Out of Africa
The Great Human Diasporas
The History of Diversity and Evolution
Perseus Publishing, 1996
About the Author: Luca Cavalli-Sforza is a geneticist who has spent his career studying human evolution. In his book The Great Human Diasporas he gives an account of our journey out of Africa that includes not only the biological point of view, but also from that of archaeology, social anthropology, linguistics, and others, in order to reduce the uncertainty of his conclusions. One of the questions he asks is “Can the history of humankind be reconstructed on the basis of today’s genetic situation?”
THE GENETIC TREE
Using sophisticated mathematical analysis of genetic data, Cavalli-Sforza and his team constructed a human evolutionary tree, one which has been confirmed and refined by the analysis of much additional data.
Plotting the genetic tree on a world map produced an approximate idea of the routes taken by modern humans in their expansion. The forks in the tree corresponded to geographical separations of populations. The sequences of the branches corresponded to that of the separations, and the position and length of the branches to the time in which the splits occurred.
Cavalli-Sforza and his team found that: the biggest branches are between Africans and non-Africans, reinforcing the prevalent view that the human species originated in Africa and later spread around the world. Non-Africans inhabit two major branches: those who lived in Southeast Asia who most likely reached Australia, New Guinea, and the Pacific Islands from there; and those who populated Northern Asia and headed eastward through Siberia to America or westward into Europe and Southern India. The extreme poles of human variation were the Africans on one end and the New Guineans and the Australian Aborigines on the other.
In trying to determine when the human populations separated, the most reliable dates concern the occupation of new continents. So far, four dates look reasonably reliable, though these can change as a result of future research.
The oldest modern human remains (the Omo remains) date from about 195,000 – 100,000 years ago and were found in southern Ethiopia. Early skulls from Africa show definite trends toward modern human forms, suggesting that Homo sapiens sapiens was born in Africa. The presence of modern human sites in the Suez area one hundred thousand years ago suggests that the journey from Africa into Asia (or less probably vice-versa) occurred at about that time, so the first diversification between Africans and non-Africans would have taken place then or a little beforehand.
(The links in this section will take you to Wikipedia. Depending on your browser preferences, close your browser window or tab to return to The Human Journey.) In human genetics, a human mitochondrial DNA haplogroup is a haplogroup defined by differences in human mitochondrial DNA. These haplogroups have led some researchers to trace the matrilineal inheritance of modern humans back to human origins in Africa and the subsequent spread across the globe. Known haplogroups are assigned the following letter codes: A, B, C, CZ, D, E, F, G, H, pre-HV, HV, I, J, pre-JT, JT, K, L0, L1, L2, L3, L4, L5, L6, L7, M, N, P, Q, R, S, T, U, UK, V, W, X, Y, and Z. The woman at the root of all these groups was the most recent common matrilineal (female-lineage) ancestor of all living humans. She is commonly called Mitochondrial Eve.
Human remains in Australia and New Guinea were dated at 55-60,000 years ago, and the team found that the genetic distance between the Oceanian Aborigines (a term for the diverse Australian and Papuan Aborigines) and the Southeast Asians is about half that between Africans and non-Africans, and their date of entry into Oceania (50-60,000 years ago) is about half that of finding modern human remains in Africa and the Middle East.
Using the same model, Europe was first occupied probably from Western Asia 35-40,000 years ago, and the occupation of America, still somewhat unclear, occurred between 15,000 and 35,000 years ago.
Since genetic distance increases with separation time, Cavalli-Sforza and associates constructed a table showing separation, date of separation, and genetic distance, using the first separation as 100.
These dates are approximate, and the statistical margin of error is still high. Yet the first three dates clearly support the idea that genetic distance increases regularly and proportionately to separation dates. The last comparison is too imprecise to be reliable. But if the genetic distance is used to calculate the date of the occupation of America, using the first three comparisons as a basis, they concluded that the date was 30,000 years ago, which is well within the range suggested by archaeologists.
Race and Racism
The concept of race itself is complex as illustrated by the attempt to answer the simple question, how many races exist on earth? We have no real answer. Each classification seems equally arbitrary. The idea of race in the human species serves no purpose. The structure of human populations is extremely complex and changes from area to area; clear distinctions are impossible.
Racists often worry about racial purity, but there are no pure races. Every group of people, no matter how small, is genetically variable. Genetic purity simply does not exist in human populations. In general, marriages between people of very different origins create a more robust line of descendants. There is absolutely no known biological disadvantage to interracial marriage.
Racism is the conviction that one race is biologically superior to the others. That is what underlies racists’ concern for the purity of the race: they do not want this superiority to diminish. But we know that no race is pure, so to think about conserving purity is absurd.