More on sponges
from NOBEL INTENT
Our cousin, the sponge. Elaborate multicellular organisms such as us humans require a careful choreography of signaling to generate all the specialized cell types that make up our various organs, and adhesion molecules to hold them together once they're generated. But multicellularity exists across a bit of a spectrum. There are organisms that form aggregates of identical cells, others that can exist both as free living cells or collections of cells that can specialize, and organisms such as sponges, which have specialized cells that are not organized into distinct tissues. Is there any clear dividing line on the evolutionary path that led to us bilateral animals?
A paper in PNAS argues that the answer is yes, and it suggests that we and the sponges fall on the same side of that line. The researchers used a series of completed animal genome sequences to look for cell signaling and adhesion genes, and found that most were distributed throughout all animals. They also looked in yeast, which are unicellular, and slime molds, which can exist as amoebas or in a group with specialized cells. They had almost none of these genes. They then searched a sponge genome, and found that they clearly grouped with the rest of the multicellular animals: in case after case, they had the genes that help us organize our multicellular existence. This also indicates that the genetic raw material for organizing diverse body forms goes back to well before the Cambrian.