The animal kingdom has gained a cousin and lost a mother.

In a fundamental reconfiguration of the tree of life, scientists now say that the last common animal ancestor is not — as was commonly believed — a sponge or comb jelly, but rather an as-yet-unknown forerunner of amoeba-like creatures called placozoans.

"It's a question that has plagued animal biologists for a couple hundred years: What could be the mother of all animals?" said Rob DeSalle, an evolutionary biologist at the American Museum of Natural History. "We've turned it upside down."

Since Carl Linnaeus put life in taxonomic order in 1735, scientists have proposed a variety of original or ur-animals, from Ernst Haeckel's hypothetical, embryo-like gastraea to larval jellyfish called planula. But in recent years, zoologists have favored either sponges or comb jellies as occupying a bifurcation point between Bilateria — creatures with bilateral symmetry, including humans and most of the animal kingdom — and the jellyfish, corals and polyps belonging collectively to Cnidaria.

Those hypotheses were based on relatively rough comparisons of anatomy and a handful of genes. Molecular sequencing and supercomputer-enabled data analysis give biologists a clearer picture of life's narrative — but DeSalle's analysis, published Monday in Public Library of Science Biology, has merely moved the mystery to another level.

Comb jellies and sponges don't appear to have predated Bilateria, but rather belong with Cnidaria in a single branch of the animal kingdom, originating in the lowly placozoans. But while some scientists, among them DeSalle, thought that placozoa might represent the last common ancestor of all animals, it appears to belong solely to this newly defined branch, leaving the ur-animal shrouded in mystery.

"It fits in with what you might think is the most basal animal. It's only got three cell layers and four cell types. Its motility is primitive. It lives in warm oceans. It's got all the earmarks of the thing that gave rise to all animal life," said DeSalle. "But that's not what the results show. And though placozoa is the ur-cousin of complicated life, we still don't know the ur-mother."

DeSalle's team fed data on 9,400 biological characteristics — from mitochondrial and nuclear DNA

sequences to RNA molecular structure — from 70 species spanning every major class of animal on Earth into a program that searched for patterns of relationships, determining which order made linear sense.

Out of this came the new tree, with DeSalle's would-be ur-animal at the start of one branch. Identified just 100 years ago crawling on an aquarium wall, placozoans have not even been studied in their ocean habitat, but its genome was sequenced in 2008.

University of Texas taxonomist David Hillis called placozoa "among the most interesting animals that almost no one has ever heard about," but was less enthusiastic about the conclusion, which he said "represents a fine distinction in how we reconstruct the ancestral animal."

But such fine distinctions are fascinating to DeSalle.

Since placozoa don't have a nervous system and jellyfish do, the findings suggest that nervous systems evolved at least twice, independently of each other. This insight could stimulate research that provides a deeper understanding of our own nervous systems.

"Evolution has done all these experiments, and when you reconstruct common ancestors, you're reconstructing the results of the experiment,"

said DeSalle. "If you want to look at the development of our brains, of our nervous system, of anything we have as a result of experiments that nature has done, the best way to do it is to reconstruct our ancestors."

DeSalle said a common animal ancestor is likely to come from Deuterostomia, a group of phyla that includes sea cucumbers.

*Citation: "Concatenated Molecular and Morphological Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern 'Urmetazoan' Hypothesis." By Bernd Schierwater, Michael Eitel, Wolfgang Jakob, Hans-Jürgen Osigus, Heike Hadrys, Stephen Dellaporta, Sergios-Orestis Kolokotronis and Rob DeSalle. Public Library of Science Biology, Jan. 26, 2009. *

Image: Trichoplax, the only known placozoan / Wolfgan Jakob

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