Route of vas deferens from testis to penis

The human body abounds with what, in one sense, we could call imperfections but, in another sense, should be seen as inescapable compromises resulting from our long ancestral history of descent from other kinds of animal. Imperfections are inevitable when 'back to the drawing board' is not an option -when improvements can be achieved only by making ad hoc modifications to what is already there. Imagine what a mess the jet engine would be if Sir Frank Whittle and Dr Hans von Ohain, its two independent inventors, had been forced to abide by a rule that said: 'You are not allowed to start with a clean sheet on your drawing board. You have to start with a propeller engine and change it, one piece at a time, screw by screw, rivet by rivet, from the "ancestral" propeller engine into a "descendant" jet engine.' Even worse, all the intermediates have got to fly, and each one in the chain has got to be at least a slight improvement on its predecessor. You can see that the resulting jet engine would be burdened with all kinds of historical relics and anomalies and imperfections. And each imperfection would be attended by a cumbersome accretion of compensatory bodges and fixes and kludges, each one making the best of the unfortunate prohibition against going right back to the drawing board.

The point is made, but a closer look at biological innovation might draw a different analogy from the propeller engine / jet engine case. An important innovation (the jet engine in our analogy) is quite likely to evolve not from the old organ that did the same job (the propeller engine in this case) but from something completely different, which performed a completely different function. As a nice example, when our fish ancestors took to breathing air, they didn't modify their gills to make a lung (as do some modern air-breathing fish, such as the climbing perch Anabas). Instead, they modified a pouch of the gut. And later, by the way, the teleosts - which means just about any fish you are likely to meet, except sharks and their kind - modified the lung (which had previously evolved in ancestors that occasionally breathed air) to become yet another vital organ, which has nothing to do with breathing: the swim bladder.

The swim bladder is perhaps the major key to the teleosts' success, and it is well worth a digression to explain it. It is an internal bladder filled with gas, which can be sensitively adjusted to keep the fish in hydrostatic equilibrium at any desired depth. If you ever played with a Cartesian Diver as a child you'll recognize the principle, but a teleost fish uses an interesting variant of it. A Cartesian Diver is a little toy whose business part is a tiny upended cup, containing a bubble of air, floating at equilibrium in a bottle of water. The number of molecules of air in the bubble is fixed, but you can decrease the volume (and increase the pressure, following Boyle's Law*) by pressing down on the cork in the bottle. Or you can increase the volume of air (and decrease the pressure of the bubble) by raising the cork. The effect is best achieved with one of those stout screw stoppers they put on cider bottles. When you lower or raise the stopper, the diver moves down or up until it reaches its new point of hydrostatic equilibrium. You can coax the diver up and down the bottle by sensitive adjustments to the stopper, and hence to the pressure.

A fish is a Cartesian Diver with a subtle difference. The swim bladder is its 'bubble' and it works in the same way, except that the number of molecules of gas in the bladder is not fixed. When the fish wants to rise to a higher level in the water it releases molecules of gas from the blood into the bladder, thereby increasing the volume. When it wants to sink deeper, it absorbs molecules of gas from the bladder into the blood, thereby decreasing the volume of the bladder. The swim bladder means that a fish doesn't have to do muscular work, as a shark does, in order to stay at a desired depth. It is at hydrostatic equilibrium at whatever depth it chooses. The swim bladder does that job, thereby freeing up the muscles for active propulsion. Sharks, by contrast, have to keep swimming all the time, otherwise they would sink to the bottom, admittedly slowly because they have special low-density substances in their tissues that keep them moderately buoyant. The swim bladder, then, is a coopted lung, which is itself a coopted gut pouch (not, as you might have expected, a coopted gill chamber). And in some fish, the swim bladder itself is yet further coopted into a hearing organ, a kind of eardrum. History is written all over the body, not just once but repeatedly, in exuberant palimpsest.

We've been land animals for about 400 million years, and we've walked on our hind legs for only about the last 1 per cent of that time. For 99 per cent of our time on land, we've had a more-or-less horizontal backbone and walked on four legs. We don't know for certain what selective advantages accrued to the individuals who first rose up and walked on their hind legs, and I am going to leave that matter aside. Jonathan Kingdon has written a whole book on the question (Lowly Origin) and I have considered it in some detail in The Ancestor's Tale. It may not have seemed like a major change when it happened, because other primates such as chimpanzees, some monkeys and the enchanting lemur Verreaux's sifaka do it from time to time. Habitually walking only on two legs as we do, however, had far-reaching ramifications all over the body, which entailed lots of compensatory adjustments. It could be argued that not a single bone or muscle, anywhere in the body, was spared the necessity to change, in order to reconcile some detail, however obscure, however out-of-the-way, and however indirectly or tenuously connected, with the major shift in gait. A similar across-the-board rejigging must attend each and every major change in way of life, from water to land, from land to water, into the air, underground. You cannot separate out the obvious changes in the body and treat them in isolation. To say that there are ramifications of every change is an understatement. There are hundreds, thousands of ramifications, and ramifications of ramifications. Natural selection is forever tweaking, adjusting the trim, 'tinkering' as the great French molecular biologist Fran├žois Jacob put it.

Here's another good way to look at it. When there's a major shift in the climate, say an ice age, you naturally expect natural selection to adjust the animals to it - grow a thicker coat of hair, for example. But the external climate is not the only sort of 'climate' we have to consider. Without any external change at all, if a major new mutation arises, and is favoured by natural selection, all the other genes in the genome will experience it as a change in the internal 'genetic climate'. No less than a shift in the weather, it is a change to which they have to adjust. Natural selection has to come along afterwards, adjusting to compensate for a major change in genetic 'climate', exactly as it would if a change had occurred in the external climate. The initial shift from a four-legged to a two-legged gait could even have been 'internally' generated rather than engendered by a shift in the external environment. Either way, it would have initiated a complicated cascade of consequences, each one of which necessitated compensatory adjustments of 'trim'.

'Unintelligent design' would have been a good title for this chapter. It might well, indeed, be a worthy banner for a whole book on the imperfections of life as a cogent indicator of the lack of deliberate design, and more than one author has independently seized upon it. Of these, because I love the robust irreverence of Australian English ('So where did Intelligent Design spring from, like a boil on a bum?') I homed in on the delightful book of Robyn Williams, doyen of Sydney science broadcasters. After complaining of the agony his own back gives him every morning, in terms that wouldn't come amiss from a whingeing Pom (don't get me wrong, I sympathize profoundly), Williams goes on, 'nearly all backs could make an instant claim on the warranty, if there were one. If [God] were responsible for back design, you'll have to concede that it wasn't one of His best moments and must have been a deadline rush job at the end of the Six Days.' The problem, of course, is that our ancestors walked for hundreds of millions of years with the backbone held more-or-less horizontally, and it doesn't take kindly to the sudden readjustment imposed by the last few million. And the point, once again, is that a real designer of an upright-walking primate would have gone back to the drawing board and done the job properly, instead of starting with a quadruped and tinkering.

Williams next mentions the pouch of that iconic Australian animal the koala, which - not a great idea in an animal that spends its time clinging to tree trunks - opens downwards, instead of upwards as in a kangaroo. Once again, the reason is a legacy of history. Koalas are descended from a wombat-like ancestor. Wombats are champion diggers, flinging great paws full of soil backwards like an excavator digging out a tunnel. Had this ancestor's pouch pointed forwards, its babies would have had eyes and teeth permanently filled with grit. So backwards it was and, when one day the creature moved up a tree, perhaps to exploit a fresh food source, the 'design' came with it, too complicated to change.

As with the recurrent laryngeal nerve, it might theoretically be possible to change the embryology of the koala to turn its pouch the other way up. But - I'm guessing - the embryological upheaval attendant on such a major change would render the intermediates even worse off than koalas coping with the existing state of affairs.

Another consequence of our own shift from quadruped to biped concerns the sinuses, which give such grief to many of us (including me at the moment of writing) because their drainage hole is in the very last place a sensible designer would have chosen. Williams quotes an Australian colleague, Professor Derek Denton:* 'The big maxillary sinuses or cavities are behind the cheeks on either side of the face. They have their drainage hole in their top, which is not much of an idea in terms of using gravity to assist drainage of fluid.' In a quadruped, the 'top' is not the top at all but the front, and the position of the drainage hole makes much more sense: the legacy of history, yet again, is written all over us.

Williams goes on to quote another Australian colleague, who shares the national gift for chucking a bonzer phrase, on the Ichneumonid wasps, whose designer, if there were one, 'must have been a sadistic bastard'. Darwin, although he visited Australia as a young man, expressed the same sentiment in staider, less antipodean terms: 'I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.' The legendary cruelty of ichneumon wasps (also the related digger wasps and tarantula wasps) is a leitmotif which will recur in the final two chapters of the book.

I find it hard to articulate what I am about to say, but it is something that I have been thinking for a while, and it came to a head during that memorable day of the dissection of the giraffe. When we look at animals from the outside, we are overwhelmingly impressed by the elegant illusion of design. A browsing giraffe, a soaring albatross, a diving swift, a swooping falcon, a leafy sea dragon invisible among the seaweed, a sprinting cheetah at full stretch after a swerving, pronking gazelle - the illusion of design makes so much intuitive sense that it becomes a positive effort to put critical thinking into gear and overcome the seductions of naive intuition. That's when we look at animals from the outside. When we look inside, the impression is opposite. Admittedly, an impression of elegant design is conveyed by simplified diagrams in textbooks, neatly laid out and colour-coded like an engineer's blueprint. But the reality that hits you when you see an animal opened up on a dissecting table is very different. I think it would be an instructive exercise to ask an engineer to draw an improved version of, say, the arteries leaving the heart. I imagine the result would be something like the exhaust manifold of a car, with a neat line of pipes coming off in orderly array, instead of the haphazard mess that we actually see when we open a real chest.

My purpose in spending a day with the anatomists dissecting a giraffe was to study the recurrent laryngeal nerve as an example of evolutionary imperfection. But I soon realized that, where imperfection is concerned, the recurrent laryngeal is just the tip of the iceberg. The fact that it takes such a long detour drives the point home with peculiar force. That is the aspect that would finally provoke a Helmholtz to send it back. But the overwhelming impression you get from surveying any part of the innards of a large animal is that it is a mess! Not only would a designer never have made a mistake like that nervous detour; a decent designer would never have perpetrated anything of the shambles that is the crisscrossing maze of arteries, veins, nerves, intestines, wads of fat and muscle, mesenteries and more. To quote the American biologist Colin Pittendrigh, the whole thing is nothing but a 'patchwork of makeshifts pieced together, as it were, from what was available when opportunity knocked, and accepted in the hindsight, not the foresight, of natural selection'.

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