Quite a while ago now, I asked the question 'Is Paris Hilton Inevitable?' Pinker covers a similar topic in his book, 'How The Mind Works' when he discusses the possibility that SETI may find humanlike intelligence elsewhere in the universe:
a SETI fan might ask, isn't it true that animals become more complex over time? And wouldn't intelligence be the culmination? In many lineages, of course, animals have become more complex. Life began simple, so the complexity of the most complex creature alive on earth at any time has to increase over the eons. But in many lineages they have not. The organisms reach an optimum and stay put, often for hundreds of millions of years. And those that do become more complex don't always become smarter. They become bigger, or faster, or more poisonous, or more fecund, or more sensitive to smells and sounds, or able to fly higher and farther, or better at building nests or dams - whatever works for them. Evolution is about ends, not means; becoming smart is just one option.Aliens will only evolve intelligence under the right conditions. And that seems simple enough. Thinking about it, there are practical examples of this in our own historical record. The dinosaurs ruled this planet for over 160 million years and yet presumably never attained civilisation. Bacteria too have been around for much longer (as have the plants) and yet again, none have seemingly stumbled into humanlike ways of doing things. If the path to intelligent creatures is in fact a very lucky one (requiring specific types of pressures to shape it) then Pinker is right in dismissing SETI as unlikely to provide us with proof of intelligent life elsewhere in the universe - in fact most life out there is likely to be very unintelligent and thus not able to send us nice signals. The universe itself is extremely big and old, and no doubt we are not the only intelligent life that has evolved or will evolve in the future. But as to whether we'll ever meet any real life intelligent aliens, only time will tell.
Still, isn't it inevitable that many organisms would take the route to intelligence? Often different lineages converge on a solution, like the forty different groups of animals that evolved complex designs for eyes. Presumably you can't be too rich, too thin, or too smart. Why wouldn't humanlike intelligence be a solution that many organisms, on this planet and elsewhere, might converge on?
Evolution could indeed have converged on humanlike intelligence several times, and perhaps that point could be developed to justify SETI. But in calculating the odds, it is not enough to think about how great it is to be smart. In evolutionary theory, that kind of reasoning merits the accusation that conservatives are always hurling at liberals: they specify a benefit but neglect to factor in the costs. Organisms don't evolve toward every imaginable advantage. If they did, every creature would be faster than a speeding bullet, more powerful than a locomotive, and able to leap tall buildings in a single bound. An organism that devotes its matter and energy to one organ must take it away from another. It must have thinner bones or less muscle or fewer eggs. Organs evolve only when their benefits outweigh the costs.