Spoiler Alert: The Placozoan is Not the Most Ancient Animal
Resembling a smudge more than an animal, a mysterious life form known as a placozoan has now joined other obscure and primitive creatures whose genomes are providing insight into how animals first arose more than 650 million years ago.
Called Trichoplax adhaerens, the creature's genetic information has surprised some scientists, who thought the organism was so rudimentary - possibly the simplest free-living animal known - that it had to be the most ancient animal ancestor. See the August 21, 2008 issue of Nature
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"Because of how simple they look, people hypothesized that Trichoplax represented an early form of animal life. Other people thought they were just simplified cnidarians - jellyfish and hydras," said first author Mansi Srivastava, a Berkeley graduate student in the Department of Molecular and Cell Biology. "We wanted to figure out where they fit with other animals, in particular the cnidarians, the sponges and more complex animals."
A microscopic pancake only three cells thick and a few millimeters across, Trichoplax looks like a multicellular amoeba. It has a top, a bottom and a one-cell thick interior, and it appears to have no nerve, sensory or muscle cells. It usually just splits into two nondescript splotches to reproduce.
Based on the genome analysis, Trichoplax is not our earliest ancestor, but seems to have separated from the animal lineage after the sponges and before the jellyfish and later "bilaterians" - animals, like humans, whose right and left sides are mirror images of one another.
Though Trichoplax is not at the base of the animal family tree, "comparing its genome with other genomes can tell us what the basic molecular toolkit was in the common ancestor of all animals," Srivatava said.
The Forgotten One
Placozoans were discovered in 1880 on the glass of a saltwater aquarium and then promptly forgotten until the 1970s, when the late German biologist Karl Grell discovered how to keep them alive and first studied their biology. Since then, placozoans have been discovered throughout the world's subtropical oceans, mostly near shore and particularly among mangroves.
Based on the new sequencing data, the genome of Trichoplax contains 11,514 genes, compared with 20,000 to 25,000 genes in humans. Despite the hundreds of millions of years separating placozoans and humans, both still have many of the same genes clustered together in their genomes. The fact that placozoans have the genes for more complex behavior makes their simple lifestyle all the more surprising.
"It's remarkable that we have the whole genome sequence but we still know so little about this animal in the wild," he said. "Hopefully, the genome sequence will stimulate more studies of this enigmatic creature."