Friday, July 16, 2010

The soma and the germ

Paul drew our attention a week or so ago to some research that appeared in the 9 July issue of Science, comparing the genome of the single-celled alga Chlamydomonas reinhardtii to the 2000-cell organism Volvox carteri.

It turns out that the two related organisms have about the same number of genes -- 14,500. Moreover the genes are remarkably similar. The conclusion seems to be: The change from single-celled life to multicellularity was no so much what "tools" organisms had as how they used them.

And what a change it was! About 700 million years ago invisibly small cells got together and soon the world was full of towering oaks and great blue whales. There is a real sense in which you and I are societies of cells that learned to cooperate and specialize to make for more efficient reproduction of the precious germs cells. It is only slightly with tongue in cheek that I'd say Chartres Cathedral, the plays of Shakespeare, Bach's Saint Mathew's Passion, and the Large Hadron Collider are just side-effects of a collectivity contrived by a string of genes to make copies of itself.

Does that sound distressingly reductionistic. It doesn't need to be. Emergence is the name of the game.

I once had the opportunity to watch the life cycle of a slime mold --- Dictyostelium discoideum -- under the microscope.

At first they are invisible individuals, an uncountable army of free-roaming, single-celled amoebas, grazing on bacteria. Like other single-celled organisms, they multiply by splitting down the middle, two from one. Their population soars. Their food becomes scarce.

Triggered by hunger, they gather in their tens of thousands, streaming like gleaming rivers to an assembly point, at last becoming visible to the eye in their slimy congregations.

Surrendering their individuality, they heap themselves into a gooey blob half-a-millimeter high. The blob falls onto its side, becoming a sluglike creature. Some amoebas know they are at the front; others bring up the rear. The front end of the slug lifts as if to sniff the wind. The newly-contrived creature slithers on a film of slime toward light and warmth.

As it slithers, the cells begin to change. The anterior cells are destined to become a stalk; the posterior cells will become spores.

A bright, warm place is found. The slithering ceases. Anterior cells push down through the spore mass, becoming a slender pillar anchored at the base, lifting a perfect sphere of spores into the air. The spores are dormant amoebas that will travel on the air to form new colonies when the sphere bursts asunder. The stalk amoebas die; they have sacrificed themselves so that others might live.

The fruiting tower is beautiful. Glittering. Translucent. An Ozmian minaret, sometimes as tall as this letter i. Fifty thousand amoebas pool their individual resources to build a reproductive spire that is as marvelous in its own amoebic way as the towers of Chartres.

What I watched on the stage of the microscope was, in a sense, a recapitulation of one of the great chapters of life on Earth, the evolution of multicellularity, with all of the glorious side effects that seems to us to be -- in our exalted sense of exceptionalism -- the point of it all.