This is where comparisons between choanoflagellates and animals come into play. The close kinship between choanoflagellates and animals means that there once lived a single-celled ancestor that gave rise to two lines of evolution — one leading to the living choanoflagellates and the other to animals. Choanoflagellates can tell us a lot about that ancestor because any characteristics that they share with animals must have been present in that ancestor and then inherited by both groups. By similar logic, whatever animals have but choanoflagellates lack probably arose during animal evolution.

There are striking physical resemblances between choanoflagellates and certain animal cells, specifically the feeding cells of sponges, called choanocytes. Sponge choanocytes also have a flagellum and possess a collar of filaments for trapping food. Similar collars have been seen on several kinds of animals cells. These similarities indicate that the unicellular ancestor of animals probably had a flagellum and a collar, and may have been much like a choanoflagellate.

But even more surprising and informative resemblances between choanoflagellates and animals have been revealed at the level of DNA. Recently, the genome sequence of one choanoflagellate species was analyzed by a team led by Nicole King and Daniel Rokhsar at the University of California, Berkeley. They identified many genetic features that were shared exclusively between choanoflagellates and animals. These included 78 pieces of proteins, many of which in animals are involved in making cells adhere to one another.

The presence of so many cell adhesion molecules in choanoflagellates was very surprising. The scientists are trying to figure out what all of those molecules are doing in a unicellular creature. One possibility is that the molecules are used in capturing prey.

Whatever the explanation, the presence of those genes in a unicellular organism indicates that much of the machinery for making multicellular animals was in place long before the origin of animals. It may be that rather than evolving new genes, animal ancestors simply used what they had to become multicellular. There may be selective advantages to forming colonies, like avoiding being eaten by other small predators. And in fact, some choanoflagellates do form multicellular colonies at stages of their life cycle.

Dr. King and her colleagues Stephen Fairclough and Mark Dayel investigated one such species to determine whether colony formation occurred by dividing cells staying together, the way animal embryos form, or by individual cells aggregating together, as some protists like slime molds do.

The scientists found that colonies formed exclusively by dividing cells staying together. They suggested that the ancient common ancestor of choanoflagellates and animals was capable of forming simple colonies and that this property may well have been a first step on the road to animal evolution.

The world is full of microbes, and we spend a lot of worry and effort trying to keep them off and out of our bodies. It is humbling to ponder that still swimming within that microscopic soup are our distant cousins.