Which critters sit at the base of the animal family tree? Now, a study  in the 13 December issue of Science provides the first-ever genome sequence of a creature called a comb jelly (a member of the phylum Ctenophora) and suggests that these animals are the most basal.
The finding "changes the way we view early animal evolution," said University of Vanderbilt researcher Antonis Rokas, who wrote an accompanying Perspective article discussing the new study.
Traditionally, scientists have thought that sponges, which are morphologically very simple, represent the earliest branch of the animal family tree. "But if the earliest branch is the ctenophore branch," Rokas explained, "this suggests that the pan-animal ancestor was much more complex than previously thought; it had a nervous system."
In this scenario, the nervous system was lost as animal evolution proceeded from comb jelly to sponge, but it was regained again when jellyfish emerged on the scene. The loss of complex cell types is not necessarily surprising, said Rokas, who noted that "body plan simplification is a frequently observed phenomenon." But this elaborate picture of cell loss and gain differs from the previous view of early animal evolution, which resembled more of a linear path of evolutionary forms from the simple to the complex.
"With the traditional animal tree of life," said the study's lead author Joseph Ryan, "it was thought that complex cell types (like nerves) arose once, and once invented, were never lost…Our study shows that this idea is not true."
"[This new view of early animal evolution] is a big deal," Rokas emphasized. "It's why decoding the ctenophore genome, and more generally, early animal evolution are attracting so much attention."
Scientists have been studying the origin and evolution of multicellular organisms for decades. But understanding the very earliest era of animal evolution has proved challenging because researchers have lacked a full genome sequence from the phylum Ctenophora. Having it would help scientists better understand the molecular changes that mounted through time to result in the diverse body types exhibited by animals today.
Andreas Baxevanis, senior author of the Science study and senior scientist in the National Human Genome Research Institute's Division of Intramural Research, and colleagues successfully decoded the genomes of two adult comb jellies colloquially named sea walnuts. The researchers collected the animals in Woods Hole, Massachusetts.
The advanced technology his team applied with the help of the National Human Genome Research Institute and the NIH Intramural Sequencing Center allowed the group to leap forward in the field, successfully sequencing the sea walnuts' gene content in a relatively short period of time.
Critically, this means the scientists now have genetic data for all four phyla thought to sit on the earliest branches of the animal family tree, including the phyla comprising sponges and the one comprising jellyfish.
Comparing the genomes of these early animals with the more than 16,000 genes of the sea walnut led to some surprising finds. For example, jellyfish and comb jellies, long envisioned as close relatives due to their morphological similarities, are more distinct than previously thought.
"This means that some of the similarities (like the nervous system) that the two phyla share may have evolved multiple times," Rokas said, explaining how they could be present at one point in the animal family tree, disappear, and then return. "Or, alternatively, the similarities they share may have evolved further back and were subsequently lost in other lineages."
Based these and other genetic finds, which helped the researchers get a clearer picture of this phylum's position on the animal family tree, Baxevanis and colleagues suggest that ctenophores are indeed the earliest branch.
Sequencing additional genomes of other early-branching animals, like sponges, will be critical to obtaining the clearest possible picture of the animal family tree trunk.