Planet Earth

The Colorful Cambrian


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The Cambrian explosion--evolution's Big Bang--began some 543 million years ago. Within a mere 5 million years the ancestors of almost all animals now living--from mollusks to humans--suddenly appeared on Earth. There has been no lack of theories to explain the phenomenon. An increase in oxygen fueled the burst, say some. A decrease in carbon dioxide, say others. Now an Australian biologist has come up with a new hypothesis: the evolution of eyes sparked the Cambrian explosion.

Andrew Parker of the Australian Museum in Sydney reached this conclusion after studying three fossil species unearthed from the Burgess Shale, 515-million-year-old sediments in British Columbia. Two of the species, Wiwaxia corrugata and Canadia spinosa, were bristle-covered marine worms. The third, Marrella splendens, was an arthropod. Using an electron microscope, Parker found that closely spaced parallel ridges covered the outer scales and spines of these animals. These ridges, which he has also found on living crustaceans and worms, act like a diffraction grating and split white light into its component colors, imparting an iridescent glow to the animal.

This discovery, says Parker, marks the first appearance in the fossil record of animals with color. At different times of day and different viewing angles, the marine creatures would have glowed blue, red, yellow, or green. Since the evolution of these worms coincides with the first appearance in the fossil record of animals with eyes, such as trilobites, the twinkling colors may have warned predators to avoid these armored, and perhaps unpalatable, animals.

Coloration wasn't the only attribute to evolve in response to eye-endowed predators, says Parker. "Before the Cambrian there were just simple animals--worms and jellyfish. Then all in one go the blueprints for all the animals around today evolved. Now, this worm, if an animal with eyes came along that wanted to eat it, wouldn't survive very long. So there were massive selection pressures for this worm to change its form into animals that could either swim, burrow, hide, or have armored parts or reflect warning colors. To do those things you have to have a completely different body plan. And by changing its body plan to move into all these different areas, it created all the different phyla we have today."

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