Oct 31 2019

Tracing Human Origins

Published by under Evolution
Comments: 0

When and where did fully modern humans first emerge? That is an interesting question that paleontologists have been chasing for decades. Now a new genetic study claims to have pinpointed that origin to northern Botswana 200,000 years ago. The claim is already getting some pushback from other experts, but the new data does add to our understanding of human origins.

The study looks at mitochrondrial DNA, a technique that has been used before. This is DNA outside the nucleus, in each mitochondria of the cell (the power factories). They are almost exclusively passed down through the maternal line, because the egg provides all the mitochondria to the embryo, while sperm generally contribute none (although one may sneak through from time to time). You can therefore use mDNA to trace maternal lines.

When this type of analysis was first done researchers found that all humans have a common female ancestor going back to about 200,000 years in Africa. This result was widely misinterpreted in the press, not helped by the fact that this alleged ancestor was deemed the “mitochondrial Eve.” If you go back far enough, everyone is related to everyone. Therefore we all have many common ancestors. What the analysis shows really is a couple of things. First, that only one mitochondrial line from this time survived to the modern day. That doesn’t mean we only have one common female ancestor. But every time a woman has only sons, her mitochondrial line dies out. This finding does suggest, however, that the human population when through a relative bottleneck at this time. Our ancestors were not spread around Africa or the world, because then each region would have its own mitochondrial lineage.

The current study is therefore nothing new. It is building on previous mDNA work, narrowing the error bars and zeroing in on the time and location of that common mitochondrial ancestor. According to ScienceMag:

Confirming earlier studies, the data reveal that one mtDNA lineage in the Khoisan speakers—L0—is the oldest known mtDNA lineage in living people. The work also tightens the date of origin of L0 to about 200,000 years ago (with a range of error of 165,000 to 240,000; previous studies had a range of error from 150,000 to 250,000), the team reports today in Nature. Because today this lineage is found only in people in southern Africa, people carrying the L0 lineage lived in southern Africa and formed the ancestral population for all living humans, says lead author Vanessa Hayes, a genomicist at the Garvan Institute of Medical Research and the University of Sydney in Australia.

“Lo” is the designation of this particular mitochondrial lineage. Narrowing down the time of our common mitochondrial ancestor with more data has not generated any controversy. What some other experts are objecting to is the claim that we can narrow down the physical location so precisely using just analysis of modern DNA. The problem with this is that people move. Migration is common in the history of humans. So just because you can trace the oldest mDNA lineage to people currently living in northern Botswana, that does not mean that the mDNA lineage originated there 200,000 year ago. It is increasingly clear that our common ancestors originated in Africa, and maybe even southern Africa, but even that statement gets less certain.

In favor of this paper’s conclusion is the fact that this region was lush wetland 200,000 years ago, with perhaps the largest lake in Africa. This area would have supported a lot of life, and would have made excellent hunting grounds. Starting aroung 130,000 years ago climate change started to turn this region into desert and salt flats, while green corridors opened up Africa to the north and the south. So, the authors speculate, our ancestor underwent a series of migrations from this central location to other parts of Africa, and then eventually to Asia and Europe.

That is a reasonable hypothesis, say critics, but the evidence does not establish it to be actually true. What we would need is fossil evidence to back it up. Even better, analysis of ancient DNA would be a tremendous help. Scientists have been able to extract DNA from fossils. We have ancient DNA (aDNA) from a 110,000 year old Denisovan. The theoretical limit of aDNA is 0.4 – 1.5 million years. DNA will accumulate post-mortem mutations over long periods of time, so the older the aDNA the less useful it is. But it seems the 200,000 year time frame is well within the limit of aDNA, so perhaps we will find a well-enough preserved specimen to further settle this debate.

While it’s great that experts are debating about our levels of certainty and alternative interpretations of the data, over the years the big picture is that the evidence has been movign in the direction of a common human ancestor at about this time, 200,000 years ago, in about this location, somewhere in Africa. This new data pushes us further in that direction.

No responses yet