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Animal family tree trunk made of jelly

Mnemiopsis leidyi, a species of comb jelly.
[Image courtesy of Stefan Siebert, Brown University]

Just which critters sit at the base of the animal family tree has been unclear, but now a new study providing the first-ever genome sequence of an ancient jelly-like creature suggests that it represents the tree's first branch.

The animal kingdom is diverse, but it didn't start out that way. The earliest animals were simple, like sponges and jellyfish.

Over time, their cells grew and changed and the simplest organisms evolved to the variety of organisms we see today.

Scientists have been studying the evolution of animals from the first multicellular organisms for decades, trying to understand how the variety of animals we see today came to be.

But understanding the very earliest moments of this process has been difficult; there are four phyla though to sit on the earliest branch of the animal family tree, but scientists have only sequenced the genomes of three.

The genomes of the jelly-like swimmers from the phylum Ctenophora have never been sequenced. And scientists have needed the genomes of the animals (known as comb jellies) to compare all four phyla and figure out which is truly oldest.

In the 13 December issue of the journal Science, Joseph F. Ryan, from the NIH's Genome Technology Branch, and colleagues made progress.

Ryan and colleagues successfully decoded the genome of a comb jelly named a sea walnut. Critically, that meant scientists now had genetic data for all four phyla -- including the phyla for sponges and jellyfish.

Comparing the genomes of all four led to some surprising finds (for example, jellyfish and comb jellies are more distinct than previously thought).

Based on these and other genetic finds, the researchers suggest that ctenophores are indeed the earliest branch on the animal family tree, stealing the slot previously thought to be held by sponges.

The authors go on to detail specific evolutionary processes by which ctenophores gave way to other early multicellular animals, including gains and losses of tissue.

These results suggest that previous hypotheses regarding the evolution of animals may require a re-evaluation.