Pushing back the origins of animals
ELEANOR LAWRENCE
If a recent analysis of animal evolution is correct,
then the famed
"Cambrian explosion" in the evolution of multicellular
animals was
not so much a Big Bang as simply the end of a long,
slow, crawl.
In the most recent issue of The Proceedings of the Royal
Society
of London, Daniel Wang and colleagues from Pennsylvania
State
University in the USA have looked at the molecular
evidence for
the timing of animal evolution. By looking at the
molecular clocks
represented by differences in the DNA sequences of some 50
genes in present-day animals, they have pushed the time
of origin
of the major animal groups (phyla) hundreds of millions
of years
further back into the Precambrian.
The apparent Cambrian explosion is one of the most
intriguing
events in evolutionary history. The Cambrian period
began 543
million years ago: about 530 million years ago, most of
the main
groups of present-day animals appear for the first time
in the fossil
record. Animals as diverse as worms, arthropods, and our
own
chordate ancestors all appear within a few tens of
millions of
years. With no apparent fossil forebears in the
Precambrian, a vast
amount of evolution and diversification of animal body
plans
seems to have been packed into a period of around 50
million
years.
Such a rapid and dramatic spurt of evolution is not easy
to explain.
Nothing like it has been seen since, and evolutionary
biologists
have often wondered whether there might be some alternative
explanation. Is the fossil record in the Cambrian just a
record of
the appearance of animals with hard skeletal parts that
fossilized
well? Is it just a multitude of tips of a flotilla of
hidden icebergs of
evolution, stretching back hundreds of millions of years?
Wang and colleagues' analysis supports the second
contention.
Instead of requiring arthropods (insects and
crustaceans) and
chordates (vertebrates and allies) to have evolved from
a common
ancestor in a mere few tens of millions of years, they
estimate that
these two lines diverged as far back as 993 million
years ago. This
gives these two very different groups of animals an
extra 400
million or so years of evolution to play with, which is
in many
ways a much more likely scenario.
Throwing their net wider, they place the fundamental
time of
divergence between animals, plants and fungi at around 1500
million years ago. Nematode worms come along at around 1200
million years ago, on their estimate, again much earlier
than the
fossil record would suggest. The most primitive animals,
the
sponges and coelenterates, would therefore have evolved
somewhere between 1500 and 1200 million years ago.
But although molecular clocks work fine in theory,
no-one knows
just how reliable they are in reality. The principle is
simple. Given
the same gene in two different animals, an insect and a
mammal
for instance, one can count the differences in the
nucleotide
sequences to estimate how long ago the ancestors of
those two
animals diverged. But previous estimates of animal
origin times
using these sorts of methods have yielded some wildly
different
timescales.
Fully aware of the problems, Wang and colleagues used a
large
number of genes in their analysis, far more than in
previous
analyses, and chose the genes carefully to avoid those
that might
not count time accurately. They calibrated their
molecular clocks
against well-established and uncontroversial timelines
in animal
evolution, such as the divergence of the reptilian
ancestors of birds
and mammals about 310 million years ago. And their
results are
indeed in rough agreement with at least some of the
previous
studies, which also place the time of animal origins far
back in the
Precambrian.
So the evidence is slowly mounting that animals were
around a
long time before the Cambrian. And this of course raises
an almost
equally difficult question ö where were they? Why is
there no
fossil record of these arthropod and chordate ancestors
in the
supposed 400 million years of their early evolution?
Were they all
soft-bodied animals that left no traces? Or, as Wang and
colleagues suggest, might they simply have been very
small? In
which case, their remains will only be found in extremely
fine-grained rocks, of which there are some examples of
suitable
age. The search is now on.
© Macmillan Magazines Ltd 1999 - NATURE NEWS SERVICE