CNN had a headline that caught my attention: “Humans nearly wiped out 70,000 years ago, study says.” Now that’s news! The gist of the Associated Press article it uses can be summed up by two quotes:
Paleontologist Meave Leakey, a Genographic adviser, asked, “Who would have thought that as recently as 70,000 years ago, extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction?”
“Dr. Spencer Wells, National Geographic Explorer-in-Residence and Director of the Genographic Project, said … “Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. Truly an epic drama, written in our DNA.””
My suspicio-meter immediately hit the jackpot. That’s a pretty tall order for a study on genetics. The mention of climate also sounds like an attempt to make the news by linking this study to global warming. I followed a link to the Genographic Project which was the source of this story. Being the responsible researchers they are (after all being part of National Geographic), they had a free link to the actual scientific paper that is the root of these findings. Of course I read it immediately.
It’s a good paper. And this is coming from someone who, let’s say, doesn’t have that much faith in genetics research. It’s highly technical of course, but the introduction and discussion are very clear. And I’m glad to report, the article is very non-sensationalist. It does contradict the popular digest of the source material on several key points:
Humans were Near Extinction
The AP article states that populations might have been as low as 2,000 around 70,000 years ago, citing another scientific paper not referenced. I tracked it down!* It actually does not say that the human population dropped to 2,000. Instead, the human variation we see today can be projected to exist in around 2,000 people 70,000 years ago. Is there a difference? Yes, because there could be untold number of people living 70,000 years ago whose unique genetic markers did not make it into our modern population. Unfortunately one of the flaws of genetic research in anthropology is that these hypothetical extinct lines cannot be detected, measured or accounted for, though they are likely to have existed. The scientific paper clearly states this, though that didn’t make it into the Genographic or AP articles.
The Wells quote is very dramatic, mentioning how ancient humans had to band together after a long split to repopulate Africa and then the rest of the world. But, the scientific article portrays this split differently. The split isn’t between caves, huts, villages, or other small groupings. The split is between the entire south Africa and east Africa! At this continental scale, there is really no personal drama as the quotes suggest. If one of the two groups had gone extinct who is to say that the other group wouldn’t have taken over and populated the world? Did the people living back then even recognize an intracontinental split between populations?
Climate is the Culprit
Even the AP article says “It is possible that this climatological shift contributed to the population splits,” (emphasis mine). In fact, there is absolutely nothing linking climate to low population in the scientific paper. The paper does mention droughts as an event that kind of coinicides with the period of low human population (actually according to the paper the droughts ended 20,000 years before the populations merged, making me wonder if they even correlate). In this case, both quotes about climate or the environment as the cause of these populations shifts are just pure speculation and “wishful thinking” if you can call it that.
It is very important to remember that correlation does not equal causation. Just because two things seem to be happening together, doesn’t mean they are related. It’s 70 degrees this fine morning and I had Chex for breakfast, but it doesn’t mean I ate Chex instead of Trail Mix Crunch (which is really good by the way) because it’s 70 degrees.
Considering material from the scientific papers, the popular quotes above exagerrate a few key points. Human population has not been determined to be around 2,000 at that time, nullifying any talk of near-extinction. Climate has not been determined to be the cause of the misinterpreted population estimate. There was a large split between two African populations, but not between individual bands. The groups eventually fused before humans crossed into Eurasia, but at such a scale of time and space that the lay-term of reunion is meaningless. So there you go, scientific research from paper to popular press and all of it’s varying forms. *sigh* if only I can write my dissertation proposal with this much speed and energy.
*Zhivotovsky LA, Rosenberg NA, and Feldman MW. 2003. Features of Evolution and Expansion of Modern Humans, Inferred from Genomewide Microsatellite Markers. Am. J. Hum. Genet. 72:1171–1186