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The Ancestor's Tale
I can't believe I'm promoting a book by Dawkins, but it actualy sounds quite
interesting. Perhaps he's rising above his usual 'ultra-Darwinian' truisms?
Some of this sounds like it could have been written by SJG....
Original at http://www.guardian.co.uk/life/
To see this story with its related links on the Guardian Unlimited site, go to
Walking back to genesis
If evolution could be re-run, how would the story end? In this exclusive
extract from his latest book, The Ancestor's Tale, Richard Dawkins goes back in
time to find out
Thursday September 02 2004
It is a conceit of hindsight to see evolution as aimed towards some particular end point, such as ourselves. A historically minded swift, understandably proud of flight as self-evidently the premier accomplishment of life, might regard swiftkind - those spectacular flying machines with their swept-back wings, who stay aloft for a year at a time and even copulate in free flight - as the acme of evolutionary progress. If elephants could write history they might portray tapirs, elephant shrews, elephant seals and proboscis monkeys as tentative beginners along the main trunk road of evolution, taking the first fumbling steps but each - for some reason - never quite making it: so near yet so far. Elephant astronomers might wonder whether, on some other world, there exist alien life forms that have crossed the nasal rubicon and taken the final leap to full proboscitude.
We are neither swifts nor elephants, we are people. As we wander in imagination through some long dead geologicial epoch, it is humanly natural to reserve a special warmth and curiosity for whichever otherwise ordinary species in that ancient landscape is our ancestor (it is an intriguingly unfamiliar thought that there is always one such species). It is hard to deny our human temptation to see this one species as "on the main line" of evolution, the others as supporting cast, walk-on parts, sidelined cameos. Without succumbing to that error, there is one way to indulge a legitimate human-centrism while respecting historical propriety. That way is to do our history backwards.
The Ancestor's Tale is cast in the form of an epic pilgrimage from the present to the past. All roads lead to the origin of life. But because we are human, the path we shall follow will be a human pilgrimage to discover human ancestors. As we go, we shall greet other pilgrims who join us at a series of rendezvous points, as we encounter the common ancestor we share with each of them.
The first fellow pilgrims we greet, some five million years ago, deep in Africa where Stanley memorably shook hands with Livingstone, are the chimpanzees. A million years further into the past, the gorillas join us, then the orang utans. Next the gibbons, then monkeys ... and so on until we finally greet the bacteria, after which all the pilgrims march together in one single backward quest for the origin of life itself, life's "Canterbury".
Following Chaucer's lead, my pilgrims, which are all the different species of living creature, have the opportunity to tell tales along the way. It is these tales that form the main substance of the book. The Dodo's Tale, on page 6, is just one of them. The Host's Return (part of which follows) summarises what I have learned during the course of the pilgrimage.
Chaucer's genial host, having guided the pilgrims from London to Canterbury and stood impresario to their tales, turned around and led them straight back to London. If I, having made the four-billion-year pilgrimage to the dawn of life, now return to the present, it must be alone, for to presume upon evolution's following the same forward course twice would be to deny the rationale of our backward journey. Evolution was never aimed at a particular endpoint.
And yet, if, to borrow a thought experiment of the American biologist Stuart Kauffman, evolution could be rerun again and again - maybe on an imaginary sample of earthlike planets - how similar would the results be?
Like any zoologist, I can search my mental database of life on this planet and come up with an estimated answer to questions of the form: "How many times has X evolved independently?" "The" eye has evolved more than 40 times, to nine different "designs". Echolocation - the trick of emitting sound pulses and navigating by accurate timing of the echoes - has evolved at least four times: in bats, toothed whales, oilbirds and cave swiftlets. Not as many times as the eye's tally of 40-60, but still often enough to make us suspect that, if the conditions are right, sonar will evolve.
To do the counts more systematically would make a good research project. What systems have evolved many times independently, like eyes? Or "several times", like echolocation? Have some things evolved only once, or not at all? I suspect that we'd find certain potential evolutionary pathways which life is "eager" to go down. Other pathways have more "resistance".
Elsewhere, I developed the analogy of a huge museum of all life, both real and conceivable, with corridors going off in many dimensions to represent evolutionary change, both real and conceivable. The corridor of eyes is wide open, almost beckoning. Other corridors are blocked off by barriers that are hard or even impossible to surmount. Evolution repeatedly races down the easy corridors, and just occasionally, and unexpectedly, leaps one of the hard barriers.
The venomous sting (injecting poison hypodermically through a sharp-pointed tube) has evolved at least 10 times independently: in jellyfish and their relatives, in spiders, scorpions, centipedes, insects, molluscs (cone shells) snakes, the shark group (stingrays), bony fish (stonefish), mammals (male platypus) and plants (stinging nettles). It's a good bet that venom, including hypodermic injection, would evolve in reruns.
Sound production for social purposes has evolved independently in birds, mammals, crickets and grasshoppers, cicadas, fish and frogs. Electrolocation, the use of weak electric fields for navigation, has evolved several times in fish and the duckbilled platypus. So has the - probably subsequent - use of electric currents as weapons. The physics of electricity is the same on all worlds, and we could bet with some confidence on repeated evolution of creatures that exploit electricity for both navigational and offensive purposes.
True flapping flight, as opposed to passive gliding or parachuting, has evolved four times: in insects, pterodactyls, bats and birds. Parachuting and gliding of various kinds evolved many times, maybe hundreds of times independently, and may be an evolutionary precursor to true flight. Examples include lizards, frogs, snakes, "flying" fish, squids, colugos, marsupials and rodents (twice). I'd put a lot of money on gliders turning up in hypothetical reruns of evolution, and a reasonable sum on true flapping fliers.
Jet propulsion may have evolved twice. Cephalopod molluscs do it, at high speed in the case of squids. The other example I can think of is also a mollusc, but it is not high-speed. Scallops mostly live on the sea bottom, but occasionally they swim. They rhythmically open and close their two shells, like a pair of snapping castanets. You'd think that this would propel them "backwards" in a direction opposite to the snapping. In fact, they move "forwards", as though biting their way into the water. How can this be? The answer is that the snapping movements pump water through a pair of apertures behind the hinge. These two jets propel the animal "forwards". The effect is so counter-intuitive it is almost comical.
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.
Spitting spiders, family Scytodidae, are a bit different again. Lacking the fleetness of a wolf spider or the net of a web spider, the spitting spider chucks a venomous glue some distance towards its prey, pinning it to the ground until the spider arrives and bites it to death. This is different from the archer fish technique of knocking prey down. Various animals, for example venom-spitting cobras, spit defensively, not to catch prey. The bolas spider, Mastophora, is different again, and is probably another unique case. It could be said to throw a missile at prey (moths, attracted by the fake sexual scent of a female moth, which the spider synthesises). But the missile, a blob of silk, is attached to a thread of silk which the spider whirls around like a lasso (or bola) and reels in.
McGavin's next candidate for an evolutionary one-off is a beauty. It is the diving bell spider, Argyroneta aquatica. This spider lives and hunts entirely under water but, like dolphins, dugongs, turtles, freshwater snails and other land animals that have returned to water, it needs to breathe air. Unlike all those other exiles, Argyroneta constructs its own diving bell. It spins it of silk (silk is the universal solution to any spider problem) attached to an underwater plant. The spider goes to the surface to collect air, which it carries in the same way as some water bugs, in a layer trapped by body hairs. But unlike the bugs, which just carry the air like a scuba cylinder wherever they go, the spider takes it to its diving bell, where it unloads it to replenish the supply. The spider sits in the diving bell watching for prey, and it stores and eats prey there, once caught.
But George McGavin's champion example of a one-off is the larva of an African horsefly called Tabanus. Predictably in Africa, the pools of water in which the larvae live and feed dry up. Each larva buries itself in the mud and pupates. The adult fly emerges from the baked mud and flies off to feed on blood, eventually to complete the cycle by laying eggs in pools of water when the rains return. The buried larva is vulnerable to a predictable danger. As mud dries out, it cracks, and there is a risk that a crack will tear right across the grub's refuge. It could theoretically save itself if it could somehow engineer a way for any crack that approaches it to be diverted around it instead. And it does indeed achieve this in a truly wonderful and probably unique manner. Before burying itself in its own pupation chamber, it first corkscrews its way down into the mud in a spiral. It then corkscrews its way back to the surface in an opposite spiral. Finally, it dives into the mud straight
down the centre between the two spirals, and that is its resting place through the bad times until water returns.
Now, you see what this means? The larva is encased in a cylinder of mud whose circular boundary has been weakened in advance by the preliminary spiral burrowing. This means that when a crack snakes across the drying mud, if it hits the edge of the cylindrical column, instead of cutting straight across the middle it goes instead in a curved bypass around the edge of the cylinder, and the larva is spared. It is just like the perforations around a stamp which stop you tearing the stamp across. Dr McGavin believes that this ingenious trick is literally unique to this one genus of horsefly.
This kind of comparative exercise, counting which things evolve often, and which seldom, might help us to predict things about life outside this planet. Which features of life are parochial, and which universal? This is a question that biologists ask less often than they should.
If, as returning host, I reflect on this whole pilgrimage, my overwhelming reaction is one of amazement. Amazement at the extravaganza of detail that we have seen; amazement, too, at the very fact that there are any such details to be had at all, on any planet. The universe could so easily have remained lifeless and simple - just physics and chemistry, the scattered dust of the cosmic explosion that gave birth to time and space. The fact that life evolved out of nearly nothing, some 10 billion years after the universe evolved out of literally nothing, is a fact so staggering that I would be mad to attempt words to do it justice. Even that is not the end of the matter. Not only did evolution happen: it eventually led to beings capable of comprehending the process, and even of comprehending the process by which they comprehend it.
This pilgrimage has been a trip, not just in the literal sense but in the counter-cultural sense I met when a young man in California in the 1960s. The most potent hallucinogen on sale in Haight or Ashbury or Telegraph Avenue would be tame by comparison. If it's amazement you want, the real world has it all. Not only is life on this planet amazing, and deeply satisfying, to all whose senses have not become dulled by familiarity. The very fact that we have evolved the brain power to understand our evolutionary genesis redoubles the amazement and compounds the satisfaction.
· The Ancestor's Tale: A pilgrimage to the dawn of life by Richard Dawkins is published
by Weidenfeld &Nicolson today at Â£25.
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