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Darkness visible: the birth of a theory

The Earth we know could once have been described as "Planet of the Worms."

Until around 543 million years ago, the onset of the Cambrian era in geologic time, all animals had the same flat, tubular shape. Then, in the proverbial blink of an eye, something changed. In what is commonly referred to as biology's Big Bang, a diversity of species emerged in a relatively short period. Instead of merely worms (and sponges and jellyfish, for you purists), the planet was suddenly inhabited by recognizable representatives of the roughly three dozen phyla that exist today, including Chordata, the one to which human beings belong.

The causes for this sudden shift have puzzled scientists since Darwin. A few years ago, thanks to a chance encounter with a bristle- worm fossil, Andrew Parker, an Australian marine biologist, thought he had the answer. His book, "In the Blink of an Eye," asserts that the event that changed the world was, in fact, the arrival of vision.

How did the ability to see spark a revolution in animal life? Parker, a researcher at Oxford University, talked from his home in Oxfordshire, England, about his new "Light Switch" theory, an idea so simple that it's stunning.

You determined that animals developed eyes at the threshold of the Cambrian era. That meant that other animals had to evolve strategies to deal with being seen, and that's why a huge differentiation of their external appearance occurred?

Yes . . . once one phylum changed, everything else had to change or be eaten.

One result was the food chain that we know today?

That's right. It kick-started the arms race.For you, this discovery began with the realization that Cambrian animals were colored?

Biologists didn't look for color in fossils because why should you?

While studying seed shrimp at the Smithsonian, you happened to wander over to some fossils and that led to your thinking about the development of the eye . . .

I was looking at reconstructions [of fossils] and drawings . . . I looked . . . under a microscope and [there were structural elements that indicated a reflective surface]. . . . The main thing was, it got me to think about color in fossils.

It's hard to imagine the world without sight. You have to make a cognitive leap . . .

I think this is why no one has ever considered this before. It's pretty obvious, really. There was a newspaper article about [my work] in Australia, and the night before the paper came out, an editor asked the writer, "Are you sure no one has ever done this before?"

Can you talk about how the world looked to animals before the Cambrian era?

In the pre-Cambrian, life was marine. Animals had light sensors so they could see the direction of light but couldn't produce an image. Water would appear dark at the bottom and light at the top, so they would have known which way was up. For us, it would be like walking into a room with your eyes closed. We'd know where the light was, but you wouldn't be able to find people standing in the room.

At the beginning of the Cambrian, it was just like opening your eyes.

How did that change things? Up until that point, the internal bodies of animals were similar, right?

Vision is the most powerful stimulus -- it's a stimulus you can't avoid. If a predator appears with excellent hearing, you just don't make a sound. With vision, the prey item has to look as if it's not there [with camouflage] or it's aggressive -- with bright colors or spines.

Before the beginning of the Cambrian era, the predator-prey interactions didn't really exist?

Right. Animals were opportunistic. They were not very mobile. They ate things that crawled along the bottom. . . . We were looking for something to explain the change in evolution. We knew that there was a big jump, from soft bodies to the animals we have today.

Today, animals look different to each other than they look to us.

Yes, UV light is an example. We can't see it. Also, we see green the best, whereas other animals see red best.

Can you talk about the contribution of Stephen Jay Gould?

I wouldn't have worked on the Cambrian without him. He single-handedly brought the Burgess Shale fossils [from a fossil lode in British Columbia] to the public's attention. . . . He came over to work with me for a few days in Oxford. . . . He was more interested in the Cambrian itself and not the explosion.

Everything changed at the onset of the Cambrian era.

The world looks today as it looked in the Cambrian. Just before it, it was extremely different. Nothing has changed since. Well, a few little things like moving onto land.

Why is the discovery of the "Light Switch" theory important?

One thing is, if we increase the hole in the ozone layer, we could think about the levels of UV rays and [future] evolution. It could favor animals that can see UV rays.

Robin Dougherty, a writer and critic, lives in Washington, D.C. She can be reached at

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