One of the things I love about doing the SGU is that my knowledge and assumptions are constantly being challenged. Teaching is the best learning experience there is.
As a scientist (a professional only in my narrow area of expertise, and an amateur in everything else) I am constantly building my internal model of scientific knowledge. I understand that in most areas of science it is a facade at best, the distilled and simplified version that is presented for non-experts. Occasionally I look behind the facade to see the frame, wiring, plumbing, etc – just to see what’s there, but I need experts to explain it all to me.
This is both exciting and a bit frustrating, as I must be content with the knowledge that I will never have the time to dive deeply enough in most areas of science to master the details. Also, whenever I turn my attention to any specific scientific question I always find that it is hopelessly complex. I could spend days reading the literature just to answer one tiny question. I am therefore constantly being reminded of the depth of my own ignorance in any developed technical area of knowledge.
I am not suggesting we should throw up our hands in defeat – just recognize the state of things. Try to formulate an understanding of a scientific topic that is accurate as far as it goes, and reasonably complete, but also recognize the incredible depth of detail, uncertainty, controversy, and evidence that underlies the understanding you have. Never confuse the two (here there be cranks).
With this in mind, I recently received the following question from an SGU listener:
I’ve been reading everything I can get my hands on about evolution since I was 13 years old. This was right around the time when a psychotic Sunday School teacher publicly (in front of the other kids) ripped my head off and crapped down the hole for asking a simple question about something I’d heard about the earth being quite a lot older than the 6000-some-odd years that was being advertised in the literature that we were being given there at the church. Sorry for the run-on sentence.
Anyway, I thought I had a pretty good handle on the ascent of man thing. But the other day, I was listening to an older SGU podcast. I think it was episode 30. Steve was saying that there was a period approximately 200,000 years ago when our ancestors went through a rather tight squeeze in numbers – something to the tune of only 2000 remaining.
I was stunned. I had not heard about that. Could you say more about this? I would particularly like to know:
· What happened to bring the down numbers so low? What was going on? What were the conditions?
· How was this determined? What fossil evidence or other is there?
· Is this generally accepted to be the case among experts? Or is this fact in dispute?
If nothing else, could you point me to a book or other source that I could read that covers this. I am very surprised that I have not stumbled on this information at some point in the past.
Thanks for all your work and I hope that you keep it going for many years.
Thanks for the question.
Genetic analysis shows that the effective population size (individuals who actually contribute to later generations) went through several bottlenecks. The one that everyone agrees on was 2 million years ago, which is the most recent speciation event that led to modern humans. Speciation events are often population bottlenecks because they tend to occur is small isolated populations.
The other bottleneck that I have read about the most was based on the mitochondrial DNA data, and shows a probable bottleneck 200,000 years ago with an effective population size of 1000-6000 individuals.
However, the various forms of genetic analysis do not give the same results, and also do not square with the paleontological evidence. For example, at times when the effective population was supposed to be very low (in the thousands) there are many fossil sites in Africa, Asia, and Europe. I have encountered three hypotheses to explain the discrepancy. One is that while there were many humans around, only one small population led to modern humans, and the rest were cousins with no living descendants. The second is that the clear bottleneck 2 million years ago restricted genetic variation in such a way that we simply do not have the resolution to detect or rule out later bottlenecks. The third is that there was a bottle with a long neck – a prolonged bottleneck with a low population for hundreds of thousands or even a million years.
Here is a summary of the genetic evidence from a recent paper:
(Ne = effective population size)
Overall, the estimates of Ne appear to be much lower than the usually quoted value of 10,000 (Takahata 1993). Earlier studies using mtDNA data suggested an Ne in the range of 1000–6000 (Rogers and Harpending 1992; Harpending et al. 1993; Sherry et al. 1994), for a population ∼200,000 yr ago (∼10,000 generations ago). Erlich et al. (1996) estimated a recent population size of ∼10,000 from HLA polymorphisms. Sherry et al. (1997) estimated an ancestral population size of ∼17,800 during the last one to two million yr from Alu repeats evolution.
And here is another recent paper with a very long discussion of the issues: http://mbe.oxfordjournals.org/cgi/content/full/17/1/2 (Try to make it through this paper if you want a good sense of how detailed and complex the science is.)
If you want to search for other information – “population bottleneck” is the key term you want to use.
I think the information above, however, answers your questions. To reiterate one point – the overall population did not necessarily decrease, just the effective population – those individuals that led to modern humans. So we may be descended from one small tribe or region, while there were early humans all over the place that simply did not lead to us.
After reading about this some more it also seems to me that this is much more controversial and unsettled than I had previously thought. So while there appears to be a strong inference from genetic information of one or more bottlenecks, there are multiple ways to interpret the data and likely this will not be settled for some time.