Saturday, May 2, 2009

Crazy sea-grapes complicate Cambrian explosion

A submarine expedition to the Bahamas has discovered a new species—the “Bahamian groma”—a single-celled organism about one inch in diameter.

The creatures are reminiscent of grapes, or balloons, or “doo-doo balls,” according to researcher Sonke Johnson of Duke University.

Although similar mega-amoebas have been discovered around the world—the first in 2000 in the Arabian Sea—these are different in one important way: they’ve made tracks across the ocean floor. Although the grapes appear to be moving too slowly to be captured with the submarine’s video capabilities, researchers are convinced the tracks they’ve made can’t be created simply by ocean currents. The organisms sometimes head out in multiple directions from one spot, and are able to go up and down the ridges and valleys of the ocean floor.

The idea that such simple organisms could have created the tracks throws a wrench in current ideas about the Cambrian explosion—the rapid expansion of multicellular, complex life that happened about 530 million years ago.

Before discovering the creatures, themselves, researchers found grape-tracks in the pre-Cambrian fossil record and assumed this meant that bilateral creatures—the types of things we normally consider capable of making tracks—were around in pre-Cambrian days.

If, instead, the Bahamian groma were responsible for the tracks, it means that the Cambrian explosion may have been even more explosive than we thought, taking animal life from amoeba to us in a shorter amount of time than previously believed.

As far as researchers know, the Bahamian groma is like a giant balloon. Filled with water and almost neutrally buoyant, it floats along the very surface of the ocean floor by “eating” the stuff in front of it and “pooping” it out behind. They’re fragile and impossible to study in captivity, however, so nobody really knows.

They clearly reproduce, “because there were sure a lot of them,” says Johnson. But that process, as well, remains a mystery.

I learned about the grapes (and stole pictures) from this blog article, which referred me to this press release as well as the original 2008 paper in Current Biology, “Giant Sea-Protist Produces Bilaterian-like Traces.”

Interestingly, wikipedia’s Cambrian explosion entry mentions that Charles Darwin saw the Cambrian explosion “as one of the main objections that could be made against his theory of evolution by natural selection,” presumably because rapid diversification during the Cambrian period contradicted his ideas about slow, successive change. He acknowledges this in "Origin," saying that “several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks,” and that this problem “at present must remain inexplicable, and may be truly urged as a valid argument against the views here entertained.”


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