Andrew Parker

IN THE BLINK OF AN EYE

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

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)