Text 144, 116 rader
Skriven 2004-09-23 16:56:00 av Anthony Campbell (1:278/230)
Ärende: Book review: In the Blink
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Andrew Parker

IN THE BLINK OF AN EYE

The cause of the most dramatic event in the history of life

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Book review by Anthony Campbell. Copyright © Anthony Campbell (2004).
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The dramatic event referred to in the subtitle of Parker's book is the
"Cambrian explosion". A period of five million years from 543 to 538
million years ago (a very short time in geological terms) witnessed the
appearance of the huge variety of animal forms we see today. To judge by
external form, before 544 mya there were three animal phyla (major
groups), but by 538 mya there were 38, most of which still exist today.
The puzzle that Parker claims to have solved is why this happened.

A variety of explanations have been advanced previously but none has
obtained general agreement. In outline, Parker's solution is that the
dramatic change occurred thanks to the evolution of complex eyes. This
in turn enabled predators to become more effective and so the prey
species had to evolve physical and behavioural defences. Hence, he
thinks there was an arms race which accelerated hugely when some of the
contestants acquired vision.

Parker develops this argument in detail in the course of his book. He
writes well (though I wish he hadn't confused "infer" and "imply");
whether his theory is correct or not, he has a fascinating story to tell
and he provides an impressive amount of interesting facts in leading up
to his conclusion.

An important element in his argument requires the appearance of new
forms in the Cambrian explosion to have been less sudden than is
sometimes supposed. The development of the different phyla was, he
believes, taking place for a long time in the Precambrian but at this
time the animals were soft-bodied and most of the changes were internal.
What happened in the Cambrian was the development of new external body
parts, particularly body armour, in all phyla in response to the
increased predation made possible by the dawning of vision.

Evidently, for this theory to be right Parker has to demonstrate that
vision was in fact lacking in the Precambrian, and much of his book is
concerned with various aspects of the evolution of sight. The evidence
for this is in part direct, being based on when eyes first appear in the
fossil record. But he also has an ingenious indirect argument, which
depends on discovering when colour adaptations first appear. This might
seem like a hopeless quest, since we cannot expect to see the colour of
Cambrian organisms directly. However, Parker has found adaptations in
the hard parts of some Cambrian animals which, he claims, can only have
existed in order to produce colours. The colour of certain present-day
creatures such as beetles and butterflies depends on effects of this
kind. He has conducted experiments to test this part of his theory.

The earliest animal with eyes seems to have been a trilobite, and this
was a predator. Other early trilobites with eyes were probably also
predators, and preyed on one another; hence they developed protective
shells although these were not as hard or spiny as those of later
trilobites. The trilobite eye was a remarkable structure, which Parker
discusses at length, and he also provides details of the construction of
a variety of other kinds of eye that have evolved.

Even if this theory is correct, we still have to explain what caused the
development of the eye. Parker discusses this question briefly at the
end of his book. One possibility is an increase in solar radiation -
more light, in fact. Another idea is an increase in the transparency of
the sea (all life was marine at this time). He touches also on the
"snowball earth" hypothesis (the idea that all, or almost all, the earth
was encased in deep ice at certain remote periods), but he thinks that,
even if this occurred, it would have been too early to affect the
Cambrian explosion although it might have played a part in the
Precambrian.

23 September 2004

See also Trilobite! (Richard Fortey)
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%T In the Blink of an Eye
%S The cause of the most dramatic event in the history of life<
%A Andrew Parker
%I The Free Press
%C London
%D 2003
%G ISBN 0-7432-5733-2
%P xviii + 361 pp
%K evolution
%O paperhack
%O illustrated
                                                                               

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