Skip to comments.Did Early Humans Go North or South?
Posted on 05/14/2005 7:58:39 AM PDT by Lessismore
By analyzing the DNA of living humans from different locations, geneticists are able to assemble a detailed reconstruction of prehistoric human colonization of the world. This research endeavor was championed by the late Allan Wilson [HN1] and his colleagues (1, 2), who led the way with their studies of maternally inherited mitochondrial DNA (mtDNA) [HN2]. Their work led to the proposal of a recent African origin for modern humans, some 5000 generations ago. Anthropologists and geneticists have since joined forces to create a broad framework of possible prehistoric human migration routes [HN3] and time scales (3-6). The two latest additions to this framework are described by Thangaraj et al. [HN4] (7) on page 996 and Macaulay et al. [HN5] (8) on page 1034 of this issue.
Our current understanding is that modern humans arose ~150,000 years ago, possibly in East Africa, where human genetic diversity is particularly high. Subsequent early colonization within Africa is supported by old genetic mtDNA and Y chromosome branches (often called "haplogroups" [HN6]) in the Bushmen or Khoisan [HN7] of the Kalahari Desert, and in certain pygmy tribes [HN8] in the central African rainforest. Early humans even ventured out of Africa briefly, as indicated by the 90,000-year-old Skhul and Qafzeh fossils [HN9] found in Israel. The next event clearly visible in the mitochondrial evolutionary tree is an expansion signature of so-called L2 and L3 mtDNA types in Africa about 85,000 years ago, which now represent more than two-thirds of female lineages throughout most of Africa. The reason for this remarkable expansion is unclear, but it led directly to the only successful migration out of Africa, and is genetically dated by mtDNA to have occurred some time between 55,000 and 85,000 years ago. Studies of the paternally inherited Y chromosome [HN10] yield time estimates for the African exodus that are in broad agreement with those derived from mtDNA.
It is at this point in the narrative that the studies by Thangaraj et al. (7) and Macaulay et al. (8) come into the picture. Which route did the first Eurasians take out of Africa? Most obvious, perhaps, is the route along the Nile and across the Sinai Peninsula leading into the rest of the world (see the figure). But if that were so, why was adjacent Europe settled thousands of years later than distant Australia? In Europe, Neanderthals were replaced by modern humans only about 30,000 to 40,000 years ago, whereas southern Australia was definitely inhabited 46,000 years ago and northern Australia and Southeast Asia necessarily even earlier (9, 10). [HN11] Or did our ancestors instead depart from East Africa, crossing the Red Sea and then following the coast of the Indian Ocean (11)? A purely coastal "express train" would conveniently explain the early dates for human presence in Australia, but would require that humans were capable of crossing the mouth of the Red Sea some 60,000 years ago. Why, then, was this feat not repeated by any later African emigrants, particularly when the Red Sea level dropped to a minimum about 20,000 years ago?
Ideally, these questions would be answered by investigating ancient fossils and DNA from the Arabian Peninsula. But because this option is currently not available, Thangaraj et al. and Macaulay et al. have centered their investigation on the other side of the Indian Ocean, in the Andaman Islands and Malaysian Peninsula. Both groups used genetic studies of relict populations known to differ substantially from their Asian neighbors to estimate the arrival time of the first humans in these locations. Thangaraj and colleagues sampled the Andamanese [HN12], who were decimated in the 19th century by diseases imported by the British and then suffered displacement by modern Indian immigration (12). Macaulay and co-workers sampled the native tribal people of Malaysia, called the Orang Asli [HN13] ("original people").
Fortunately, the two teams arrived at compatible conclusions. In the Andaman Islands, Thangaraj et al. identified the M31 and M32 mtDNA types among indigenous Andamanese. These two mtDNA types branched directly from M mtDNA, which arose as a founder 65,000 years ago. This time estimate for the arrival of M founder mtDNA is matched by that of Macaulay and co-workers. These investigators found mtDNA types M21 and M22 in their Malaysian data set. These M types are geographically specific branches of M that branched off from other Asian mtDNA lineages around 60,000 years ago. Thus, the first Eurasians appear to have reached the coast of the Indian Ocean soon after leaving Africa, regardless of whether they took the northern or the southern route. Interestingly, the adjacent Nicobar Islands do not harbor any old mtDNA branches specific to the islands. Instead, their mtDNA has a close and hence recent genetic relationship (on the order of 15,000 years or less) with the mtDNA of other Southeast Asian populations. This is not unexpected given the more Asian appearance of the Nicobar islanders.
Macaulay and colleagues go two steps further and estimate the prehistoric migration speed of early humans along the coast of the Indian Ocean; they also estimate the likely population size of the emigrant population. Comparing genetic dates of founder types between India and Australia, and assuming a 12,000-km journey along the Indian Ocean coastline, they suggest a migration speed for the first Eurasians of 0.7 to 4 km per year. This value is of the same order of magnitude as genetically dated inland journeys of migrant populations during the last Ice Age, 60,000 to 10,000 years ago (6) [HN14].
One intriguing question is the number of women who originally emigrated out of Africa. Only one is required, theoretically. Such a single female founder would have had to carry the African L3 mtDNA type, and her descendants would have carried those mtDNA types (M, N, and R) that populate Eurasia today. Macaulay et al. use population modeling to obtain a rough upper estimate of the number of women who left Africa 60,000 years ago. From their model, they calculate this number to be about 600. Using published conversion factors, we can translate this estimate into a number between 500 and 2000 actual women. The authors' preferred estimate is several hundred female founders. All such estimations are influenced by the choice of parameters and by statistical uncertainty; hence, it is understood that the true number could have been considerably larger or smaller. Improved estimates will involve computer simulations based on informed scenarios using additional genetic loci.
Time is short if researchers wish to secure data on dwindling indigenous populations such as the Andamanese and the Orang Asli. The studies by Macaulay et al. and Thangaraj et al., which are devoted to the peoples inhabiting the "southern route" along the Indian Ocean, are therefore very welcome. We hope that the new findings will inspire archaeological exploration between the Arabian Peninsula and Southeast Asia in search of the remains of the first Eurasians 50,000 to 100,000 years ago.
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The authors are at the McDonald Institute for Archaeological Research, University of Cambridge, Cambridge CB2 3ER, UK. E-mail: email@example.com
The theory is that in order for less and less sunlight available as people moved north to make vitamin D, their skin lost melanin. In the early part of the last century (before vitamin D fortification of milk) an outbreak of rickets (D deficiency disease) occurred in Chicago among African American infants because they did not get enough sun.
As far as modern human evolution goes, the key is the isolation of a population. With all the genetic mixing nowadays, it would be hard for much genetic change over time to only affect certain populations.
I thought this bit was interesting: We hope that the new findings will inspire archaeological exploration between the Arabian Peninsula and Southeast Asia in search of the remains of the first Eurasians 50,000 to 100,000 years ago.
I know there's a lot of digs on the Sub-Continent, but what about the Arabian Peninsula?
But the children of the ones that went North died out, while the children of the ones that went South continue to reproduce, to this day.
That's the point of the story. You, I, and everybody else on Earth, are the descendents of the ones who went South. At least, this is what DNA is telling us.
We don't know what color the original Africans were. They may have had lighter skin and straighter hair than present day Africans, and got darker while everybody else got lighter.
Good point. The 'stay-at-home' Africans may have gone through their own series of changes that were different than those that affected the 'out-of-Africans.' The sub-Saharan Africans are the most 'different' from all the other people outside that region.
I googled on this topic and found the following article: The Biology of Skin Color. What's interesting is that there are at least two real feedback mechanisms (ie. ones that affect survival) due to ultraviolet sunlight: vitamin B and D. Nowadays, women when they're anticipating pregnancy, are advised to take B in prenatal vitamins to help reduce neural tube defects. Vitamin D is the rickets.
The article says that the earliest humans in Africa (pre-modern humans) were lighter skinned but as they lost body hair became darker skinned to reduce exposure to strong ultraviolet sunlight. Why? Because UV light decreases the amount of B in a woman's body, leading to more neural tube defects in children. IOW, there was evolutionary pressure in favor of darker skin and inhibiting lighter skin. This pressure goes away in Europe.
The UV connection with Vitamin D has the evolutionary pressure favoring lighter skin in Europe because D is needed for strong bone growth. Dark skin blocks UV from the Sun, leading to weaker bones in lower-light climates like Europe.
This all sounds perfectly plausible. I don't have the biological credentials to judge whether its accurate or not. Still, are these two pressures strong enough to produce such significant changes in only 30K years???
Yes, that's what the article I found assumes. The earliest humans (pre-modern man), were lighter skinned but had much more hair. As the amount of hair decreased, the skin darkened to reduce overall UV exposure. The mechanism was intense UV sunlight destroying vitamin B (folic acid) in lighter skinned, pregnant women's bodies, thus causing neural tube defects in babies.
Note: apparently there are documented cases of white women using tanning salons while pregnant and having NTD babies possibly due to the reduced B. The bad thing is that B is most important in the early stages of a PG, before it becomes obvious. Yikes!
As I understand the studies, a very large majority of European and North African (semetic?) people share a common marker. That marker is not found in others, so a break away by people of common heritage is implied.
Further, it is possible (?) to crudely date the arrival of that marker in places that form a pathway out of Africa, into the middle east & south china, and then into europe. Other studies have identified migration of he same marker into south America at a date well before the advent of "Native Americans" because...
The native americans appear to have used a different and later route which can also be mapped using similar tracking methods to map a different mtDNA marker.
I don't see much of a hoax in that and the possibility that both male and female ancestors might contribute to mtDNA is a red herring - if a marker exists and can be passed on, it seems a valid clue.
It is, after all, merely informed guess work.
According to anthropologist Marvin Harris (bless his soul), yes. He said that at some point both these features became sexual preferences. In Europe, lighter skin produced healthier babies and light skin became beautiful. The opposite was occurring in Africa where those with lighter skin were getting ugly and deadly skin cancer and blacker became beautiful.
In ancient times, due to starvation, mothers would have to choose which child to feed and save...they would choose the healthier. There were many such pressures working on skin pigmentation.
The African Bushmen are physically different than all other humans on earth. The females have an 'apron' over the genetial area and the males have a semi-erect penis. Also, the Bushmen are lighter skinned, have some features resembling Mongoloids and their children even have 'Mongoloid spots' which is common among the Asians of today. (The 'spots' disappear with age on all)
Seems like very good speculation. It provides a mechanism to favor specific characteristics *and* inhibit others. I think the main problem in teaching evolution is the way it's described. Evolution, imho, is due to the killing off of less favored by pressure. Europeans aren't white because they developed that way, its that the ones with darker skin died off. The reverse is true in equatorial regions. I think that current Evolution teaching implies that the genes of offspring are *changed* by the evolutionary pressure.
I guess the thing I wonder about most is this: what evolutionary pressures are working today? Who is being favored by them? When will the next jump occur (like that from pre-modern to modern man)? Will it be natural or manmade?
That is good. But it places the birth of agriculture in a different spot than I thought.
You're confusing the data with the popular presentation. The latter is a hoax, regardless of your unfamiliarity with it.
Fascinating stuff. I read up on the Khosians. When showed pictures of other ethnic groups, they identified the Vietnamese as being the same as themselves. If you look at them they do bear some resemblance, and look more like them than they do their classical negro neighbors. It makes me think that both very dark and very light humans are the offshoots. The skull shape and other mongoloid traits are a different matter though.
I've read 2-3 books on them myself. They are the ancient people of Africa but, their myths would place their origins somewhere around the Mediterranean. I've read (somewhere?) that one anthropologist speculates that they are the leprachauns(sp) of Irish legend. They do have pointy ears, are reclusive and dance around fires all night, lol.
Whether or not the mtDNA represents the only true human woman (with some leakage from the males) alive or simply the tiny minority of women whose genes survive to the present day is purely a matter of faith. How can we ever know which is the case?
I am more concerned that they get the dates right. There is a lot of pressure to keep dates old enough so that they seem "respectable" to the evos. What is instead of "the first (surviving?") woman " being around 150 years ago it was only 50,000? The article assumes the Quazfeh finds are modern humans, but they don't look modern to me, and there is no way to check them by DNA analysis.
The whole dating thing is based on mutation rates. How do we know mutation rates today have been constant for the last 150,000 years? The Vela Super Nova sent cosmic rays at highly elevated levels about 30,000 years ago. That should have raised mutation rates for many thousands of years. If the male DNA can enter the mitochondria, wouldn
't that cause more "changes" than the chance mutation rate? The more changes it has, the older they assume it is.