Science: Let there be light

Was the Cambrian diversity of life `switched on'

by colour vision? The book of Genesis may have

had the answer all along.

It was arguably the most important event in the history of life on Earth, yet biologists have been scratching their heads over it ever since Charles Darwin formulated his theory of evolution.

Why did life bump along for thousands of millions of years with nothing much more complicated than a jellyfish ever evolving, until suddenly, about 570 million years ago, it went through an explosive period of immense diversity that can still be seen in animals today?

Nobody has ever really explained this ``Cambrian explosion'' satisfactorily, but one British-born scientist working in Australia believes he has the answer.

Andrew Parker, a researcher at the Australian Museum in Sydney, believes the explanation for the huge diversification of life during the Cambrian period was the sudden invention of colour vision. Parker believes it was like a ``light switch'' that turned the spotlight on living organisms and ultimately inspired the huge range of body plans that exist today in animals as diverse as butterflies and beluga whales.

At the heart of his theory is the discovery of microscopic grooves on fossils of the Cambrian period, which, Parker believes, had the same effect on light as the grooves of a compact disc. When light shines on these grooves, it becomes diffracted - reflected in a distorted way - so that the surface shines with bright, iridescent colours. It is the same physical phenomenon that makes a CD or a hologram on a credit card shimmer like a rainbow.

Parker's controversial idea is simple enough. With the sudden appearance of bright coloration in the otherwise drab world of early life forms, a new dimension was added to the forces of natural selection, the power behind evolutionary change. Colour vision made it easier to see and be seen, which is good for attracting mates and detecting food, but bad for prey species who want to avoid predators. As Darwin himself remarked in his 1859 classic The Origin of Species: ``Whenever colour has been modified for some special purpose, this has been, as far as we can judge, either for direct or indirect protection, or as an attraction between sexes.''

The importance of the Cambrian explosion in the history of life cannot be underestimated. For about 3.5 billion years, evolution had produced nothing particularly complicated in terms of body architecture. Suddenly, a biological explosion occurred, when the Cambrian period started. (It takes its title from the ancient name for Wales, where rock sediments of this age were first identified in the 19th century).

Biologists called it the Big Bang of evolution, but were puzzled as to what had caused it. ``With few possible exceptions, the Cambrian explosion represents the most dramatic event in the history of life on earth,'' Dr Parker says. ``However, until recently we could not explain what lit the Cambrian fuse, and why evolution did not continue at this frantic pace.''

It was a period when, in a relatively short period of less than 10 million years (a blink of an eye, in terms of life's long history), living organisms underwent an astonishing change in diversity.

Before the Cambrian explosion there were only one or two basic phyla - body plans for living organisms. Within the space of a few millions of years, however, the Cambrian explosion had resulted in the creation of 38 new phyla of animals - and no more phyla have come about since then.

Before the Cambrian period, animals floated around as mere blobs of living tissue, or lived virtually sedentary lives on the sea floor; they were no more animated than the average bathroom sponge. After the explosion of diversity, life forms walked and crawled, swam and burrowed, and generally took on the diverse array of behaviours and lifestyles that we now associate as marking the essential difference between animal and vegetable life forms.

The most important source of information about what happened in the Cambrian period comes from a quarry in the Canadian Rocky Mountains. Excavations there have exposed a large slab of sedimentary rock known as the Burgess Shale. It is within this rock that scientists have discovered the diverse nature of Cambrian life forms.

Simon Conway Morris, a Cambridge palaeontologist who has made a lifetime's study of the Cambrian period, says in his latest book, The Crucible of Creation, that the Burgess Shale is a monument to the understanding of how life evolved. ``It represents a reference point and a benchmark, a point of common discussion and an issue of universal scientific interest. Just as Darwin's finches on the Galapagos Islands exemplify the recognition of the central role of adaptation... so the Burgess Shale is becoming the icon for those who study the history of life,'' writes Conway Morris.

The unique nature of the Burgess Shale fossils is that they have preserved the soft body parts of an astonishing array of animals whose bizarre appearance are straight from the pages of science fiction. There is the mysterious Hallucigenia, so-called because it reminded biologists of a bizarre dream; the creatures walked on stilt-like legs, with flexible siphons emerging from their backs. There is Anomalocaris, a voracious predator that swam in the shallow seas of the Cambrian using wing-like fins. Anomalocaris had two vicious jaws for gripping and crushing prey and large eyes for detecting its next meal.

It was another bizarre animal from the Cambrian, called Wiwaxia, that has inspired Andrew Parker's light-switch theory. Wiwaxia looks like a Viking helmet, with two sets of spines emerging from the side. Experts such as Conway Morris believe that it crawled slowly on the sea floor, and used its spines for protection against predators.

Parker found microscopic grooves in the spines of Wiwaxia which gave the surface a corrugated appearance and presumably were initially used for strengthening. But because the distances between the grooves are on a par with the wavelength of light, Parker believes the grooves would also have acted as a diffraction grating, distorting white light into a range of iridescent colours. According to Parker, this development would given Wiwaxia an advantage because the bright colours would have advertised its defensive spines, and so warned off predators such as Anomalocaris.

Parker has found similar diffraction gratings on other animals of the Cambrian period, and believes he has uncovered a crucial development that accelerated the evolution of colour vision and, with it, the adaptation of new lifestyles and, ultimately, a range of radically different types of animals.

``Light, in general, is the most powerful stimulus in most of today's environments," Parker says. "Before the Cambrian, light, though present, could not have had a major effect on the lives of multicellular animals because they were all blind.''

``At the beginning of the Cambrian, the lights were effectively `turned on' for these animals. Then, for the first time, animals would have been `lit up', and therefore obvious to predators.

"Animals suddenly had to adapt to a stimulus that was to become the most important of all: light. The inevitable chaos resulted in an explosion in evolution,'' he says.

Experts in evolutionary theory are taking Parker's idea seriously, although they are understandably reluctant to give him their full backing just yet. Stephen Jay Gould, the Harvard evolutionist who popularised the Burgess Shale story in his award-winning book, Wonderful Life, says the idea is ``most interesting''. Derek Briggs, professor of palaeontology at Bristol University, who knows more about the Cambrian period than probably anyone else, says that Parker is on to something. ``This may well have been part of the story,'' he says.

If Briggs is right, then Parker may have stumbled across the answer to a problem that has foxed generations of biologists.

The most important moment in life on Earth may turn out to have been the turning on of a simple light switch that suddenly bathed the biological world in all the colours of the rainbow.