Ancestor For All Animals Identified

Jan. 27, 2009 -- A sperm-looking creature called monosiga is the closest living surrogate to the ancestor of all animals, according to new research that also determined animal evolution may not always follow a trajectory from simple to complex.

Yet another find of the study, published in the latest PLoS Biology, is that Earth may have given rise to two distinct groups of animals: bilaterians -- animals with bilateral symmetry, like humans -- and non-bilaterians, which include corals, jelly fish, hydra, unusual, often poisonous, creatures known as cubozoans, and other organisms.

Free-living, unicellular organisms called choanoflagellates, however, could be on every person's family tree, so long as it was a gigantic one.

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"It is clear that the choanoflagellates -- living representative is monosiga -- are the best candidate for the nearest relative of animals," co-author Rob DeSalle told Discovery News.

"So a choanoflagellate-like organism could be looked at as a probable common ancestor for animals," added DeSalle, curator at the Sackler Institute for Comparative Genomics at the American Museum of Natural History.

He and his colleagues compiled data from multiple gene sequences derived from many sources to find over 9,400 variable characters that contain parsimony information, which collectively refers to the shared, derived traits that help biologists infer species relationships on the tree of life.

They determined that so-called "simple" and "lower" tier animals, such as corals and jellyfish, evolved in parallel to "higher" animals, like seemingly more complex insects and even humans. On the tree of life, monosiga then currently holds the root position for the latter group.

The new research completely shakes up the non-bilaterian animal ordering. Previously it was thought that either super simple-structured or comb jellies were at the root of the non-bilaterian animal tree. Instead, complete outsiders -- placozoans -- have been placed in that basal position.