A limb, a feather, or a flower is a marvel, but not a miracle.

From One Cell to Trillions

In every human body roughly ten trillion cells—brainless units of life—come together to work as a unified whole. "It's a complex dance," says Nicole King, a biologist at the University of California, Berkeley, requiring organization and constant communication. And it began more than 600 million years ago when organisms containing just one cell gave rise to the first multicellular animals, the group that now includes creatures as diverse as sea sponges, beetles, and us. It turns out that some of those single-celled ancestors were already equipped for social life.

King studies some of our closest living single-celled relatives, known as choanoflagellates. Choanoflagellates are easy to find. Just scoop some water from a local creek or marsh, put a few drops under a microscope, and you may see the tadpole-shaped creatures flitting about. You can tell them apart from other protozoans by a distinctive collar at the base of their tail.

When King and her colleagues examined the proteins made by choanoflagellates, they found several that were thought to be unique to animals—molecules essential to maintaining a multicellular body. "It really blew our minds," says King. "What are these single-celled organisms doing with these proteins?"

Some of the proteins normally create what King calls "an armlock between cells," keeping animal cells from sticking together randomly. King and her colleagues are running experiments to figure out how choanoflagellates use these adhesive proteins—perhaps to snag bacteria for food. Others play a role in cell-to-cell communication. Choanoflagellates, which presumably have no need to talk to other cells, may use these proteins to sense changes in their environment.

The discoveries suggest that many of the tools necessary to build a multicellular body already existed in our single-celled ancestors. Evolution borrowed those tools for a new task: building bodies of increasing complexity.