Freethought & Rationalism Archive

Sauron · 06-09-2003, 10:56 PM

http://news.bbc.co.uk/2/hi/science/nature/2975862.stm

Quote:

When humans looked over the edge

By Dr David Whitehouse
BBC News Online science editor

Humans may have come close to extinction about 70,000 years ago, according to the latest genetic research.

The study suggests that at one point there may have been only 2,000 individuals alive as our species teetered on the brink.

This means that, for a while, humanity was in a perilous state, vulnerable to disease, environmental disasters and conflict. If any of these factors had turned against us, we would not be here.

The research also suggests that humans (Homo sapiens sapiens) made their first journey out of Africa as recently as 70,000 years ago.

I quote this article, knowing full well that some fundibot is going to find a way to turn this story into a proof of the bible (i.e., "Adam & Eve" or "Noah's progeny repopulating the earth") , in spite of the obvious bad reasoning.

lpetrich · 06-09-2003, 11:57 PM

Actually, this bottleneck is consistent with Stephen Jay Gould's "Punctuated Equilibrium", that new species emerge from small offshoot populations of existing ones.

So this evidence of a bottleneck could be evidence of the speciation event in which our species emerged from a predecessor one.

Oolon Colluphid · 06-10-2003, 03:03 AM

Quote:

Originally posted by lpetrich
Actually, this bottleneck is consistent with Stephen Jay Gould's "Punctuated Equilibrium", that new species emerge from small offshoot populations of existing ones.

Yeah, it's consistent... so? Whilst I bow to your much greater knowledge, lpetrich, how does this particularly support PE? New species emerging "from small offshoot populations of existing ones" is just one version of normal, eg allopatric, speciation. It may well produce a punk-eek-like effect in the fossil record... but only if there's eeks before and after the punk, no?

This is not really any different from, say, a few founder individuals of finches on the first Galapagos island. It's just a bottleneck, no need to involve PE.

PE is merely an observation of patterns -- some patterns, some times -- in the fossils. It isn't, as far as I've heard, a mechanism... or really anything except something for creationists to get worked up about.

Heehee... Just to stir up the hive some more, I'd say that PE is just another bee that Gould had in his on-backwards bonnet. It's a fuss about little: a storm in a D-cup -- ie something that gets on people's tits by causing unnecessary confusion.

Cheers, Oolon

MrDarwin · 06-10-2003, 06:13 AM

Quote:

Originally posted by lpetrich
Actually, this bottleneck is consistent with Stephen Jay Gould's "Punctuated Equilibrium", that new species emerge from small offshoot populations of existing ones.

So this evidence of a bottleneck could be evidence of the speciation event in which our species emerged from a predecessor one.

Except that there was no "speciation event" at that time (that we're aware of). Humans (i.e., Homo sapiens) already existed, and I'm not aware of any significant changes in morphology, tool making, etc. that happened around that time.

ps418 · 06-10-2003, 06:22 AM

Here's the AJHJ article's abstract:

Quote:

We study data on variation in 52 worldwide populations at 377 autosomal short tandem repeat loci, to infer a demographic history of human populations. Variation at di-, tri-, and tetranucleotide repeat loci is distributed differently, although each class of markers exhibits a decrease of within-population genetic variation in the following order: sub-Saharan Africa, Eurasia, East Asia, Oceania, and America. There is a similar decrease in the frequency of private alleles. With multidimensional scaling, populations belonging to the same major geographic region cluster together, and some regions permit a finer resolution of populations. When a stepwise mutation model is used, a population tree based on TD estimates of divergence time suggests that the branches leading to the present sub-Saharan African populations of hunter-gatherers were the first to diverge from a common ancestral population (71142 thousand years ago). The branches corresponding to sub-Saharan African farming populations and those that left Africa diverge next, with subsequent splits of branches for Eurasia, Oceania, East Asia, and America. African hunter-gatherer populations and populations of Oceania and America exhibit no statistically significant signature of growth. The features of population subdivision and growth are discussed in the context of the ancient expansion of modern humans.

Zhivotovsky et al, Features of Evolution and Expansion of Modern Humans, Inferred from Genomewide Microsatellite Markers. Am. J. Hum. Genet., 72:1171-1186, 2003.

You can download this paper, and other papers by the same group, at coauthor Noah Rosenberg's website.

Patrick

ps418 · 06-10-2003, 07:00 AM

Quote:

Originally posted by MrDarwin
Except that there was no "speciation event" at that time (that we're aware of). Humans (i.e., Homo sapiens) already existed, and I'm not aware of any significant changes in morphology, tool making, etc. that happened around that time.

There was no speciation event, but there was big change in the complexity of cultural artifacts around 50k years BP, though obviously that could not have lead to a major reduction in genetic diversity at 70k years BP. Maybe the cultural revolution began earlier than 50k years BP or the reduction in genetic diversity happened later than 70k years BP.

Some anthropologists BTW have tried to tie this rapid cultural change to the emergence and subsequent fixation of the FOXP2 allele associated with enhanced language production, though this is pretty speculative, and I have no idea if it is correct or not.

Patrick

MrDarwin · 06-10-2003, 07:06 AM

Patrick, I'm referring to changes in morphology, etc. as being indicators of speciation, not as causes of it. If in fact there is evidence of major cultural change at or shortly after that time, it does in fact provide evidence for a speciation (or subspeciation or speciation-like) event. And as we know from present-day organisms, speciation does not necessarily result in obvious morphological changes.

ps418 · 06-10-2003, 11:49 AM

There has been some speculation as to whether the massive Younger Toba eruption played a role in the genetic bottleneck. It is by far the largest explosive volcanic eruption to occur during the existence of H. sapiens, and the date of the eruption, ~70k yrs BP, is consistent (though merely consistent) with the genetic data. The volume of the Younger Toba Tuff is ~2800km3, compared to about 1km3 for the Mt St Helens eruption. Whether this eruption played a role in the bottleneck or not, it was definitely a whopper.

Quote:

The volume of the youngest eruption is estimated at 2,800 cubic km, making the eruption the largest in the Quaternary. Pyroclastic flows covered an area of at least 20,000 square km. Up to 1200 feet (400 m) of Young Toba Tuff is exposed in the walls of the caldera. On Samosir Island the tuff is more than 1800 feet (600 m) thick. Ash fall from the eruption covers an area of at least 4 million square km (about half the size on the continental United States). Ash from the eruption has been recovered from deep-sea cores taken in the Bay of Bengal and in India, roughly 300 miles (500 km) inland (1,900 miles, 3100 km from Toba). Rose and Chesner suggested the ash may have reached central Asia and the Middle East.

Toba, Sumatra, Indonesia

Patrick

MrDarwin · 06-10-2003, 12:33 PM

Patrick, if a volcanic eruption in Indonesia was massive to cause the near-extinction of humans thousands of miles away, wouldn't we expect many other animal species to show similar evidence of genetic bottlenecks at about the same time?

ps418 · 06-10-2003, 12:40 PM

Quote:

Originally posted by MrDarwin
Patrick, if a volcanic eruption in Indonesia was massive to cause the near-extinction of humans thousands of miles away, wouldn't we expect many other animal species to show similar evidence of genetic bottlenecks at about the same time?

Yes, I would certainly assume so. I wonder how many other species are genetically sufficiently characterized enough to look for such evidence?

Patrick

06-09-2003, 11:57 PM	#2
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	Expected from Punc-Eq? Actually, this bottleneck is consistent with Stephen Jay Gould's "Punctuated Equilibrium", that new species emerge from small offshoot populations of existing ones. So this evidence of a bottleneck could be evidence of the speciation event in which our species emerged from a predecessor one.

06-10-2003, 07:06 AM	#7
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	Re: Re: Re: Expected from Punc-Eq? Patrick, I'm referring to changes in morphology, etc. as being indicators of speciation, not as causes of it. If in fact there is evidence of major cultural change at or shortly after that time, it does in fact provide evidence for a speciation (or subspeciation or speciation-like) event. And as we know from present-day organisms, speciation does not necessarily result in obvious morphological changes.

Thread Tools	Search this Thread
Show Printable Version	Search this Thread: Advanced Search

06-10-2003, 12:33 PM	#9
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	Patrick, if a volcanic eruption in Indonesia was massive to cause the near-extinction of humans thousands of miles away, wouldn't we expect many other animal species to show similar evidence of genetic bottlenecks at about the same time?

Freethought & Rationalism Archive

The archives are read only.