The common ancestor of all animals may have resembled a certain absorbent, yellow, porous someone who lives in a pineapple under the sea.
The evidence lies in the genes, not the pants.
A complete genetic catalog of the sponge Amphimedon queenslandica suggests that the first animals already had a complex kit of genetic tools at their disposal. Sponges harbor between 18,000 and 30,000 genes — roughly the same number as humans, fruit flies, roundworms and other animals, an international team of researchers reports in the Aug. 5 Nature.
Comparison of the sponge’s genetic blueprints with those of other animals reveals that sponge genes are lined up in the same way as those of other animals. Analyses in the new study also support the idea that sponges form the base of the animal branch of the evolutionary tree, says April Hill, an evolutionary developmental biologist at the University of Richmond in Virginia who was not involved in the work.
“That makes them a pretty important group,” Hill says.
Recently, some scientists had suggested that comb jellies, not sponges, were the first multicellular animals (SN: 4/5/08, p. 214).
Sponges don’t make certain types of organs, such as muscles, nerves and epithelial tissues like skin or gut linings, which help form a barrier to the outside world in more complex animals. Yet proteins that nerve cells use to communicate and connect with each other are among those encoded in sponges’ genes, the researchers say. So are proteins needed for epithelial tissues. Sponges also have some genes that are important in other animals for helping the immune system tell an animal’s own cells apart from foreign cells.
“The thing that really captivates me the most is that so many gene families evolved between the unicellular organisms and the animals,” says Hill. “You see a lot of innovation.”
One thing that really struck researchers, says lead author Mansi Srivastava of the Whitehead Institute in Cambridge, Mass., was that genes shared between humans, sponges and other animals are some of the very genes involved in cancer. “So cancer is really a disease of multicellularity,” she says. “Cancer arises when multicellularity is interfered with.”
Srivastava and her colleagues also note that sponges have 705 genes — more than any other animal — encoding kinases, proteins that attach a phosphate molecule onto other proteins. The researchers don’t know why sponges would need so many of the proteins.