SCIENCE

Human Migration: ‘Out of Africa’
By Dr. Rizwana Rahim
P. O. Box 60267, Chicago
IL 60660-0267

The completely sequenced human genome shows that we -- some six and a half billion Homo sapiens sapiens on earth -- are 99.9% genetically alike. The rest (0.1%) accounts for our differences (facial features or physiognomy, color of our skin, eyes, hair, etc). But where did we all come from is still being debated.
The view held by most paleo-anthropologists is that modern humans came ‘out of Africa’ roughly 200 thousand years ago (TYA) – not in just one big wave but several small ones over time. And, this has been increasingly supported by genetic information, in addition to fossil remains found around the world. [The other view, ‘multi-regional’ theory, that evolution occurred concurrently in different places around the world, lacks similar genetic support].
In these population genomics studies, two genetic approaches are involved. One deals with analysis of DNA in mitochondria, which are mostly responsible for cellular metabolism and energy. Each cell has many mitochondria in the cytoplasm outside the nucleus. Each mitochondria contain its own DNA, and since it is completely sequenced (only 16,500 bases; much smaller than nuclear DNA) it helps us follow the mutations it accumulates over time. While nuclear DNA of father recombines with mother’s, mtDNA does not. Instead, mtDNA with its accumulated mutations is passed intact from the mother to the child. Even though focusing on a small fraction of mitochondrial DNA (mtDNA) where mutation rate is highest (‘the hot spot’ or ‘D-loop’, about 7% of total mtDNA) is criticized, but the rest of mtDNA (15,345 bases) is not all that big either to follow and normalize the rate either.
The other approach deals with Y-chromosome (the chromosome found only in males); most of it is transferred from father to son.
These two lines of evidence go all the way to our ‘initial’ ancestors. Mutations occurring at steady frequency over time in subsequent generations over thousands of years and all around the world are the telltale signs or the markers used in tracing the history. So, following the markers in populations around the world, one can make some ‘educated’ guess of the generalized human wanderlust.
Comparing mtDNA of women around the world, it was found that the diversity in African women was twice as much as found elsewhere. This led the scientists to suggest that modern humans could have lived twice as long in Africa as anywhere else. Based on other information and extrapolating back in history, they estimated that all human beings may be descendants of a ‘Mitochondrial Eve’ who must have lived with a “Y Chromosome Adam’ in Africa, some 171.5 thousand years ago (TYA). There were other African hunter-gatherers ancestors, but the hereditary chain of mothers going back to that ‘Eve’ seems unbroken. Ancestral DNA markers seem to be present most often in some African tribes: San (southern Africa), the Biaka Pygmies (central Africa) and in a few East African tribes.
The oldest modern human bones were found in Omo Kibish, Ethiopia (estimated age 195 thousand years old (TYO). Another relic found in Klasies River Mouth in southern Africa is estimated to be 120 TYO.
What seems clearer now is that between 50 to 70 TYA, a small wave of these Africans (about 1,000) may have been first to migrate out of Africa. These emigrants carried a genetic marker (M168) on their Y chromosome, which is shared by all non-Africans. They followed three generalized paths out of Africa. They didn’t have to cover long distances in each generation, but over the millennia (and gradual acclimatization, development of tools, other skills, etc) they can spread far.
One path, up the Nile Valley, across the Sinai Peninsula, north on to the Levant: Artifacts have been found in Qafzeh cave, Israel (several human remains, 100 TYO, discovered in the 1930s), and Pestera cu Oase (human mandible discovered in 2002 in southwestern Romania, about 40 TYO) and farther into Europe. There was some overlap in Europe with Neanderthals, but there was no interbreeding, and the modern humans had genetically diverged from Neanderthals 500 to 600 TYA.
Another route took them to north of Asia through Siberia to the Bering Strait and across to Alaska. Some of the notable finds on this route have been Zhoukoudian (Shandingdong) ‘Peking Man’ skull discovered in 1929, some 11-18 TYO, and human tools and artifacts, 30-40 TYO, in Yana River, north in the Arctic Siberia. These people in Northern Asia were the ones that migrated into the Americas.
When did this all occur is still being unclear, but possibly between 15-20 TYA, when the sea-levels were low (during the last Ice Age) and Siberia was connected by land to Alaska. People took the West coast routes, because at that time most of North America was covered with ice. The sites of human artifacts, from north to south America, include: Kennewick, WA (Kennewick Man, discovered in 1996 (9.5 TYO), Spirit Cave, a burial ground in Nevada (9.4 -9. 5 TYO), Clovis, a spear-point found in New Mexico (13.5 TYO), Meadowcroft Rockshelter, on the Ohio River in PA (12-19 TYO) and Monte Verde in southern Chile (14.8 TYO). This migration occurred in not just one but two to three distinct waves (15-20 TYA).
An old genetic marker (M9) is found common in the Y chromosomes of Eurasian populations of Central Asia and Middle East. The genetic data also shows similarities between the western Eurasian groups and people of India; it is possible that there was some migration between Asia and Europe, 30-40 TYA.
For the path through Asia to Australia, migrant Africans probably walked across the mouth of Red Sea (between the Horn of Africa and Arabia), which was probably frozen 70 TYA when the last Ice Age began. Perhaps they took a coastal route along the Arabia Peninsula, to India and then (with more tools and skills) to Australia. Their path is also strewn with some fossils and other relics: in Malayasia (human skull and others in Niah Cave; 40 TYO), and two in Australia (Malakunanja near the northern tip of Australia; foot prints and other relics; 50 TYO) and in Lake Mungo in southeastern Australia (human skeletons cremated remains in Lake Mungo, found in 1969 of a man and a woman in western NSW, 45 TYO, with underlying layers of soil bearing older artifacts). This may be the earliest evidence of modern humans in Australia. Except for genetic traces in the Australian aborigines, and in some indigenous groups (along the way in the Andaman Islands, Malaysia, Papua New Guinea), no other signs are yet found of their 8,000-mile journey from Africa to Australia.
The 5-year ‘Genographic Project’ launched by the National Geographic magazine and other sponsors is designed to pursue the population genomics, with the help of 11 centers worldwide where the geneticists will draw DNA from the blood and cheek swabs of the people from at least a thousand indigenous populations (as well as from fossils found). By following mutations in populations and piecing together a line age, they plan to reconstruct a molecular phylogenetic tree and show human migrations around the planet. National Geographic (March, 2006) has a cover story on this project and TIME magazine (March 13) has an exclusive on Kennewick Man.
References:
1. National Geographics: ‘The Greatest Journey Ever Told: The Trail of our DNA. March 2006
2. TIME Magazine: The Untold Saga of Early Man in America, 13 March 2006


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