Nature has an interesting short review this week about the search for the urmetazoan, the last common ancestor of all animals. The search isn't for a fossil, of course, since the urmetazoan would have been minute, soft, and rather nondescript, but is instead a plunge into the genomes of extant simple forms, like choanoflagellates and sponges, to extract the common elements of their molecular circuitry. There's a brief summary of what we've learned so far, but what I found most enticing is the promise of what is to come.

Like choanoflagellates, the urmetazoan's single-celled ancestors may have used signals relayed by proteins called tyrosine kinases to sense changes in the outside world. Cell adhesion might have helped unicellular organisms to form simple colonies — colonies that may have been an intermediate step between single cells and true multicellularity. The urmetazoan may then have recruited these genes for new purposes. "Evolution is an extremely dynamic system and paradoxically a very lazy one," says palaeobiologist Simon Conway Morris at the University of Cambridge, UK, who studies the origins of metazoans. "It will co-opt whatever it can."

But this raises a puzzling question. If the toolkit was already there, why didn't animals evolve sooner? The answer seems to be that the catalyst for multicellular animal life may not have been genetic but environmental — in the form of rising global oxygen levels.

Whatever the trigger, researchers hope that the completion of the choanoflagellate genome sequence, expected later this year, will yield fresh insights into the biology of this momentous transition. Genome sequences are also expected for the sea anemone Nematostella vectensis and the simplest known living animal, Trichoplax adhaerens. Trichoplax is just two cells thick, has only four cell types and looks like a giant multicellular amoeba. Its genome promises to be the smallest of any animal yet measured and should define the minimum set of genes needed for animal life. Sea anemones, which have a more advanced body plan, could yield information on the genetic mechanisms underlying body-plan formation.

Isn't that exciting? Predictions from evolutionary biology are leading us directly to detailed analyses of the function and origin of fine details of the molecules in diverse organisms—and that is the power of good science.

Pilcher H (2005) Back to our roots. Nature 435:1022-1023.