Tuesday, April 22, 2008
One thing creationists, including the Intelligent Design sort, inevitably get around to is claiming that there is some insurmountable difference between "microevolution" and "macroevolution." The usual trope is that microevolution, as evidenced by such observations as the changes over human time spans in "Darwin's Finches" or the Peppered Moth, is nothing more than minor adaptations to the environment, different in some unspecified way from those necessary to result in a different "kind" of animal. Along the way, they usually wind up misrepresenting how scientists use the terms and quote mining them to boot.
Ultimately, the distinction the creationist try to make is that microevolution is observed and macroevolution is not. This is, of course, wrong. First and foremost, it fundamentally misrepresents how science is done. We need not directly observe events or processes in order to draw inferences about them that, in turn, support conclusions we justly have high confidence in. We need not directly observe the core of the sun in order to know, beyond reasonable doubt, that there are fusion reactions taking place there. Nor do we insist on seeing a germ actually making a person ill in order to be sure that germs cause disease. We have massive amounts of evidence for common descent and the microevolution / macroevolution ploy, even if it plays well to the unsophisticated, is a measure of the desperation the creationists are driven to in their denial of the results of science.
However, I've been reading Neil Shubin's Your Inner Fish and he mentions an example that may put the lie even to the creationist's strawman version of science. Shubin's subject is the formation of multicellular creatures. He notes three essentials to form bodies: 1) molecules (most importantly, collagen) that lie between the individual cells; 2) molecules that cause the cells to stick together; and 3) molecules that move between the cells sending "messages" (actually setting off chemical reactions) to other cells that it is time to die or divide or to make new molecules.
Why did it take so long, some 3 billion years, for multicellularity to arise? Building bodies takes a great deal of energy and it may be that the correct set of circumstances to make it worthwhile was complex -- what Shubin calls "a perfect storm." But one factor might have been quite simple:
Perhaps bodies arose when microbes developed new ways to eat each other or avoid being eaten? Having a body with many cells allows creatures to get big. Getting big is often a very good way to avoid being eaten. Bodies may have arisen as just that kind of defense.
When predators develop new ways of eating, prey develop new ways of avoiding that fate. This interplay may have led to the origin of many of our bodybuilding molecules. Many microbes feed by attaching and engulfing other microbes. The molecules that allow microbes to catch their prey and hold on to them are likely candidates for the molecules that form the rivet attachments between cells in our bodies. Some microbes can actually communicate with each other by making compounds that influence the behavior of other microbes. Predator-prey interactions between microbes often involve molecular cues, either to ward off potential predators or to serve as lures enticing prey to come close. Perhaps signals like these were precursors to the kinds of signals that our own cells use to exchange information to keep our bodies intact.
We could speculate on this ad infinitum, but more exciting would be some tangible experimental evidence that shows how predation could bring about bodies. That is essentially what Martin Boraas and his colleagues provided. They took an alga that is normally single-celled and let it live in the lab for over a thousand generations. Then they introduced a predator: a single-celled creature with a flagellum that engulfs other microbes to ingest them. In less than two hundred generations, the alga responded by becoming a clump of hundreds of cells; over time, the number of cells dropped until there were only eight in each clump. Eight turned out to be the optimum because it made clumps large enough to avoid being eaten but small enough so that each cell could pick up light to survive. The most surprising thing happened when the predator was removed: the algae continued to reproduce and form individuals with eight cells. In short, a simple version of a multicellular form had arisen from a no-body.