31 March 2011

The eyes have it

Cosmos Magazine
It’s a tempting idea: the development of sight triggered an evolutionary arms race that created life as we know it today. But there were doubts … until now.
jesus christ lizard eye

Credit: iStockPhoto

Suddenly, about 530 million years ago, and for no obvious reason, the range and variety of life forms went ballistic.

This was during the Cambrian period, and it represents life’s ‘big bang’: at no other time in the Earth’s history has there been such a profusion, such procreative exuberance, and such an overwhelming diversity of living things in so short a time scale.

Not surprisingly, this event has become known as the Cambrian explosion. Prior to this, animals were exclusively soft-bodied and worm-like, as they had been for the entire 300 million years before that.

But in the somewhat less than five million years that followed (a blink of an eye in geological terms), nearly all the major animal groups on Earth today independently evolved hard body parts.

The big question has long been: why? What exactly lit the Cambrian fuse? My own theory – known as the Light Switch Theory – has been that the development of vision was the spark that started it all: once this capability arose, it allowed predators to identify prey – and, naturally, those predators did very well indeed.

This triggered an arms race (or, if you prefer, a defence race): detecting predators became rather important to survival. Thus, vision became a dominant force in evolution and resulted in the eyes we have today.

The first animal to evolve vision was the trilobite, a distant extinct 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 – in fact, the first active predator – with visual search capabilities.

If the first eye is added to the geological timescale, the order of events becomes, sequentially, the introduction of vision, followed closely by the Cambrian explosion. But just how closely did the one follow the other, precisely? Only fossils can provide an answer to that question.

Until recently there has always been a slight hiccup with the theory: Canada – where the same fossil deposit that gave rise to the Light Switch Theory, also left room for doubt.

The age, condition and diversity of the Burgess Shale – a rich collection of fossils discovered nearly a century ago in the Canadian Rockies – provides a snapshot of community life during the Cambrian explosion, with all manner of lifestyles existing in one place buried together simultaneously in a mud slump.

The date was 515 million years ago. Yes, the evidence from the Burgess Shale fossils fitted the Light Switch Theory: hard external parts had evolved in many animal groups after vision had been introduced, but there was a 15-million-year gap between the arrival of the first eye and the Cambrian explosion.

To strengthen the theory, that gap in the fossil record had to be filled. Disturbingly, some fossils had even been found that looked like hard-bodied shells, and these pre-dated eyes.

Only later did it emerge that these were the remains of soft-bodied animals that had become hardened after death; their post-mortem ‘shells’ had been produced by unusual preservation conditions. What was really needed to confirm the theory was another, older Burgess Shale. The search for more fossils to support the theory began.

In 2004, palaeontologists from Britain, China and Sweden published their combined reports of expeditions to the hills and lakes of Yunnan Province, in south-west China.

This province is home to the remarkable Chengjiang fossils, and these did bridge the gap in the record. From ordinary beginnings more than 20 years ago, the Chengjiang fossils have grown in stature to rival those of the Burgess Shale.

Since the first specimen was uncovered in 1984, they have been found to be preserved equally well, and to reveal even the fine needlelike spines of some creatures. They also match the Burgess Shale fauna for diversity.

So they do provide a second excellent snapshot of life in the Cambrian – but this time the fossils are dated at 525 million years. Now we know for certain the Cambrian explosion took place just after the introduction of vision. That’s the evidence the theory needed.

So what types of 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 and they include some of the most bizarre life forms one could imagine.

Choia is a sponge, but resembles a miniature 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, and Microdictyon, a ‘velvet-worm,’ could be 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. Even the tiniest spines, thinner than a human hair, are preserved in exquisite detail. The arthropods (to which crabs belong) are well represented in the Chengjiang fossils. Some are shaped just 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), a pair of anthropomorphic ‘hands’ and a fish’s tail protruding from the rear of its two-part shell. Enigmatic forms also existed there, so peculiar we cannot even begin to guess at their phylogeny; one fossil is a ball of spines, another resembles a vase-shaped pincushion. Then there is a feather attached to a discus; a half-worm, half-squid; and a badminton shuttlecock that has, disturbingly, a chameleon-like ‘tongue’.

Anomalocaris appears like an armoured cuttlefish, with just two spiky arms, but is considered unusual because of its size: this animal grew to well over a metre, as opposed to the meagre centimetres of most of its neighbours. With large eyes and a shape that suggests a capacity for speed and acceleration, 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 chordate, one of the key early ancestors of humans.

One of its relatives of 10 million years later, Pikaia, from the Burgess Shale, occupied the finale of Stephen Jay Gould’s book, 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 humans would not have evolved to examine and analyse its fossils today.

As they represent the products of the Cambrian explosion, the Chengjiang fossils tell us precisely when evolution’s big bang took place, and we now know the only animal with hard body parts that existed before this event was the trilobite (that is, the trilobite with eyes – life’s first eyes).

And now, as a result of the Chengjiang fossils, we are armed with better and more accurate dates for our commonly found trilobite, and we have been able to determine that it pre-dates the finale of evolution’s big bang by just five million years – exactly the length of time taken for all the Precambrian worms to evolve their hard parts.

Suddenly, the hard body parts that evolved during the Cambrian explosion seem they might be adaptations to a new world with sight. Some animals evolved shells and spines – some with bright colours – a visual warning 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 now well and truly bridged.

Since the Cambrian period, vision has been paramount to life. Today, more than 95 per cent of all multicellular animals possess eyes. Walk into a field full of animals and very few can be seen: life is adapted to sight.

Not just the simple light receptors that existed before the Cambrian, which could not form an image. Vision is behind all the food webs of today – there would not be active predation without it.

Since I proposed the Light Switch Theory, many scientific colleagues from around the world have written supporting comments; as yet, none has found reasonable evidence to counter the idea. The late Stephen Jay Gould, the celebrated Harvard palaeontologist, was among those who spread the word.

Today we live in a world full of adaptations to vision, adaptations found even within the few per cent of species that lack image-forming eyes. Today’s animals are also well adapted to taste, smell, hearing and touch – and some researchers suggest adding these senses to the mix – but only vision entered at one precise moment, which is a prerequisite for any trigger of an explosive event.

And the precise moment at which vision emerged 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.

Gawne challenged some ideas given for why the eye (and in particular a pair of eyes) should evolve, but at the same time admitted, “The idea that once one animal had functional vision, all the other critters 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 man who shared a Nobel Prize for discovering the structure of DNA – after reading the theory.

“Your arguments seem very plausible to me,” he continued, in a letter last year. “I would have thought that detailed genetic (sequence) studies should settle the matter.” But then, of course, he would say that.

Andrew Parker is an evolutionary biologist at the University of Oxford, and author of In the Blink of an Eye.

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