Jared Diamond - The rise and fall of the third chimpanzee

A second, stronger argument is that life on Earth is characterized by what biologists term convergent evolution. That is, seemingly whatever ecological niche or physiological adaptation you consider, many groups of creatures have 'converged by evolving independently to exploit that niche, or to acquire that adaptation. An obvious example is the independent evolution of flight by birds, bats, pterodactyls, and insects. Other spectacular cases are the independent evolution of eyes, and even of devices for electrocuting prey, by many animals. Within the past two decades, biochemists have recognized convergent evolution at the molecular level, such as the repeated evolution of similar protein-splitting enzymes or membrane-spanning proteins. It is now difficult to pick up any issue of any journal in any field of biology without encountering further examples. So common is convergent evolution of anatomy, physiology, biochemistry, and behaviour that whenever biologists observe two species to be similar in some respect, one of the first questions they now ask is: did that similarity result from common ancestry or from convergence? There is nothing surprising about the seeming ubiquity of convergent evolution. If you expose millions of species for millions of years to similar selective forces, of course you can expect similar solutions to emerge time and time again. We know that there has been much convergence among species on Earth, but by the same reasoning there should also be much convergence between Earth's species and those elsewhere. Hence although radio communication is one of those things that happens to have evolved here only once so far, considerations of convergent evolution lead us to expect its evolution on some other planets as well. As the Encyclopaedia Britannica puts it, 'It is difficult to imagine life evolving on another planet without progressing towards intelligence.

That conclusion brings us back to the puzzle I mentioned earlier. If many or most stars have a planetary system, and if many of those systems include at least one planet with conditions suitable for life, and if life is likely eventually to evolve where suitable conditions exist, and if about one per cent of planets with life include an advanced technical civilization ~ then one estimates that our own galaxy alone contains about a million P^nets supporting advanced civilizations. But within only a few dozen light-years of us are several hundred stars, some (most?) of which surely have planets like ours, supporting life. Then where are all the flying saucers that we would expect? Where are the intelligent beings that should be visiting us, or at least directing radio signals at us? If intelligent beings from elsewhere had visited Earth after literate civilizations began to develop here several thousand years ago, those beings would probably have searched out the most interesting civilizations on Earth, and we would now have written records of the visit. If the visitors had arrived in the pre-literate or prehuman past, they might have colonized Earth, and we would know of it as an abrupt arrival of drastically different life forms in our fossil record. We are bombarded by Hollywood films depicting such visits, and by tabloids actually claiming them. You will see the headlines at any US supermarket checkout counter: 'Woman kidnapped by UFO', 'Flying saucer terrorizes family', and so on. But compare that pseudo-bombardment, or our expectations, with reality. The silence is deafening. Something must be wrong with the astronomers' calculations. They know what they are talking about when they estimate the number of planetary systems, and the fraction of those likely to be supporting life. I find these estimates plausible. Instead, the problem is likely to lie in the argument, based on convergent evolution, that a significant fraction of biotas will evolve advanced technical civilizations. Let's scrutinize more closely the inevitability of convergent evolution.

This brings me at last to woodpeckers. The 'woodpecker niche' is based on digging holes in live wood and on prying off pieces of bark. It is a terrific niche that offers much more food than do flying saucers or radios. Thus, we might expect convergence among many species that evolved independently to exploit the woodpecker niche. The niche provides dependable food sources in the form of insects living under bark, insects burrowing into wood, and sap. Since wood contains insects and sap all year round, occupants of the woodpecker niche would not have to migrate.

The other advantage of the woodpecker niche is that it provides a terrific place for a nest. A hole in a tree is a stable environment with relatively constant temperature and humidity, protected from wind and j rain and desiccation and temperature fluctuations, and concealed and protected from predators. Other bird species can pull off the easier feat of digging nest holes in dead wood, but there are many fewer dead trees than live trees available. Many other species nest in natural holes, but such holes too are few in number, quickly become known to predators, get reused year after year, and breed infections. Hence it is a big advantage to be able to excavate a clean new nest hole in a live tree, instead of having to use a dead tree or natural hole. Other birds often pay tribute (unsought by woodpeckers) to that advantage, by usurping woodpeckers' holes.

All these considerations mean that if we are counting on convergent evolution of radio communication, we can surely count on convergent evolution of woodpecking. Not surprisingly, woodpeckers are very successful birds. There are nearly 200 species, many of them common. They come in all sizes, from tiny birds the size of kinglets up to crow-sized species. They are widespread over most of the world, with a few exceptions that I shall mention later. They do not have to migrate in winter. Some species have even exploited their woodpecking skills to live in treeless places, excavate nest holes in the ground, and feed on ants. While the earliest known fossil woodpeckers date only from the Pliocene (about seven million years ago), molecular evidence indicates that woodpeckers evolved about fifty million years ago.

How hard is it to evolve to become a woodpecker? Two considerations seem to suggest, 'Not very hard'. Woodpeckers are not an extremely distinctive old group without close relatives, like egg-laying mammals. Instead ornithologists have agreed for a long time that their closest relatives are the honey-guides of Africa, the toucans and barbets of tropical America, and the barbets of the tropical Old World, to which woodpeckers are fairly similar except in their special adaptations for woodpecking. Woodpeckers have numerous such adaptations, but none is remotely as extraordinary as building radios, and all are readily seen as extensions of adaptations possessed by other birds. The adaptations fall into four groups.

First and most obvious are the adaptations for drilling in live wood. These include a strong, straight, chisel-like bill with a hard, horny covering at the tip; nostrils protected with feathers to keep out sawdust; a thick skull; strong head and neck muscles; a broad base of the bill, and a hinge between that base and the front of the skull, to help spread the shock of pounding; and possibly a brain/skull design like a bicycle helmet, to protect the brain from shock. I hese features for drilling in live wood can be traced to features of other birds much more easily than our radios can be traced to any primitive radios of chimpanzees. Many other birds, such as parrots, peck or bite holes in dead Wood. Some barbets can actually excavate in live wood, but they are much more, clumsier, and less neat than woodpeckers and peck from the side rather than straight. Within the woodpecker family there is a gradation of n" ing ability—from wrynecks, which cannot excavate at all, to the many Woodpeckers that drill in softer wood, to hardwood specialists like sapsuckers and the pileated woodpecker.