Wednesday, March 29, 2006

Fossilized Embryos

Much of what scientists learn about the evolution of Earth's first animals will have to be gleaned from spherical embryos fossilized under very specific conditions, according to a new study.

Purported animal embryo fossils have been reported continuously over the last 12 years, mainly by paleontologists working in China. Scientists disagree about whether the fossils are actually animal embryos or even if they are animals.

"The fossils look great. The problem is, if you know anything about embryos, their fossilization just doesn't seem likely," said IU Bloomington Professor of Biology Rudolf Raff, one of the report's authors. "It's like trying to fossilize soap bubbles. Some investigators showed that these fossils are being preserved with calcium phosphate, but they haven't explained how embryos could survive long enough for that to happen. We do that."

The dead spherical embryos do not last long under normal seawater conditions. If the cell's own degradation processes don't destroy it, nearby bacteria will. The embryos must die in the presence of a so-called "reducing" substance, such as hydrogen sulfide. Such reducing substances slow or stop the internal degradation processes that occur very soon after cells die and also inhibit voracious bacteria.

Hydrogen sulfide is known to have been present in the environment half a billion years ago, at the time of the animal kingdom's origin, and when the embryos died, because sulfur-containing pyrite ("fool's gold") is found in the fossils. Today, hydrogen sulfide exists in significant concentrations at the bottom of the ocean, especially in the vicinity of deep sea vents. Because hydrogen sulfide is extremely toxic (and flammable), the scientists used beta-mercaptoethanol, which exerts similar effects.

The scientists also found that embryo size does not influence the fossilization process. Sea urchin species whose animal embryos are small (a tenth of a millimeter) were just as likely to be fossilized as those of larger cousins (about half a millimeter).

Much mystery surrounds the sudden appearance of animals in the fossil record, between 500 and 600 million years ago. Within a few million years, the fossil record goes from zero evidence of animals to great diversity in animal forms, including anomalocarids and trilobites. Harvard University biologist and historian Stephen Jay Gould brought this "Cambrian explosion" to the popular consciousness in 1990 with his book Wonderful Life.

Images from HERE.