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Member (Idle past 6505 days) Posts: 3085 From: Munich, Germany Joined: |
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Author | Topic: Neanderthals and Cro-Magnon | |||||||||||||||||||||||
sfs Member (Idle past 2563 days) Posts: 464 From: Cambridge, MA USA Joined: |
quote: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.
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Jor-el Inactive Member |
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.
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.~ Again I say that I'm just learning the basics so I'm going on what I've been reading on the net as well as asking questions of those who know alot more than I. We are the sum of all that is, and has been. We will be the sum of our choices.
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sfs Member (Idle past 2563 days) Posts: 464 From: Cambridge, MA USA Joined: |
quote:Do they cite any particular papers? quote: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.
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Lights Inactive Member |
Kind of an ameteur here but very interested in the topic anyhow.
What about the bottleneck caused by Toba? I understand that Toba erupted around 74,000 years ago. What are the other bottlenecks mentioned?
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sfs Member (Idle past 2563 days) Posts: 464 From: Cambridge, MA USA Joined: |
quote: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.
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Jor-el Inactive Member |
Population bottlenecks and patterns of human polymorphism
Toba catastrophe theory National Geographic, Volcanology: Big Chill Discovery Channel feuture, Supervolcanoes Late Pleistocene human population bottlenecks Here are some of the sites I visited that give the possibility of a bottleneck in recent human history. There are also many sites saying the exact opposite. It is interesting to note that researchers cannot agree on this issue but that is due mainly to the impossibility of actually finding physical proof either way, so in the end all we are left with are a series of indicators that people use to fit one theory or the other. Since DNA studies cannot at the present time indicate whether this really happened or not, I think I'll just go by logic and accept that there were population bottlenecks in the past and that Toba was one such circumstance. I was hoping to find more conclusive proof of the matter but that does not seem to be forthcoming in the near future. The main reason that I find this logical is due to the date that mankind first appeared in its present physical form. I've heard dates ranging from 100 000 years to 50 000 years. Now taking into account that Toba straddles these values it could be concludes that the events are linked. A major decrease in population and DNA diversity will invariably cause mutations in the the population due to an insufficient gene pool. These mutations could have caused a leap in intelligence as well as physical adaptation to the cicumstances of a volcanic winter, which contributed to the start a new period of glaciation. I recently read a paper on this although I can't find the site now. It talks about genetic mutation in reduced population sizes that after the population stabilizes then become the norm for the species in question. this doesn't just apply to animals but to humans as well. Edited by Jor-el, : Forgot to add a small point to the discussion We are the sum of all that is, and has been. We will be the sum of our choices.
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Jackie Inactive Member |
Does this help?
Inbreeding is for slow news days - john hawks weblog
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Jor-el Inactive Member |
shortened link
I'm not certain but I think this is it. Edited by AdminJar, : shorten link We are the sum of all that is, and has been. We will be the sum of our choices.
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Lights Inactive Member |
Thanks so much for the information!
I too think there is no real way to prove a bottleneck such as the Toba event is supposed to have caused, but yes, I do think they have happened and will hapeen. Again, thanks for all your help! ~Lux Vestra--Let Your Light Shine~
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sfs Member (Idle past 2563 days) Posts: 464 From: Cambridge, MA USA Joined: |
quote: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.
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Jor-el Inactive Member |
Ok, You lost me with all the technical stuff.
I'll take your word for it that the study is outdated in part. Does any new study refute the hypothesis put forward by people like Henry Harpending among others, and prove that this hypothesis is based on a flawed and false premise at this time? We are the sum of all that is, and has been. We will be the sum of our choices.
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Peleg Inactive Member |
Now here are archaic features we could "chew" the fat over!
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pop  Inactive Member |
okay let us first discuss the Neanderthals
1-They were human beings appeared suddenly about 100 000 years ago and disappeared or mixed with races 35 000 years ago.Some researchers call them Homo neanderthalensis. 2-Their skeletons and their cranial capacities are bigger(according to my studying
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Peleg Inactive Member |
Hmmm maybe the traits are a form phenotypic survival mechanisms triggered by severe climate, predator-prey relationships, and persecutions--and add a dash of inbreeding?
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MarkAustin Member (Idle past 3845 days) Posts: 122 From: London., UK Joined: |
Some time ago, I read a book (can't track it down - may have been a (now returned) library book), which put forward the "beachcomber" theory for the spread of humanity. On this thesis, on Out of Africa 2, modern or near modern man turned right along the coast (the other way was blocked by glaciation, and settled in a pattern of coast and river following. Europe was settled late (IIRC consistent with the European bottleneck) when a relatively (in geological terms) climate change opened a path from the upper Tigris/Euphrates round the Black Sea and into Europe. Thus the population of Europe would come from the relatively small number who made it through, thus explaining the population bottleneck as a founder effect.
I'll see if I can track down the reference. Mark Austin For Whigs admit no force but argument. |
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