February 16, 2010

Darwin Days: Biodiversity Explosion

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In school, Doug Erwin trained to think like a snail. This isn’t an insult to the his mind; it’s a tribute to his ability to think in tens of millions of years. He is a senior scientist, the curator at the Smithsonian Institution, and the president of The Paleontological Society.

As curator of the Smithsonian’s Burgess Shale exhibition, Erwin understands the relationship between the Cambrian Explosion and biodiversity. The Burgess Shale contains remarkably well preserved fossils from the Middle Cambrian Epoch, which lasted from 520 to 512 million years ago. The “Cambrian Explosion,” or “Cambrian Radiation,” was an intense period of animal diversification, lasting over 20 million years. Life on Earth evolved from sponges, stromatolites (photosynthetic microbial communities) and microbial organisms into fish, vertebrates and other diverse lifeforms.

The Cambrian Explosion created morphological diversity, including an increase in the number of unique body forms.

The Burgess Shale provides a “wonderful menagerie of animals,” some so fantastical that Erwin says, “Spielberg can’t do better.” For example, Opabinia is four to five inches long, has five eyes on stalks, a proboscis (a long flexible snout), and lateral gill flaps.

Erwin desires to illuminate the construction of this biodiversity.

Erwin focuses on ecological networks. “Ecosystem engineering” is environmental modification by a species that does not aid that species.

“Niche construction” alters an environment to benefit a species. For instance, when a beaver builds a dam, it allows his progeny “to have a better chance of surviving the winter and produce more offspring,” Erwin said.

Erwin believes “ecological inheritance” of these networks generates biodiversity. As one generation passes genes to the next, environments are too passed on. Physical and chemical changes in the environment impact fitness and natural selection.

According to this theory, the “Cambrian Explosion” was actually a “Cambrian Substrate Revolution” in the structure of sediments.

Cambrian worms disturbed sediments where surface was “well-laminated,” Erwin said. The burrowing of Cambrian worms increased primary productivity (production by photosynthetic species), resulting in spillover effects on unrelated species.

“These spillover effects are the clue to what happens in the Cambrian Radiation,” he explained.

Oxygenation through sponges also contributed to the Cambrian Explosion. Erwin cites the evolution of sponges and bioturbation as the two ecological adaptations with the largest spillover effects.

Spillover effects from simple, early organisms created a world for new, diverse generations. Via natural selection, Earth’s earliest organisms gave rise to the major lineages of animal life.

Original Author: Jing Jin