As usual we need much
more data but we do know two things.
1
A small population entered Eurasia and
immediately generated three separate linages that continued coherently
thereafter while rapidly populating every plausible niche in a short period of
time. As they were hunter gathers, they
delineated a hunting range and calved to enter new hunting ranges. Living was good and it is safe to presume
that this process was generational. Each
generation calved and struck out for a new range. As a range could easily be two or more
thousands of square miles early on, it is not hard to see a rapid expansion.
2
Any population expansion occurred through
the capture of females. It appears that
the Neanderthals did not succeed in this.
Thus it appears that this was an aggressive overlay of established
populations that bypassed, and competed but ultimately overran and absorbed as
populations grew.
As usual this is the
description of technical advantages been exploited and not countered before a
population advantage makes it all mute.
In our own world, indigenous peoples have adapted but in the face of a
hundred to one population imbalance, must in time be substantially absorbed
into the homogenous whole. It can take a
thousand years but it is inevitable as the imbalance is simply too great and
genetic enclaves are presently avoided.
Arabian flights: Early
humans diverged in 150 years
Humanity's
dramatic race across the Old World after it left its African cradle has been
told countless times. But for a true sense of the rapidity of events, look no
further than the Y chromosome. The most comprehensive analysis of the Y yet
shows that within 150 years, an evolutionary blink of an eye, the first
migrants to make it into Eurasia split into three distinct groups that can
still be identified today.
Men
inherit their Y chromosome from their fathers. Like all other chromosomes, the
Y mutates over time so the more distant the relationship between two men, the
more genetically distinct their Y chromosomes. In 2000, biologists
used this fact to construct a family tree of all men, which shows how human populations
around the world are related.
But
this tree was built using the information from a small samples of DNA on the Y
chromosome, which revealed only around 100 sites of genetic variation that
could help establish familial relations. Advances in gene technology mean it is
now possible to build a more comprehensive tree based on information from the
entire Y chromosome.
69
dudes
David
Poznik and Carlos
Bustamante at
Stanford University in California, together with their colleagues, analysed the
entire Y chromosome of 69 men from Africa, Eurasia and Central America. They
found more than 11,000 sites of variation, allowing them to build a much more
detailed tree.
For
ideas on how he could use the new tree, Poznik talked to Peter Underhill, another researcher at Stanford.
Underhill has been studying the Y chromosome for years and helped build the
first Y tree in 2000.
"He'd
always had in the back of his mind this problematic area in the tree that he'd
never been able to crack," says Poznik.
That
area of the Y tree describes the crucial period when modern humans began going
global, sometime after they left Africa between 70,000 and 50,000 years ago.
The earlier family tree showed that the three major Eurasian genetic groups
that survive today emerged from those pioneering migrants, who were probably
living in the Arabian peninsula. But exactly how the ancestors of those three
groups relate to one another, and how quickly they diverged after leaving
Arabia was not resolved.
The
new analysis reveals that just a single genetic mutation differentiated the
three when they first arose. "The fact that we have exactly one variant
separating the three indicates a timescale on the order of 150 years for the
split," says Poznik.
A
mountain high enough
Those
near-identical genetics imply that the early Eurasians split into two
populations, and then in little more than a century – before those two
populations could accrue more than one new mutation – one of them split in two
again to create three in total. Although these populations probably appeared
around 50,000 years ago, cultural or geographical barriers like mountain chains
have kept them recognisably distinct to this day.
The
speed of these events make sense, says Ornella
Semino at
the University of Pavia in Italy. "When modern humans spread out of
Africa, they encountered multiple different environments that stimulated the
fragmentation of the first migrant group," she says.
The
first Eurasians were significant for another reason: their pioneering
exploration brought them into contact with
Neanderthals for the first time.
Studies within the last few years have shown that interbreeding did occur between the
two species,
although there is no evidence a Y chromosome from a male Neanderthal passed
into the Homo sapiens gene pool.
Or
rather, there is no evidence so far. "In theory there may be a Y
chromosome from a Neanderthal still present among humans," says Bustamante
– especially since a different analysis earlier this year suggested,
remarkably, that a Y chromosome from another
now-extinct species of human might be circulating in today's male population.
Michael Hammer at the University of Arizona
in Tucson and his colleagues identified a Y chromosome unlike any other in a
small group of people who can trace their roots back to Cameroon.
It
is so different from any other Y chromosome so far found that it may actually
have entered the modern gene pool via an ancient human that lived in western
Africa, says Bustamante. "It might be our first glimpse of archaic
interbreeding within Africa."
Hammer,
who was not involved in the new study, says the unusual Y chromosome he found
is too rare and too new to science to have featured in the latest analysis. But
Bustamante says he would "absolutely love" to add it to the data set
in future.
No comments:
Post a Comment