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Thursday, June 4, 2009

The study of time as it relates to archaeology is, to coin a phrase,

The study of time as it relates to archaeology is, to coin a phrase,
moving right along. The pottery clock, the various "rock" clocks,
added to the radiocarbon system now have a companion in the
examination of the entire mtDNA molecule.

"We can settle the debate regarding mankind's expansion through the
Americas. Researchers have been estimating dates from mtDNA that are
too old for the archaeological evidence, but our calculations confirm
the date to be some 15,000 years ago, around the time of the first
unequivocal archaeological remains.


"Furthermore, we can say with some confidence that the estimate of
humanity's 'out of Africa' migration was around 60-70,000 years ago –
some 10-20,000 years earlier than previously thought."


New 'Molecular Clock' Aids Dating Of Human Migration History


ScienceDaily (June 4, 2009) — Researchers at the University of Leeds
have devised a more accurate method of dating ancient human migration
– even when no corroborating archaeological evidence exists.


Estimating the chronology of population migrations throughout
mankind's early history has always been problematic. The most widely
used genetic method works back to find the last common ancestor of any
particular set of lineages using samples of mitochondrial DNA (mtDNA),
but this method has recently been shown to be unreliable, throwing 20
years of research into doubt.


The new method refines the mtDNA calculation by taking into account
the process of natural selection - which researchers realised was
skewing their results - and has been tested successfully against known
colonisation dates confirmed by archaeological evidence, such as in
Polynesia in the Pacific (approximately 3,000 years ago), and the
Canary Islands (approximately 2,500 years ago).


Says PhD student Pedro Soares who devised the new method: "Natural
selection's very gradual removal of harmful gene mutations in the
mtDNA produces a time-dependent effect on how many mutations you see
in the family tree. What we've done is work out a formula that
corrects this effect so that we now have a reliable way of dating
genetic lineages.


"This means that we can put a timescale on any part of the particular
family tree, right back to humanity's last common maternal ancestor,
known as 'Mitochondrial Eve', who lived some 200,000 years ago. In
fact we can date any migration for which we have available data," he
says.


Moreover, working with a published database of more than 2,000 fully
sequenced mtDNA samples, Soares' calculation, for the first time, uses
data from the whole of the mtDNA molecule. This means that the results
are not only more accurate, but also more precise, giving narrower
date ranges.


The new method has already yielded some surprising findings. Says
archaogeneticist Professor Martin Richards, who supervised Soares: "We
can settle the debate regarding mankind's expansion through the
Americas. Researchers have been estimating dates from mtDNA that are
too old for the archaeological evidence, but our calculations confirm
the date to be some 15,000 years ago, around the time of the first
unequivocal archaeological remains.


"Furthermore, we can say with some confidence that the estimate of
humanity's 'out of Africa' migration was around 60-70,000 years ago –
some 10-20,000 years earlier than previously thought."


The team has devised a simple calculator into which researchers can
feed their data and this is being made freely available on the
University of Leeds website.
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