underlying assumption that the Khoe-San lived in the same place for hundreds of thousands of years is unfounded. Their colleagues in the field point out that making a relationship between 200,000-year old mitochondrial DNA and modern human features in the same time period is absolutely unheard of. The researchers here have not carried out any bone study, either. Thus many others question their final hypothesis.
All that can be said is that the earliest mitochondrial DNA was in southern Africa 200,000 years ago. Beyond that, we don’t know if the people from whom it came remained in the region all these thousands of centuries, which is almost incredible. Neither do we know that any ancient population is identically linked to any modern one. It is equivalent, says, Scerri, to saying that “the Khoe-San are evolutionary relicts who have neither changed nor moved geographically for tens or even hundreds of thousand years.” Calling such a statement both “factually incorrect” and “ethically problematic”, she makes it clear that the present study doesn’t convince her, or a lot of others, that its conclusion is the right one.
Chan, E.K.F., Timmermann, A., Baldi, B.F. et al. Human origins in a southern African palaeo-wetland and first migrations. Nature (2019) doi:10.1038/s41586-019-1714-1, https://www.nature.com/articles/s41586-019-1714-1