Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: cheryl j on 25/05/2013 02:40:59
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According to the Globe and Mail (May 23, Social Studies, Micheal Kesterton):
“In 2004, statistician Joseph Chang, computer scientist Douglas Rohde and writer Steve Olson used a computer model of human genetics to show that anyone who was alive 2,000 to 3,000 years ago is either the ancestor of anyone who’s now alive, or no one at all. Think about that: If a person alive in 1,000 BC has any descendents alive today, they have all of us – even people from different continents and isolated populations.”
Is this right? Are we really all that closely related?
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That sounds pretty iffy to me. Aboriginal populations, for example, were around in Australia from 40,000 years ago; they stayed there without much further mixing; so I can't see how they fit the argument above...
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That sounds pretty iffy to me. Aboriginal populations, for example, were around in Australia from 40,000 years ago; they stayed there without much further mixing; so I can't see how they fit the argument above...
That was my first reaction, and I was mainly considering the Native and Inuit populations of Canada. While there has been some intermarriage, I'm sure in remote communities there must be some individuals who don't have genes from Europeans who settled here.
After googling it for a while I did find the original source in Nature which is a pretty respectable journal. But I still don't understand how it can be right.
Here is the abstract of the original study:
Letters to Nature
Nature 431, 562-566 (30 September 2004) | doi:10.1038/nature02842; Received 30 December 2003; Accepted 14 July 2004
Modelling the recent common ancestry of all living humans
Douglas L. T. Rohde1, Steve Olson2 & Joseph T. Chang3
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
7609 Sebago Road, Bethesda, Maryland 20817, USA
Department of Statistics, Yale University, New Haven, Connecticut 06520, USA
Correspondence to: Douglas L. T. Rohde1 Email: dr@tedlab.mit.edu
Top of page
If a common ancestor of all living humans is defined as an individual who is a genealogical ancestor of all present-day people, the most recent common ancestor (MRCA) for a randomly mating population would have lived in the very recent past1, 2, 3. However, the random mating model ignores essential aspects of population substructure, such as the tendency of individuals to choose mates from the same social group, and the relative isolation of geographically separated groups. Here we show that recent common ancestors also emerge from two models incorporating substantial population substructure. One model, designed for simplicity and theoretical insight, yields explicit mathematical results through a probabilistic analysis. A more elaborate second model, designed to capture historical population dynamics in a more realistic way, is analysed computationally through Monte Carlo simulations. These analyses suggest that the genealogies of all living humans overlap in remarkable ways in the recent past. In particular, the MRCA of all present-day humans lived just a few thousand years ago in these models. Moreover, among all individuals living more than just a few thousand years earlier than the MRCA, each present-day human has exactly the same set of genealogical ancestors.
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Sure, but it's still a model and shows that it's feasible, but doesn't prove it's true, for reasons that we've mentioned above and which they acknowledge as a shortcoming of the random mating model.
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I think the problem is that until about 400 years ago, there were a great number of very strict ethnic separations.
American Indians, African Blacks, Spanish Basque, East Asian, etc.
It is quite possible that genes have spread throughout an entire region, but some physical separation was enough stop the spread of certain genes. Recognizable ethnic groups may also cause barriers.
Anyway, I don't believe that I have any Black African genes, or native Australian Aboriginal genes. Of course, the "out of Africa" hypothesis indicates that while in modern times, one may not have had gene mixing, all of us would have had ancient African heritage.
South America developed somewhat differently than North America. At times I've wondered what exactly the "Hispanic race" is. It is quite possible there was much greater mixing of European and Native races in South American than occurred in North America.
It is quite possible that 2000 years from today, there will be so great of population mixing that such boundaries will no longer exist.
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According to the abstract, the model assumes "a randomly mating population", so while it might be applicable to some human populations, unless capturing 'historical population dynamics in a more realistic way' means allowing for the isolation of populations (in which case their figures don't make sense), it surely can't apply to all living humans. I interpret the result as saying each randomly mating (sounds like dogging!) human population a 'few thousand years' old or more will have an MRCA 'a few thousand' years ago.
But what do I know? :)
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Clifford - it's impossible that you don't have any black African genes because that's where we began, a few hundred thousand years ago... Hence they are our ancestors. As are the 3 million year old remains that are turning up in Kenya and South Africa...
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... they are our ancestors. As are the 3 million year old remains that are turning up in Kenya and South Africa...
But the majority of ancient hominids aren't (weren't?) our ancestors, although we share a MRCA. We're the single lucky hominid branch to make it to the present...
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Oddly enough, I was sitting in the dentist's office yesterday and picked up a magazine (Canada's History April-May 2013) to read an article about John Lennon and Yoko Ono's "Bed-in" at a Montreal hotel in 1969, but it also had an article about this very same ancestor thing - "Can You Relate" by Paul Jones. (Yes, I stole the magazine, but I will return it. I have to go back anyway.) This article says the MRCA or most common recent ancestor lived "within the past several thousand years" for "99.999 percent of humans." It uses the term "IAP" or Identical Ancestors Point for the moment "when every living ancestor was the ancestor of either no one or everyone living today." This article puts the IAP at five to 15 thousand years ago, but it doesn't cite a particular study.
The author offers this explanation:
"Why the surprising recent estimates for the MRCA and IAP ? The answer lies in the phenomenon of pedigree collapse. In our classic understanding of pedigree, an individual has two parents, four grandparents, and so on, with every successive generation twice as large. Common sense tells us that this exponential growth can't go on indefinitely. Pedigree collapse is the mathematical explanation for the common sense reality that you can't have more ancestors than there were people at the time. Inevitably, and frequently, in any pedigree, cousins of some degree marry one another and whenever they do, overlapping segments of their ancestries are duplicated in their progeny's pedigree. For much of human history, marriages between second or third cousins was the norm. The upshot is that you don't have to go too far back in within any population to find that everyone who had descendents is an ancestor, often many times over."
None of which explains why remote or isolated groups account for so little in these models. And if the model is screwed up, it's odd that they couldn't fix it between 2004 and 2013.