Tuesday, January 27, 2009

First Animal

Trichoplax adharens, the living member of the Placozoan phylum, is a microbe-eating blob of several thousand cells. If you looked closely, you might just see it with naked eye. This tiny, humble creature may have been Earth's first animal. Placozoa are thought to have evolved early in the development of multicellular organisms. Trichoplax likely represents the oldest living animal group.

New comparative genomics studies demonstrate that animals with bilateral symmetry (like us) split from other animals (like placozoans and jellyfish) early in metazoan evolution. This suggests, intriguingly, that a nervous system developed twice in animal history, in parallel evolution. Placozoa like Trichoplax have the genetic hardware to form a nervous system, but apparently not the inclination.

(image: Trichoplax, the extant Placozoan species, W. Jakob, via livescience.com)