You can see it in their eyes

Andrew Parker reveals why scientists now believe that the arrival of creatures with sight kick-started evolution's 'big bang'

Around 530 million years ago, in the Cambrian period, the animal kingdom exploded into life. This was the time of the "Cambrian explosion" - evolution's "big bang". Prior to this animals were all soft-bodied, like worms, as they had been for the past 300 million years. Then, in less than five million years, nearly all the major animal groups on Earth today independently evolved hard parts.

Around 530 million years ago, in the Cambrian period, the animal kingdom exploded into life. This was the time of the "Cambrian explosion" - evolution's "big bang". Prior to this animals were all soft-bodied, like worms, as they had been for the past 300 million years. Then, in less than five million years, nearly all the major animal groups on Earth today independently evolved hard parts.

The big question is, why? What lit the Cambrian fuse?

When we consider another event of the Cambrian period, an interesting parallel emerges. Around 540 million years ago, the Earth was the land of the blind. Then something of note happened, something that would change life forever - an animal evolved image-forming eyes. That animal was a trilobite, a distant relative of spiders and shrimps. Before long it had also evolved swimming capabilities and strong, grasping limbs and mouthparts - it had become an active predator. The first active predator, with visual search capabilities on Earth.

The lights had been switched on for animal behaviour (sunlight did exist before this event, but it could not be used by animals to find or recognise each other). Now, if the first eye is added to the geological timescale, the order of events becomes the introduction of vision, first, followed closely by the Cambrian explosion, second. But followed how closely, precisely? Only fossils could answer that question.

It was the age, condition and diversity of the "Burgess shale" fossils that led to the formulation of the "light switch theory". These famous fossils were discovered nearly a hundred years ago in the Canadian Rockies. They represent a whole community of Cambrian animals, buried together in a mud slump to provide a rock photograph of community life at that time, with all manner of lifestyles in the shot.

That time, though, was 515 million years ago. Yes, the Burgess Shale fossils do inform that in the Cambrian, hard external parts had evolved in many animal groups, but some 15 million years after vision had been introduced. The evidence fitted, but there was something of a gap between the first eye and the Cambrian explosion event.

To strengthen the light-switch theory, that gap had to be filled. The search for more fossils began.

Some fossil "shells" were found that even pre-dated eyes. But these emerged only as soft bodied animals that became hardened when they died - they were altered by unusual preservation conditions. The animals were without shells when they lived.

Another, and younger, Burgess shale was still needed. This year, palaeontologists from Britain, China and Sweden published their combined reports of expeditions to the hills and lakes of Yunnan Province, southwest China. This region is home to the Chengjiang fossils, and these did bridge the gap. The Chengjiang fossils have emerged to rival those of the Burgess shale. Since the first specimen was found in 1984 we have known that they are preserved equally well as to reveal their finest, needle-like spines. But the new data shows that they also match the Burgess shale fauna for diversity. They provide another snapshot of life taking place in the Cambrian, but this time 525 million years ago. Now we know that the Cambrian explosion took place just after the introduction of vision. That's the evidence we needed.

So what animals are represented in the Chengjiang fossils? They may belong to the same groups of animals that exist today, but they certainly don't look the same. They appear as the most bizarre life forms one could ever imagine. Choia is a sponge, but looks like a tiny traditional Chinese hat with needles radiating from the edges; Xianguangia is a relative of today's jellyfish and corals, but looks like an octopus stuck headfirst in a beaker, with tentacles waving above; Microdictyon, a "velvet worm", appears like a caterpillar walking on long legs with rows of shields protecting its body from attacks from above.

Then there is a whole assortment of worms from other animal groups with spines projecting from various parts and into different directions. Again, even the tiniest spines, thinner than a human hair, are preserved in exquisite detail.

The arthropods, to which crabs and flies belong, are well represented in the Chengjiang fossils. Some are shaped like speed-cycling helmets. But one of the most bizarre is Occacaris, which can only be described as a swimming pea with a pair of beady eyes and bull's horns (antennae), and a pair of anthropomorphic "hands" and a fish's tail protruding from its two-part shell.

Enigmatic forms also exist, in that they are so peculiar we cannot even guess as to which animal groups they belong. One fossil is simply a ball of spines, another is a vase-shaped pin-cushion. Then there is a discus with a feather attached, along with a half-worm-half-squid, and also a badminton shuttlecock with a chameleon-like "tongue". Anomalocaris appears like an armoured cuttlefish, with just two spiky arms, but is unusual also for its size - this animal grew to well over a metre, as opposed to the meagre centimetres of most of its neighbours.

With its large eyes and shape suggestive of speed, it was top of any Cambrian food chain. The rather fish-like Myllokunmingia, on the other hand, can be placed into an animal group, and one close to home - the chordates. This is in fact the first known of the chordates, the ancestor of humans. Its relative of 10 million years later, Pikaia from the Burgess shale, occupied the finale of Stephen Jay Gould's Wonderful Life. If this animal had failed to make it through the Cambrian and start a new branch of the evolutionary tree, the story goes, we would not be here to examine its fossils today. As the end product of the Cambrian explosion, the Chengjiang fossils tell us precisely when this event took place. Now we know that the only animal with hard parts to predate evolution's "big bang" is the trilobite. That is, the trilobite with eyes - life's first eyes. And now, armed with accurate dates for this trilobite, we know it predates the finale of evolution's "big bang" by just five million years - the length of the Cambrian explosion.

Suddenly, the hard parts that evolved during the Cambrian explosion appear as simply adaptations to a new world with sight. Some animals evolved shells and spines, some with bright colours, to visually warn of their new armour. Others evolved streamlined appearances and swimming oars to enlighten trilobites that they could not be caught. Again, the gap in the evidence is well and truly bridged.

Since the Cambrian, vision has continued to tower over the laws of life. Today, more than 95 per cent of all multicelled animals possess eyes. Walk into a field full of animals and very few can be seen - life is adapted to sight. Sight, that is. Not just the simple light receptors that existed before the Cambrian, which could not form an image. Vision is behind the food webs of today - there would be no active predation without it.

Since the publication of the hardback edition of my book In the Blink of an Eye many researchers have written with supporting comments, yet none has found reasonable evidence to counter the light switch theory. Stephen Jay Gould was among those who spread the word. However, some researchers suggest adding other senses to the mix, too.

Today we do live in a world full of adaptations to vision, adaptation found even within the few per cent of species that do not possess image-forming eyes. But living animals are also well adapted to taste, smell, hearing and touch. However, as the other senses staggered into the history of life, it is only vision that entered at one, precise moment - the requirements to trigger an explosive event. And that moment just so happens to correspond with the beginning of evolution's "big bang".

A critic's view was provided from Timothy Gawne of the University of Alabama. Dr Gawne challenged some ideas given for why the eye (and in particular a pair of eyes) should evolve, but at the same time admits: "The notion that once one animal had functional vision, then every other critter had to adapt, is quite powerful." The new evidence from China shows that they did adapt, immediately.

"As I see it ... a predator [a trilobite] evolved for the first time a working visual system, and was so successful that the other organisms had to evolve hard parts to survive, which produced the so-called Cambrian explosion," wrote the late Francis Crick, the co-founder of the structure of DNA, after reading In the Blink of an Eye. He continued, in a letter last year: "Your arguments seem very plausible to me." "I would have thought that detailed genetic [sequence] studies should settle the matter," he added. But then he would.

Dr Andrew Parker's book "In the Blink of an Eye" is now available in paperback, published by Simon and Schuster, £7.99