- 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.
January 2007 