SOME INSIGHTS INTO THE ORIGIN OF MULTICELLULAR ANIMALS
Most animals that you have encountered are worms, insects, snails, clams, fish, lizards, snakes, amphibians, birds, or your fellow mammals. Some of you may have gone scuba diving and increased your repertoire of animals including corals, jellyfish, tunicates, and sponges. The least complicated in body plan of all these animals belongs to the sponges. They have only a few cell types. One of these, known as collar cells, have a filament (called a flagellum) that whips around inside a fringe-like collar attached to the cell. The flagellum sweeps up bacteria and smaller protozoa and digests them inside the cell. Very similar to these specialized cells of the sponges are protozoan single cells called choanoflagellates (a very fancy name for collared cells). When I was a graduate student at Indiana University taking a course in invertebrate zoology, I imagined that these choanoflagellates might have been ancestors of sponges in a distant evolutionary past.
In 2008 a team of California scientists coordinated by Nicole King and Daniel Rokhsar published a DNA analysis of a species of choanoflagellates, Monosiga brevicollis. It has about 9200 genes, which is about one half what most multicellular animals have (we have 23,000 genes in a sperm or egg). Monosiga has split genes, like humans, but its genes have slightly fewer “junk DNA” insertions (an average of 6.6 per gene compared to 7.7 in our genes). What were more interesting were the functions of some of Monosiga’s genes. They had several associated with cell adhesion (what makes cells stick to one another as they do in our muscles or skin), cell recognition, and immunological defense. Before King and Rokhsar’s work, these genes were thought to be unique to multicellular animals and missing in single celled protozoa.
The authors compared Monosiga’s genes with genomes of other plants, animals, and fungi. They found that of 24 genes associated with multicelled animals, only 3 were shared in fungi and plants, but Monosiga shared 16 of them. This is a striking association that suggests many of the components for animal evolution were present in protozoa like Monosiga. Very likely around 600 million years ago, an era called the Precambrian, the first gathering of cells by adhesion took place and by the Cambrian era the proliferation of animal types appeared in the fossil record. Future studies of the 125 known species of choanoflagellates and related protozoa and simple animal forms may reveal the way these different body plans came into being.
The difference between my vague suspicions some fifty years ago and the molecular and biochemical analysis possible today is stunning. Evolution studies are now providing models of how cellular processes, embryonic processes, and genetic processes participate in bringing about an evolution that predates the fossilized skeletal remains of more advanced forms of animals first appearing about 500 million years ago.