Researchers have uncovered what they think is a sudden diversification of life at least 30 million years before the Cambrian period, the time when most of the major living groups of animals emerged. If confirmed, the find reinforces the idea that major evolutionary innovations occurred in bursts.
The main points of Charles Darwin's theory of evolution, which he carefully laid out in The Origin of Species 149 years ago, have stood the test of time. But where Darwin assumed that natural selection proceeds slowly and orderly--much the way Isaac Newton imagined a clockwork universe--modern investigations have shown that the process more resembles the chaotic world of quantum physics. Scores of new groups of species can appear within a few million years. By far the biggest and most famous of these events is the Cambrian explosion, a period between 542 million and 520 million years ago, when due to some still-unknown cause, the ancestors of nearly all extant groups, or phyla, of animals appeared.
Now a team of paleontologists from Virginia Polytechnic Institute and State University in Blacksburg think they've found a second great explosion of life. The researchers have performed the most detailed analysis yet of strange fossils from Australia, known as the Ediacara biota. They represent the oldest known multicellular organisms, which emerged about 575 million years ago.
The morphology of the Ediacara organisms is so different from that of previous life forms and from one another that they must qualify as several distinct new classes of species, the team argues tomorrow in Science. Many of the organisms, which resembled leaflike fronds and fractal forms, emerged abruptly over about 25 million years during the Avalon period, so the team has named the event the Avalon explosion.
The event is "a perfect match in time" to a sudden infusion of oxygen into the oceans, which may have sparked the explosion of marine biodiversity, says geobiologist and co-author Shuhai Xiao. Another possible stimulus, he suggests, is a warming of the ocean that occurred back then as an ice age was ending. Whatever the cause, there was one big difference between the Avalon and Cambrian explosions: The Cambrian produced groups that endure to this day, Xiao says, whereas the Ediacaran forms soon vanished.
The idea that the Ediacara fossils evolved a wide range of shapes and forms very quickly seems "reasonable and sound" in the context of evolutionary history, says evolutionary biologist Andrew Knoll of Harvard University. But that might not be the most striking aspect of the find. Rather, he explains, it's that these creatures evolved "much the same way as in later evolutionary radiations, large and small," suggesting that explosions in diversity might share similar dynamics.
Paleobiologist Richard Aronson of the Dauphin Island Sea Lab in Alabama says the study will spark new questions. Why, for example, was the Avalon explosion quashed, he asks, "but the Cambrian explosion prevailed and gave us life as we know it."