All for one and one for all: multicellular organisms have arisen more than once, each time through an intricate dance of cooperation and conflict - The Evolutionary Front

Natural History, Feb, 2002 by Carl Zimmer

Life, death, and reproduction differ greatly on these two levels. When we have children, we continue to exist as individuals until our own bodies fail and die. A single-celled microbe, on the other hand, typically reproduces by dividing into two or more "daughter" cells. Once it divides, there is no parent left. While human bodies don't divide this way, each of our individual cells do. As they divide, they give rise to lineages of cells that create and maintain our entire body. But all these lineages are doomed to extinction when we die--except for those that give rise to the few eggs or sperm that create our children.

In multicellular organisms, the cells that can foster a new generation are called germ cells, while all the others are called somatic cells. This division of labor can work only if all the players--the cells--obey the rules. But cheating can be extremely attractive. Imagine a primitive colony of microbes that reproduces by budding off little packages of germ cells (a method used by a number of primitive animals, including corals). Each of these packages of germ cells then grows into a new individual. Suppose a mutant gene arises in one of the microbes in the colony, guaranteeing that the microbe will become a germ cell rather than dying off with the rest of the colony. This "cheater gene" might bring so much reproductive success that more and more cells become germ cells, until ultimately a colony (or body) has too few somatic cells to function. This would defeat the whole advantage of multicellular organization and lead to a dead end.

Another way for cells to cheat in such a colony is to replicate madly. Instead of restraining themselves for the good of the colony, these cheaters exploit their fellow cells for their own benefit. Such cheaters are an all-too-common threat to our own well-being. They strike millions of people every year and are best known by their common name: cancer. Cancer cells can mutate at a much higher rate than normal cells, and those that can take advantage of our bodies--by, for example, stimulating the growth of blood vessels that supply them with food--are rewarded by natural selection. Ultimately, of course, their evolution is doomed, because they can never survive outside their host. But natural selection is, by definition, a blind process that cannot recognize a dead end on the road of evolution.

Michod and his graduate students have suggested that these kinds of conflicts between individual cells and the entire body favor the evolution of what they call conflict mediators--genes that bring the interests of the cells and the body into line. Such conflict mediators, they hypothesize, might ensure that an organism develops from a single cell instead of a package of cells. This would reduce the competition between the organism's cells, because they descend from a single common ancestor. Since the cells share identical genes, the reproductive success of any one of them means success for all. Cheaters would pay a higher evolutionary cost for betraying their fellow cells because they would be harming their cousins rather than strangers.