Article: Molecular clock tied to fossil record

From: Robert Karl Stonjek (rstonjek_at_bigpond.net.au)
Date: 10/18/04 

Date: Mon, 18 Oct 2004 21:55:04 +0000 (UTC)

Molecular clock tied to fossil record
Emma Marris
Evolutionary trees may finally provide answers everyone can agree on.

A way to date prehistoric events using molecules from living creatures is
finally becoming precise enough to be useful. A team of scientists has
improved on a 'molecular clock' system that can fix a rough date for the
last common ancestor of two separate species.

Determining when two branches of living things parted company is not an easy
task. For more recent events, or for bigger animals, there might be a fossil
record: a set of bones that represent a last common ancestor or first
separate species. These can then be dated by the rocks around them or by
carbon dating.

But what if there is no fossil record? Several decades ago it was first
proposed that if DNA accumulates mutations at a constant rate, then you
should be able to measure the differences between the DNA from two
present-day species and extrapolate back to a time when the DNA was
identical - to when one species became two.

There were problems, though. The rate of change was first calculated for
vertebrates, using fossil vertebrates to calibrate the scale, but it then
turned out that evolution progresses at a different rate in different groups
of organisms, so the vertebrate rate gave wacky dates when applied to
anything else. And it was not clear how constant the rate of mutation was
over time for any group.

Worse, dates given by the molecular clock consistently disagreed with the
fossil record, tending to give estimates that were much older, by as much as
several hundred million years.

Full Text at Nature
http://www.nature.com/news/2004/041011/full/041011-2.html

Posted by
Robert Karl Stonjek

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