Born in Africa_ The Quest for the Origins of Human Life - Martin Meredith [87]
But the most dramatic movement of all was by a small group which left Africa altogether.
By 60,000 years ago, Homo sapiens had long been accustomed to a beachcombing lifestyle, exploiting food resources from the sea. During periods of prolonged drought in the interior, Africa’s coastal terrain offered a ready refuge. Several sites in South Africa—Klasies River, Blombos Cave and Pinnacle Point—have yielded evidence of seaside living. Another site which gained attention in 2000 is the Abdur reef in Eritrea, on the western shore of the Red Sea. A research team led by an American geophysicist, Robert Walter, reported discovering large middens, or refuse dumps, of shells from clams and oysters, interspersed with obsidian flake tools, dating from about 125,000 years ago. What researchers found of particular interest is that the Abdur reef is in the same neighbourhood as the location from which a group of African emigrants are thought to have crossed from Africa into Arabia.
At the southern end of the Red Sea, as it narrows at the straits of Bab al-Mandab, the distance that separates the coast of Africa from the Arabian peninsula is only fifteen miles. At times of global cooling in the past, when the Red Sea has fallen to levels some 200 feet lower than today, the distance would have been no more than seven miles, with a scattering of islands and reefs offering stepping stones across the straits into new terrain. This crossing point is considered the most likely route taken by Africans expanding into the world beyond.
The group making the crossing, according to genetic evidence, numbered only a few hundred. One estimate by geneticists in 2005 using mitochondrial DNA data put the figure at most as 550 women of child-bearing age, and probably far fewer. Another estimate suggests a much smaller number, a total of no more than 150 emigrants. What is certain is that the emigrant group contained only a fraction of the full genetic diversity of the existing African population. From the point of their departure, there is a clear divergence between the genetic inheritance of Africa’s population and the population of African emigrants who went on to populate the rest of the world.
All women in sub-Saharan Africa belong to one of three branches of the mitochondrial-DNA tree, known as L1, L2 and L3; all women outside Africa belong to two daughter lineages of L3, known as M and N. The likelihood is that a branch of L3 left Africa in a single migration, giving rise to M and N as they made their way eastwards to the Indian subcontinent. A similar pattern emerges from Y-chromosome data. All male lineages outside sub-Saharan Africa carry a Y-chromosome mutation known as M 168.
Most geneticists agree that the crossing took place between 60,000 and 50,000 years ago. A study in 2007 led by Andrea Manica of Cambridge University put forward a date of 55,000 years ago. Generation by generation, the emigrants expanded along the coastlines of southern Arabia to India, leaving behind their genetic footprints. Traces of the mitochondrial M lineage are still commonly found in southern Arabia. Y-chromosome genealogy shows that M 168 soon began to diversify. One branch of emigrants known as M 130 continued their coastal journey eastwards from India, eventually reaching the lost continent of Sunda—the single land mass at the southern end of Asia that once incorporated the Malay Peninsula and the islands of Sumatra, Java and Borneo. Beyond Sunda, some crossed by raft or boat to Australia, then part of another lost continent, Sahul, that included New Guinea and Tasmania. About 60 per cent of Australian aboriginal men have an ancestry directly linked to M 130.