Algae and Identity: Volvox Species Helps to Demystify Evolution Towards Multicellularity

Trillions of cells make up the human body, and it is interesting to view each one as a living entity of its own. One can imagine their body as a community of trillions of tiny lives, one-celled organisms living and working, reproducing and dying. Such imagining leads one to question how one-celled organisms, like bacteria, archaea, protozoa and yeasts, evolved into such complex societies. Did they begin with individual identities which were lost over increasing cooperative specialization, or did a larger body evolve from the separation of multiple nuclei within the same membrane? Observing the behavior of a certain species of green algae puts these speculations into perspective.

Many unicellular organisms learned over billions of years that there is safety in numbers. An interesting example would be Vibrio fischeri, a luminescent bacteria that does not used energy to turn on its lights until it is cued by the presence of others like itself, inside of the Hawaiian bobtail squid, which then looks like a mere glow on the water to predators.

Cells within the family of protists called Volvocacae, however, depend on one another more drastically. They live in spherical communities of tens of thousands and, though each cell would appear to have the capability, they cannot live alone. The cells are, however, considered individuals, which resemble Chlamydomonas, but are either one of two types: somatic cells (with flagella) or reproductive cells (without flagella). When the presence of nutrient is detected, the flagella turn counterclockwise and the whole community moves in a straight line; in the presence of danger, the flagella turn clockwise and back in a seemingly random motion that disperses the community and triggers the release of daughter cells, which fall down to the floor to await a safer time to emerge. The cells are also differentiated by region in that eye-spots are more developed at one end to encourage the algae's movement towards light.

The Volvox species of communal unicellular organisms is so specialized that it is nearly its own organism. There are three major theories as to how multicellular organisms came to be: 1.) Symbiosis, 2.) Cellularization, and 3.) the Colonial Theory. The third is the most accepted, due in part to close observations of the Volvox species of algae, whereby increased specializations decreased the independence of individual cells. Volvox being the most efficient shape in nature, this step in evolution was suspended for our present scientific speculations.

Of course, no one theory provides a complete picture of something so complex as the human body. Surly an evolutionary process so complex might have involved some cellularization, which has been shown to exist in the insect kingdom. And many scientists believe that our cells' mitochondria might have evolved from invasive bacteria that found a symbiotic niche. Studies of living fossils like our ancient algae will continue to color our biological history.