If the world had been different, would humans still have evolved? Without doubt, says Simon Conway Morris.
Imagine if evolution were run again. Would something as strange as a human again emerge? Would humans still predominate? Would life even be carbon-based? Well, all life on Earth is composed of carbon chains based in a water medium, and it is difficult to imagine an alternative. Water itself is an extraordinary molecule with uncanny properties. Look at a glass of gin and tonic, and think how odd it is that the ice is floating. Imagine an alternative world in which ice sank or water was more viscous, less transparent, or had a lower thermal capacity. Well, you wouldn't be able to think about it because you couldn't be here.
Biochemistry is universal; there isn't any choice but for our biosphere to be based on DNA and proteins and to depend on photosynthesis powered by chlorophyll. The evolution of animals is also of little surprise, given how much of the molecular architecture is available in primitive organisms. Did you know a third of the genes found in the brain of some invertebrates are present in plants and yeast, both of which lack any noticeable mental capacities? The emergence of not only animals but even human-like intelligence is pre-ordained from the evolution of the first cell, and may even be inherent in the Big Bang.
It is widely agreed that for intelligences to emerge, animals must be able to process sensory information. What better example than the eye? A classic case of how unrelated animals can independently develop similar traits to meet the same demands, be it in the ocean or on land. The astonishing similarity between the camera eye and that of the octopus is justly famous, but in total the camera eye has evolved independently at least seven times.
Of course, there are plenty of other sorts of eye. The compound eye, most familiar in the fly eye, is an excellent design, and has evolved independently at least four times. But you should be grateful you aren't stuck with a compound eye. To provide the same visual acuity as your camera eye, it would have to be more than 1m wide. And such evolutionary convergence occurs in all sensory systems. Thus basic mechanisms of olfaction and audition are identical in insects and vertebrates.
That's also true of those sensory modalities that to us are very alien.
Think, for example, of either echo-location or electrical sensation. Both are convergent. It turns out that evolution has few choices. Yet common sense would tell us that the likelihood of one biosphere even remotely resembling another is impossible; in principle, the number of biological alternatives is truly astronomical. Well, that is incorrect. The majority of the supposed alternatives simply don't work. That is why convergence is ubiquitous. The exciting prospect is that if evolution has to navigate a landscape where nearly all alternatives are maladaptive, then this hints that buried beneath the apparent chaos of biology is a deep and universal structure.
This navigation explains not only convergence but also the ease with which complex structures evolve. Take the placenta: a remarkable structure so complicated that surely it could evolve only once, in mammals. Not at all; similar placentas have evolved in some lizards.
The main evolutionary properties necessary to be a humanoid are warm-bloodedness, sophisticated vocalisation and song, bipedality, cultural transmission, tool-making and intelligence. All are convergent. Just consider the last two in the list. That chimps use tools and even show rudimentary cultures is scarcely surprising. For something uncannily familiar to us, consider dolphins. The convergences between dolphin and ape intelligences are remarkable, yet not only is living in the ocean about as different as you can get, but the convergences even extend to social structure. Dolphin and chimp societies are remarkably similar. So if evolution navigates repeatedly to the same solutions, then the emergence of a humanoid is as inevitable as any other sort of convergence. If it hadn't been you, then sooner rather than later another group would have pulled off the same trick.
But what about giant meteorite impacts like the one that destroyed the dinosaurs? No impact, no dinosaur extinction, no opportunity for the mammals to take over the world, and so ultimately no you either? Hardly.
Remember, when the dinosaurs ruled the earth there were plenty of mammals and birds. Supposing there had been no giant impact? About 20 million years later, the Earth began to spiral into an ice age. It would be warm-blooded mammals and birds that would have taken over the temperate and polar regions. More intelligent, more agile - the end result is inevitable. The hunters would move into the tropics and the extinction of the dinosaurs would begin. Mass extinctions only postpone, they cannot cancel. Now is the time to separate the incidental from the central problems of biology, and renew our search for life's deep structure.
Simon Conway Morris works in the department of earth sciences, University of Cambridge. His book Life's Solution: Inevitable Humans in a Lonely Universe has just been published by Cambridge University Press, £18.95.