When animals hit the big time
Sitting in his sprawling office under the eaves of the Earth sciences building at the University of Cambridge, Nicholas Butterfield seems a typical denizen of the ivory tower. A specialist in ancient ecology, Butterfield spends most of his time pondering the finer points of life in seas that vanished hundreds of millions of years ago. But perhaps he isn't so far removed from the daily concerns of the rest of us after all; his biggest problem, it turns out, is sex.
It's not what you're thinking. His computer screen saver displays radio telescope data, not earthly delights. But as the discoverer of the earliest evidence of sexual reproduction--a red alga called Bangiomorpha pubescens fossilized in the act some 1.2 billion years ago--Butterfield has had to confront a central mystery of life's history on Earth. Textbooks say that sexual reproduction should speed up evolution. So why, he asks, did it take 600 million years more for modern animals--active, many-celled creatures like us--to make their first impression in the fossil record?
Scientists have been posing versions of that question for over a century. And the puzzle has only deepened with recent discoveries showing that evolution had laid the groundwork for modern animals tens of millions of years before their sudden burst of evolutionary exuberance, known as the Cambrian explosion. New data pouring in from fossil beds, biology labs, and computer simulations of Earth's history have sparked a matching explosion of new theories, some invoking a drastic change in the ancient environment and others a trigger within life itself, such as the first eyes. This week, researchers are gathering in Seattle to compare notes, promote their pet ideas, and perhaps plot a strategy to solve the persistent riddle once and for all.
Late starters. For the first 2.5 billion years of life's history, the oceans were home to little more than bacteria, and what dry land there was remained lifeless. A few simple animals showed up on the seafloor about 575 million years ago. But while these soft-bodied "Ediacaran" creatures could grow to several feet long, they had simple, plantlike body plans and remained rooted in place on the bottom, filtering food particles out of the seawater.
And then suddenly, 543 million years ago, the shallow coastal seas of the Cambrian era started to explode into a vibrant bestiary of crawling, swimming, voraciously snacking animal life. In a geological eye blink--10 million to 20 million years--most Ediacarans died out and were replaced by what would eventually become everything from animal-rights activists to the creatures they worry about.
The Burgess Shale fossil beds in eastern British Columbia, discovered in 1909, gave science its first good look at the Cambrian explosion. Now a steep climb up the side of Mount Field, the flaky rock was once a coastal seafloor, and it preserves in intricate detail an otherworldly community of armored trilobites, spiny worms, and fearsome swimming predators, including the 2-foot monster Anomalocaris. Yet the Burgess Shale animals aren't quite as bizarre as they seem. They had the eyes and legs and distinctive left-and-right-hand body plans that define modern animals. To try to understand where they came from, scientists are now peering even further back into the geological record. "You can't understand the Cambrian without looking at the ecology and evolution of what came before," says Butterfield.
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