Your Body - The Fish That Evolved. Dr. Keith Harrison

       Evolution in Practice

       T he body shape of any one species at any one time is a collection of features its history has handed down to it. If a species is to survive a change in its environment or to exploit a new way of life, natural selection can only act on the body that exists. It cannot always address a species’ problems in the most efficient way because there may not be the genes for this. If the species is to survive, natural selection must work with the tools history has given it. It’s time for another analogy.

      If your ship sank without warning and you were washed alone onto a desert island, you would have to survive with whatever you had in your pockets at the time. The best survival aids for the situation might be a 36-piece set of woodworking tools and a copy of Island Survival (4th edn) by R. Crusoe, but you don’t have these. You would have to use what you did have as best you could and adapt or die.

      In your pocket you might have a coin. You sharpen the edge on a stone and use it to whittle points onto arrows for hunting and fishing. A knife would have been better but you don’t have one. The coin will do the job and, in fact, the raised lettering that indicates the denomination helps your grip when cutting. Later you find a better way of sharpening your arrows. You no longer need the coin as a cutting tool but you do need a weight for your fishing line. The ideal solution would be a split lead shot but you don’t have any lead. Instead, you bend the coin double and clamp it over the line. Now the raised lettering aids the grip of the coin on the line. You have found two important uses for something that was initially for a completely different purpose; a purpose of no relevance whatever to your current situation. The coin was not ideal for either job but it worked. You catch fish and you survive. Meanwhile, the coin bears the scars of its past history. It still has the lettering and a mainly curved edge from its life as a coin, and the sharp edge from its life as a cutting tool. The lettering is now useful for a reason never intended when it was first developed; the curved and sharpened edges serve no purpose in a weight, but do no harm.

      What we do by craft, evolution does as an automatic consequence of natural selection, but the results can be similar. Like the coin and its lettering, the bodies of many animals include structures now employed for purposes that were not their original use. The teeth of sharks are modified skin scales; the wings of birds are modified arms; the membranes of bats’ wings evolved as skin to cover the body but later extended to provide a large aerodynamic surface. Also, like the curved and sharp edges of the fishing weight, animals’ bodies contain structures which serve no purpose today but which were evolved by ancestors who did need them. These have passed down through the generations ever since. You are probably sitting on the remnants of a tail, a tail evolved and used by your ancestors but now just a line of bones inside your body at the bottom of your spine. Another example would be the dewclaw of dogs. This is the small redundant toe which is seen above the foot and doesn’t touch the ground. The ancestors of dogs had five full toes but, as the dog evolved to move faster, one toe shrank and moved up the leg away from the others (reduction in toe number is common in animals which have evolved to move quickly over the ground – see p127). Dogs now have only four functional toes. Dewclaws serve no purpose today and dog breeders often have them surgically removed. If the dog’s ancestors had remained subject to the pressures of natural selection, rather than forming a relationship with humans and allowing us to control the evolution of their bodies, it is possible the dew claw would eventually have disappeared completely, or at least become wholly internal, like our tails.

      Evolution is not a perfectionist

      Once a feature has been lost in evolution, it is almost impossible to resurrect it. Natural selection only acts on what is visible to it so it is more usual for evolution to provide solutions to new problems by modifying parts of the body currently in use. For example, birds evolved from early reptiles which evolved from early amphibians which, in turn, evolved from early fish. Early fish, amphibians and reptiles all had long tails and these were originally used for swimming. In their evolutionary journey into the air, birds lost their heavy, bony tails and replaced them with long, light tail-feathers. Only the stump of the ‘parson’s nose’ remains. When some birds returned to a watery existence and began to swim again (penguins for example), they did not re-evolve a long swimming tail, they used the limbs that propelled them when they flew, their wings, and now fly underwater. In response to this, the bones of the wing have become stronger and heavier than those of other birds to cope with the density of water, which is much harder to push through than air, and as a result penguins have lost the ability to fly in air. Penguins also float and swim on the surface of the sea but again a swimming tail did not re-evolve. Here the hind feet became webbed and push the bird forward as they do when it walks on land. The actions of paddling are very like those of walking.

      The ideal engineering solution to the problem of how to propel a bird underwater might have been to give it a fish-like tail, but evolution does not plan and design bodies. Evolution is what happens because there is selection; it is not why there is selection. The penguin adapted to its new habits and survived because each generation was edited by natural selection. Small incremental changes modified the average appearance of penguins in such a way that they survived as underwater fish hunters. Nature does not demand the perfect solution, only a solution that works.

      This gives us the answer to an age-old question that has plagued many mothers: ‘Why is giving birth so painful?’ The brutal explanation is, ‘Because it doesn’t have to be painless.’ Evolution doesn’t care whether birth is agony or ecstasy as long as it is successful. As long as healthy babies continue to be born, the accompanying pain has no relevance to natural selection, regardless of its relevance to the mother. The current degree of childbirth pain is probably a relatively recent phenomenon, increasing only over the last few million years as human brains have increased in size. With the brains of babies in the womb enlarging relative to their ancestors, a larger head has had to pass through the same size of birth canal. As this has continued to happen successfully there has been no drive for natural selection to increase the size of the canal. The result is pain.

      ‘Nature abhors a vacuum’

      This is a phrase often used by biologists, but its meaning is not immediately obvious. As Darwin noted, species compete for resources. However, it’s always easier to make a living if you have no competitors, so, if an opportunity exists in nature which is not already being exploited, something will invariably evolve to exploit it.

      This is very like our commercial world today.