Posted on 04/23/2005 8:30:41 PM PDT by Lessismore
MILWAUKEE, WISCONSIN--About 1200 researchers gathered near the shores of Lake Michigan here from 5 to 9 April to discuss early Englishmen, the birth of modern humans, and Stone Age weapons.
In the past 15 years, a flood of genetic data has helped propel the Out of Africa theory into the leading explanation of modern human origins. DNA from mitochondria (mtDNA), the Y chromosome, and ancient humans each suggest that the ancestors of all living people arose in Africa some time after 200,000 years ago, swept out of their homeland, and replaced archaic humans around the globe without mixing with them. But at a genetics symposium, two independent groups presented data from the X chromosome hinting that modern humans interbred with other human species: The teams found possible traces of archaic hominids in our genes. "Just as the Y and mtDNA data seemed to have settled it, the new data revive the question [of interbreeding]," says Stanford University's Joanna Mountain, co-organizer of the symposium. "The controversy is not settled." Geneticists Makoto Shimada and Jody Hey of Rutgers University in Piscataway, New Jersey, presented an intriguing haplotype--a set of genetic mutations inherited together--that appears to have ancient roots in Asia rather than Africa. Shimada sequenced a 10.1-kilobase noncoding region in 659 individuals from around the world. Overall, the genetic variations were most frequent in Africa, just as expected if our ancestors were a subset of ancient Africans who migrated out of that continent. But one rare variant, appropriately named haplotype X, appeared in nine individuals from Europe to Oceania but was entirely absent in Africa. Shimada estimated that the haplotype arose 1 million years ago, long before the modern human exodus from Africa. "Haplotype X is difficult to explain by the recent African origins model," says Shimada. "It's very old, it's rare, and it is widespread outside of Africa."
In independent work, geneticist Michael Hammer of the University of Arizona in Tucson offered a similar example. Hammer and postdoc Dan Garrigan identified a 2-million-year-old haplotype in the RRM2P4 region of the X chromosome that is common in East Asia but vanishingly rare in Africa. Their work, published 2 months ago in Molecular Biology and Evolution, raises the possibility that the haplotype arose in very ancient Asian populations, presumably of Homo erectus, an ancient human once found across Asia. "This is what you'd expect if you had introgression" between modern humans and H. erectus, Hammer said.
But at this point several other explanations are possible. Hey of Rutgers acknowledges, for example, that haplotype X may be present in Africa but was missed by spotty sampling in that continent. "Simply observing those [examples] is not sufficient to rule out one model or another," cautions Mountain. "What you need is 10 or 50 loci--one or two is not sufficient." Hammer, for one, thinks that these preliminary data do "speak to some archaic admixture. The few [loci] we've done so far are so suggestive that it gives me great excitement to continue sequencing more loci."
btt
I don't buy it, at least not on such slim evidence. The admixture hypothesis presumes that humans moving out of Africa bred with Asian hominids at least 2 million years removed from those in Africa. If so, the genetic imprint should be much more dramatic and evident ("what you need is 10 or 50 loci--one or two is not sufficient"). I can see interbreeding with Neanderthals, but not with Homo erecti based on this slight evidence.
It's also worth noting that "vanishingly rare" is not the same as nonexistent. That a haplotype appears in 53% of Chinese but only 0.5% of Africans (in a survey with 7.37% margin of error) does not necessarily signify as much as it may appear. To state this differently, there are tens of millions of Asians today descended in some part from a literal handful of Ghengis Khan's comrades - and their gene dissemination was much more recent and diluted in a far larger population.
In other words, all this means so far is that some Southeast Asian alpha male was descended from a "vanishingly rare" African...
One more point worth noting is that there are a number of small, very insular African clans. It may just (quite easily) be the case that when you survey 177 people out of a population of more than 800 million that you miss discovering the band with, say, an 80% abundance of this haplotype.
Thanks for reminding me to do the Nat Geo thing.
Beyond that, this article blows me away as well.
PS. One more point worth noting is that a haplotype dated to originate two million years ago (as in the second study) would be from before Homo erectus itself moved out of Africa..
What you are bringing up indirectly is feed forward evolution, (usual evolution is feedback). Humans may especially have been using this for many tens of thousands of years.
We have lots to learn and with proper DNA research we'll find all the answers. I just hope I live long enough to see/learn it.
PPS. One last point I'd like to note is that a small survey which turns up just one person with a given haplotype could just as easily miss that person. All of a sudden the 4-5 million people represented by that person have 'dropped' to zero, and "vanishingly rare" is now "nonexistent" - although it's not. And, with a 7% margin of error, your extrapolated 4-5 million might actually be more like 60 million.
Even 4-5 million if accurate establishes that the given haplotype has been floating around on the continent for quite a long while.
So, where did Eve come from? Was she a chicken or an egg?
Thanks for the ping!
In the halls of Congress and the courts. They are evolving into nincompoops at a far greater speed than previously realized.
What's your evidence for that conclusion?
However, you should be able to tell roughly how recently that "common ancestor" lived, by comparing the amount of silent mutations (among other markers) which had accumulated in those "regional" genes. It should be pretty easy to tell apart a gene that was "foundered" into a population from the time of Genghis Khan (800 years ago) from one that entered the population a million years ago.
Note that although the article doesn't explain the methods, it does say that one of the identified haplotypes arose "1 million years ago" and another was a "2-million-year-old" genetic sequence -- they probably used methods like the ones I described to determine the approximate "age" of those sequences (which in cases like this, would be the time of the last common ancestor before it spread into a population, not necessarily the date that the sequence actually originated).
Yes.
But at a genetics symposium, two independent groups presented data from the X chromosome hinting that modern humans interbred with other human species
I have always wondered if children born with Down's Syndrome are actually a throw back.
LOL! She appears to be bi (bye?).
I have no clue to what exactly they are looking at to divide us into "tribes" other than some X-linked markers have been consistent for mega years. I find it pretty fascinating.
To be clear, I only noted Genghis Khan (who alone has some 32 million+ descendents) to demonstrate how a "vanishingly rare" trait can become extraordinarily common if the right person happens to have it.
Let's hypothesize two "vanishingly rare" traits floating uniformly around the primordial African populace. As humans migrate out, some equally "vanishing" proportion carry said traits out along with them. Then, for whatever reason, the "vanishingly rare" traits begin propagating at a higher rate. Once the humans have migrated over whatever range, that pivot can take place anywhere: i.e., all else being equal, it's just as likely to take place in Fujian as it is to take place in Axum.
So, if one discounts pure chance, the question you're left with is why it would appear in Asia and not in Africa. I can throw out one hypothesis right off the bat: The Toba eruption 73,000 years ago induced both a crash in the global population and severed the Southeast Asian remnant from the ancestral African homeland. This may be nothing more than a chance disproportionate survival of these two traits in that now-isolated population as compared to the surviving humans that had remained in Africa.
And that's just one random conjecture. The ultimate point that I'm advancing is that you should see a much more dramatic divergence if non-Africans were a hybrid of Homo sapiens & Homo erectus whereas Africans remained 'purely' Homo sapiens. That's what this hypothesis is suggesting and I just don't think there's anywhere near enough evidence to support that conclusion.
No. It's a structural defect caused by having two chromosome 21 in the egg -- the older the female the more often this happens.
When fertilized by sperm, this will produce a zygote with three (two from the abnormal egg + one from the sperm) 21st chromosomes. Thus the technical name "trisomy 21".
A related defected is "monosomy X" or Turner's Syndrome.
Normally, a male will have an X and a Y chromosome, females with two X'es. In Turner's Syndrome, a baby girl develops with only a single X chromosome.
Intellect is usually normal but growth is stunted, abnormal bone development in the chest and abnormal or no menstruation -- and usually the woman is infertile.
Thank you.
I sent in my cheek scrapings last week. I'll let y'all know whether there lurks some Hottentots or Huns in Pharmboy's genes.
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