|
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
Richard Dawkins
Thursday September 02 2004
The Guardian
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 £25.To order a copy at the offer
price of £22 plus p&p call Guardian Book Service on 0870 836 0875
Copyright Guardian Newspapers Limited
|