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Saturday, January 6, 2007
Reading List: Before The Dawn
- Wade, Nicholas. Before The Dawn. New York: Penguin Press, 2006. ISBN 1-59420-079-3.
- Modern human beings, physically very similar to people alive today, with spoken language and social institutions including religion, trade, and warfare, had evolved by 50,000 years ago, yet written historical records go back only about 5,000 years. Ninety percent of human history, then, is “prehistory” which paleoanthropologists have attempted to decipher from meagre artefacts and rare discoveries of human remains. The degree of inference and the latitude for interpretation of this material has rendered conclusions drawn from it highly speculative and tentative. But in the last decade this has begun to change. While humans only began to write the history of their species in the last 10% of their presence on the planet, the DNA that makes them human has been patiently recording their history in a robust molecular medium which only recently, with the ability to determine the sequence of the genome, humans have learnt to read. This has provided a new, largely objective, window on human history and origins, and has both confirmed results teased out of the archæological record over the centuries, and yielded a series of stunning surprises which are probably only the first of many to come. Each individual's genome is a mix of genes inherited from their father and mother, plus a few random changes (mutations) due to errors in the process of transcription. The separate genome of the mitochondria (energy producing organelles) in their cells is inherited exclusively from the mother, and in males, the Y chromosome (except for the very tips) is inherited directly from the father, unmodified except for mutations. In an isolated population whose members breed mostly with one another, members of the group will come to share a genetic signature which reflects natural selection for reproductive success in the environment they inhabit (climate, sources of food, endemic diseases, competition with other populations, etc.) and the effects of random “genetic drift” which acts to reduce genetic diversity, particularly in small, isolated populations. Random mutations appear in certain parts of the genome at a reasonably constant rate, which allows them to be used as a “molecular clock” to estimate the time elapsed since two related populations diverged from their last common ancestor. (This is biology, so naturally the details are fantastically complicated, messy, subtle, and difficult to apply in practice, but the general outline is as described above.) Even without access to the genomes of long-dead ancestors (which are difficult in the extreme to obtain and fraught with potential sources of error), the genomes of current populations provide a record of their ancestry, geographical origin, migrations, conquests and subjugations, isolation or intermarriage, diseases and disasters, population booms and busts, sources of food, and, by inference, language, social structure, and technologies. This book provides a look at the current state of research in the rapidly expanding field of genetic anthropology, and it makes for an absolutely compelling narrative of the human adventure. Obviously, in a work where the overwhelming majority of source citations are to work published in the last decade, this is a description of work in progress and most of the deductions made should be considered tentative pending further results. Genomic investigation has shed light on puzzles as varied as the size of the initial population of modern humans who left Africa (almost certainly less than 1000, and possibly a single hunter-gatherer band of about 150), the date when wolves were domesticated into dogs and where it happened, the origin of wheat and rice farming, the domestication of cattle, the origin of surnames in England, and the genetic heritage of the randiest conqueror in human history, Genghis Khan, who, based on Y chromosome analysis, appears to have about 16 million living male descendants today. Some of the results from molecular anthropology run the risk of being so at variance with the politically correct ideology of academic soft science that the author, a New York Times reporter, tiptoes around them with the mastery of prose which on other topics he deploys toward their elucidation. Chief among these is the discussion of the microcephalin and ASPM genes on pp. 97–99. (Note that genes are often named based on syndromes which result from deleterious mutations within them, and hence bear names opposite to their function in the normal organism. For example, the gene which triggers the cascade of eye formation in Drosophila is named eyeless.) Both of these genes appear to regulate brain size and, in particular, the development of the cerebral cortex, which is the site of higher intelligence in mammals. Specific alleles of these genes are of recent origin, and are unequally distributed geographically among the human population. Haplogroup D of Microcephalin appeared in the human population around 37,000 years ago (all of these estimates have a large margin of error); which is just about the time when quintessentially modern human behaviour such as cave painting appeared in Europe. Today, about 70% of the population of Europe and East Asia carry this allele, but its incidence in populations in sub-Saharan Africa ranges from 0 to 25%. The ASPM gene exists in two forms: a “new” allele which arose only about 5800 years ago (coincidentally[?] just about the time when cities, agriculture, and written language appeared), and an “old” form which predates this period. Today, the new allele occurs in about 50% of the population of the Middle East and Europe, but hardly at all in sub-Saharan Africa. Draw your own conclusions from this about the potential impact on human history when germline gene therapy becomes possible, and why opposition to it may not be the obvious ethical choice.
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