Neanderthals and Cro-Magnon

quote:

---

I know this an old topic but could you explain to me how this bottleneck 40000 yrs ago could relate to another such occurence 74000 yrs ago. The eruption of the supervolcano Toba in Sumatra, Indonesia, that is supposed to have been the cause of the last ice age according to some articles I've read.

​

It seems to me that these events have happened fairly often in human history. Although we haven't been hit that hard by anything major in the last 10000 yrs or so.

---

The bulk of genetic evidence is that there has been no substantial bottleneck in the human population as a whole for at least the last 100,000 years. Specifically, there hasn't been a bottleneck in most African populations studies. Non-African populations all show signs of substantial bottlenecks, presumably associated with the founding of those populations (probably including the Out of Africa migration itself).These conclusions are based largely on studies of the X chromosome and the autosomes, since they preserve many independent records of demographic history, unlike the mtDNA and the Y chromosome, which only record one each.

quote:

---

Well that seems to go against most of what I've been reading in relation to mtDNA studies as well as most documentaries on supervolcanoes which I've got on dvd. As a matter of fact they say in many instances that these sudden losses in population have happened at least 3 times if not more. 74 000 - 40 000 - 11 000 kyrs respectively.

---

Do they cite any particular papers?

quote:

---

Since most of these studies in one way or another support the "Out of Africa" theory there may be a tendency for opponents of this theory to dilute these findings.~

---

No, that's not the issue. All the studies I'm talking about show a small effective population size for humans (which supports Out of Africa), and they're pretty much all interpreted in a context of African origins. They just don't show any evidence for substantial changes in African population sizes, prior to the recent massive expansion. There are one or two that suggest an earlier phase to the expansion, but nothing suggests a decrease in population size.Of course, the small effective population size could be taken by itself as evidence that there was a bottleneck, but it's weak evidence, since we could also just have had a small population for a long time. It is true that modern humans probably had a smaller effective population size than Homo erectus, but that observation doesn't suggest the kind of dated bottleneck you're talking about here.

quote:

---

What about the bottleneck caused by Toba? I understand that Toba erupted around 74,000 years ago.

---

I don't know of any reason to think that there was a bottleneck associated with Toba. It's an interesting hypothesis, and is consistent with the data as far as I know, but I don't see any real genetic support for it.

quote:

---

Population bottlenecks and patterns of human polymorphism

---

This paper deals with the kind of evidence that could point to a bottleneck, but their central piece of data is no longer true. They use a bottleneck to explain why mitochondrial DNA shows an excess of rare alleles (negative Tajima's D) while nuclear DNA shows an excess of higher frequency alleles (positive Tajima's D). Larger studies have now shown conclusively that, overall, nuclear DNA also has an excess of rare alleles. For example, Stephens et al (Science, 293:489-493 (2001)) found a negative Tajima's D value for 281 out of 313 genes they studied. A global negative Tajima's D suggests a population expansion (which of course has happened for humans), and that effect dominates the genetic picture for African populations.So, clear evidence for expansion, but nothing obviously requiring a bottleneck.Edited by sfs, : Edited to improve clarity.