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

Viewed under a microscope, a drop of pond water is a liquid game park, squirming with bacteria, amoebas, and other exotic wildlife. One of the prettiest creatures you may catch sight of is a tumbling, crystalline, globe-shaped alga known as volvox. Reaching up to three millimeters in diameter, volvox is essentially a shell formed from cells. Each cell is equipped with a quivering tail, and these appendages spin the globe through water. The alga's gelatinous core holds a few gigantic cells, like jewels encased in a translucent Faberge egg. For several days of its brief life, volvox basks in the sunlight, using photosynthesis to capture the energy of the sun. Meanwhile, the gigantic cells--the eggs of volvox--start dividing and turning into embryos. When they're ready for life on their own, they digest their way out of their parent's body, which dies after a few more days.

As alien as volvox may seem, we humans actually share a bond with it. Like us, it belongs to that special fraternity of species that are made of many cells living together in a single body. On a planet where life got its start as single-celled microbes, the dawn of multicellular life was a milestone in the history of evolution. And by studying how multicellular creatures such as volvox took that step, scientists can gather clues about how our own bodies evolved.

Multicellularity has emerged dozens of times during the past 2 billion years. Our own multicellular lineage, the animals, probably descends from protozoans that lived in loose colonies, as do some choanoflagellates, the closest living single-celled relatives of animals. The earliest animals (which, according to fossil evidence, emerged at least 600 million years ago) may have consisted of cells glued together with adhesive proteins, as is the case in the most primitive living animals, the sponges. And, like sponges, they may have reproduced by releasing small chunks that developed into full-blown organisms. Eventually animals evolved a system in which only a relatively few specialized germ cells (eggs in females, sperm in males) played a role in reproduction.

Other lineages that became multicellular took on dramatically different forms. Plants, for example, started out as green algae, while mushrooms evolved from several lineages of single-celled fungi. Some single-celled species have evolved the ability to become multicellular in times of crisis. When their food runs out, slime molds (protozoans common in the soil) swarm together, forming a stalk topped by a spore-packed bulb. Wind and water can carry the spores away to a more promising place, where they can revive and go on with their single-celled lives.

Because multicellularity is an experiment that's been run many times in the history of life, scientists may be able to discover some universal rules for how it comes about. Some of the most interesting recent results have come out of the laboratory of Richard Michod at the University of Arizona. Michod and his team have built complex mathematical models of the transition and compared them With processes in the natural world. They conclude that multicellularity arises through an intricate dance of cooperation and conflict.

For microbes, joining together has plenty of potential rewards. First off, multicellular organisms can enjoy the benefits of being big. For example, being big is a good defense, because once you get bigger than the microbes around you, they can't simply swallow you whole. Big organisms can also evolve novel means to swim or crawl faster toward food and away from danger.

On its own, a single cell faces inherent obstacles to becoming larger. In order to keep up its metabolism, it needs to pump molecules in and out through the cell wall. This exchange gets less efficient as a cell expands, because its volume increases far faster than its surface area. Multicellular creatures can skirt this physical constraint by keeping their individual cells small but gluing them together into big bodies. Of course, pathways need to be kept open to ensure that the individual cells are bathed in the necessary fluids.

Size isn't the only advantage of the multicellular lifestyle. A single-celled microbe often has to be a jack-of-all-trades, capable of finding food, defending itself against parasites and predators, withstanding sudden swings in its environment, and creating new copies of itself. By contrast, an organism built out of millions of cells is free to transform them into various types that can carry out such different tasks as detecting light, digesting proteins, or forming a skeleton. Through evolution, each type of cell--whether it is a foot-long neuron in a spinal cord or a keratin-producing cell in a fingernail--can become exquisitely specialized for its job.

So why hasn't all life become multicellular? One important reason may be that whenever a multicellular life-form emerges, natural selection begins to operate on two distinct levels. One level is that of the individual cell, and the other is that of the whole body. And what is advantageous for one may well turn out to be disastrous for the other.