Travel Reference
In-Depth Information
But as we move back, to a time before the Cambrian when the fossil record
becomes uncertain, it is as if we were walking of a clif . Without fossils and
relying only on molecular evidence, we can be certain that dif erent groups
of organisms had a common ancestor, but we are not sure when that ances-
tor lived. The further back we go, the greater the uncertainty over timing
grows.
Part of this is the fault of the DNA sequences, which can diverge at dif er-
ent rates. For example, when Kevin Peterson of Dartmouth University and
his colleagues built evolutionary family trees using vertebrate and mollusk
DNA, they found that the vertebrate branches of the tree were only half as
long as the mollusk branches. It seems that vertebrate DNA in general has
evolved at only about half the rate of mollusk DNA. We do not know why
these rates are dif erent, and why these dif erent rates have been maintained
for well over half a billion years.
Despite these dii culties, many groups of scientists have used the grow-
ing library of DNA sequences to probe the distant past. They have been cau-
tious, extrapolating back from well-supported dates, and using a variety of
assumptions about the rates of DNA evolutionary change. Some of these
studies estimate that the common ancestor of scuba divers and cuttlefi sh
might have lived as much as a billion years in the past. Others, using dif erent
statistical methods, arrive at the more recent date of 650 million years ago,
a mere hundred million years before the start of the Cambrian. Still other
estimates fall between these extremes. But none of the dates are so recent
that they fall within the Cambrian itself. The consensus is that these animal
lineages did indeed begin to diverge at some point in time well before the
start of the Cambrian.
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Thus, by the beginning of the Cambrian, the fossil and DNA evidence agree
that much diversifi cation had already taken place, though the exact nature of
that diversifi cation remains a mystery. At the time of the Cambrian explo-
sion, environmental changes allowed each of these already-divergent lineages
of small soft-bodied organisms to grow larger and evolve various hard parts
such as skeletons or shells, so that they were more likely to be fossilized. Thus,
the Cambrian explosion was not really sudden. In the memorable phrase of
Simon Conway Morris of Cambridge University, it had a long fuse.
