September 17, 2004

Dawkins: The Ancestor's Tale

The prolific Richard Dawkins has released yet another book, this one on the heels of "A Devil's Chaplain: Reflections on Hope, Lies, Science and Love." His latest book, titled "The Ancestor's Tale : A Pilgrimage to the Dawn of Evolution," chronicles the remarkable diversity of Earth's organisms throughout time.

Dawkins recently allowed the Guardian to publish an excerpt from his book. Here's a taste:
But how about things that have evolved only once, or not at all? The wheel, with a true, freely rotating bearing, seems to have evolved only once, in bacteria, before being finally invented in human technology. Language, too, has apparently evolved only in us: that is to say at least 40 times less often than the eye. It is surprisingly hard to think of "good ideas" that have evolved only once.

I put the challenge to my Oxford colleague the entomologist and naturalist George McGavin, and he came up with a nice list, but still a short one compared with the list of things that have evolved many times. Bombardier beetles of the genus Brachinus are unique in Dr McGavin's experience in mixing chemicals to make an explosion. The ingredients are made and held in separate (obviously!) glands. When danger threatens, they are squirted into a chamber near the rear end of the beetle, where they explode, forcing noxious (caustic and boiling-hot) liquid out through a directed nozzle at the enemy. The case is well known to creationists, who love it. They think it is self-evidently impossible to evolve by gradual degrees because the intermediate stages would all explode. What they don't understand is that the explosive reaction requires a catalyst: gradually increase the dose of catalyst, and you gradually escalate the explosion, from nothing to lethal.

Next in the McGavin list is the archer fish, which may be unique in shooting a missile to knock prey down from a distance. It comes to the surface of the water and spits a mouthful at a perched insect, knocking it down into the water, where it eats it. The other possible candidate for a "knocking down" predator might be an ant lion. Ant lions are insect larvae of the order Neuroptera. Like many larvae, they look nothing like their adults. With their huge jaws, they would be good casting for a horror film. Each ant lion lurks in sand, just below the surface at the base of a conical pit trap which it digs itself. It digs by flicking sand vigorously outwards from the centre - this causes miniature landslides down the sides of the pit, and the laws of physics do the rest, neatly shaping the cone. Prey, usually ants, fall into the pit and slide down the steep sides into the ant lion's jaws. The possible point of resemblance to the archer fish is that prey don't fall only passively. They are sometimes knocked down into the pit by the particles of sand. These are not, however, aimed with the precision of an archer fish's spit, which is guided, with devastating accuracy, by binocularly focused eyes.

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