Now that evolution has become an issue in the presidential campaign (in the May 3 debate among Republican presidential hopefuls, when moderator Chris Matthews asked if any candidates did not “believe in” evolution, three hands—Tom Tancredo’s, Sam Brownback’s and Mike Huckabee’s—shot up), it is always amusing when biologists put another brick in the solid wall that is evolution. The latest comes from a study in which researchers discovered clues to the evolutionary origins of the nervous system.
For anyone who just arrived from Neptune, the “nuanced” stance against evolution—that is, the one that doesn’t make you look like a complete Neanderthal—is to note that of course you know that microevolution occurs, with bacteria evolving resistance to antibiotics and mosquitoes to pesticides, for instance. It’s macroevolution—in which one species evolves into another—that gives you pause since, after all, who has seen such a thing?
The intelligent design camp also argues that some biological structures are just too darn sophisticated to have evolved through random mutation and natural selection. They must therefore have been designed by an intelligent agent. In particular, since complex structures have lots of components, how could the components have been just hanging around for eons waiting for the final component to emerge? Think of it this way: if you don’t already have all the other components of a mousetrap, why would you keep a spring around? A spring is only useful if you also have the base, the bar and the rest. This is the argument called “irreducible complexity,” and it has proved very persuasive to the public.
It’s always dangerous to base your argument on some version of “scientists have never found X” (with X in this case being components of a complex structure existing and serving a function before the rest of the components showed up). That’s because those darn scientists keep making discoveries. If you want to say they “have never found . . . ,” you’d better understand that what you really mean is “they haven’t found it yet.”
Which brings us to the latest discovery in evolution: DNA needed to make synapses, the sophisticated junctions between neurons, in none other than the lowly sea sponge. Considered among the most primitive and ancient of all animals, sea sponges have no nervous system (or internal organs of any kind, for that matter), notes Todd Oakley, assistant professor in the Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara. But, he adds, they “have most of the genetic components of synapses.”
The first neurons and synapses appeared something like 600 million years ago, in a group of animals called cnidarians which, today, include hydra, sea anemones and jellyfish. Sea sponges are even older. “We look at the evolutionary period between sponges and cnidarians as the period when the nervous system came into existence, about 600 million years ago,” says Ken Kosik, co-director of UCSB’s Neuroscience Research Institute.
He, Oakley and the rest of the team listed all the genes known to be operative in synapses in the human nervous system. They then examined the sponge genome. “That was when the surprise hit,” said Kosik. “We found a lot of genes to make a nervous system present in the sponge.”
What were genes for synapses doing in a sponge, which has no neurons and therefore no synapses? This is where the irreducible-complexity crowd makes a fatal error: they assume that whatever the function of a biological component (gene, protein, biochemical pathway . . . ) today must have been its function in the past. Maybe you noticed that my mouse trap example above wasn’t very persuasive; even without a base and a bar, a spring can be a useful little device. So it goes with biological systems. For instance, of the 42 proteins known to make up the bacterial flagellum, 40 have been found to serve as ion channels or something else in bacteria. It is therefore perfectly plausible that they really were hanging around—serving some function that would have allowed evolution and natural selection to keep them around generation after generation—until they all got together and formed a flagellum.
So it seems to be with the genes for synapses. The sea sponge did not use them for their current purpose, but that doesn’t mean the genes had no use. “We found this mysterious unknown structure in the sponge, and it is clear that evolution was able to take this entire structure and, with small modifications, direct its use toward a new function,” said Kosik. “Evolution can take these ‘off the shelf’ components and put them together in new and interesting ways.”