That is something which any science fiction writer wishing to create a plausible planetary fauna has to remember: evolution can only work with what it has to start with and, generally speaking, organs which are lost cannot be regained. The reason birds have no forelegs is because all land vertebrates, of which they are descendants, have only four legs. On the other hand, if birds possessed four legs as well as a pair of wings, you could bet your bottom dollar that all the mammals, reptiles, and amphibians would have six legs.
Not only that, but all land vertebrate limbs are based on the five-digit system. Many of them have lost some digits, but only a few, such as the giant panda, have gained a sixth - and the extra one develops differently from the others. If you examine the fossil record, you will find that the ancestors of the horse had more toes, and gradually lost them over time. If you examine the horse's embryo, you will find that it starts off with five toes and then loses them.
Have you ever wondered why whales have a horizontal tail fin while the tail fins of fish are vertical? Fish swim by lateral undulations. This method was then transferred to land vertebrates. If you watch a lizard, you will see that its legs sprawl out to the side, and it walks by swinging its body from side to side. Mammals, on the other hand, have managed to get their legs tucked in vertically under their bodies, allowing a more efficient gait. They flex their backbones vertically when they walk and run. So when they moved back to the sea, they maintained that method. Whales and dolphins undulate vertically. So do seals and otters.
Let's now examine a genuine fictional fauna. Edgar Rice Burroughs may have been a pulp writer, but his series on Barsoom (Mars) nevertheless display a brilliant creative imagination. But he committed three biological sins: (1) interplanetary hybrids, (2) two intelligent species, and now (3) evolutionary anomalies. It was a fine touch to make his animals multilegged, but it is puzzling to note that the calots and banths had ten limbs, the thoats eight, the white apes, apts, and Green Martians six, and the Red Martians who, despite laying eggs, nevertheless had breasts, possessed no more limbs that the earthly humans they resembled.
I am trying to imagine how this could come about. With little creepy-crawlies it is not so difficult. Common worms are made up of segments, and usually the segments carry an appendage. These have been lost in the common or garden earthworms, but you can still see them in marine worms. One step up the evolutionary ladder stand the arthropods: the familiar insects, spiders, scorpions, and so forth, which have their skeletons on the outside. They also consist of segments, each with their own appendage, a phenomenon easiest to see in the centipedes, millipedes, and prawns. Indeed, several segments have been squeezed together up front to produce a head, with the appendages converted into mouthparts. Creatures like this can easily vary in the number of legs, because simple genetic mechanisms exist to add or subtract segments.
It is another matter with large animals containing internal skeletons. It is hard to see how they could be segmented, and legs are far too complex to simply duplicate. Fish do possess segments of a sort; their bodies are made up of bands of muscles called myotomes. They still exist in us today. The bony stiffening between the myotomes have become ribs and vertebrae.
As you are aware, fish have a pair of fins at both the front and the rear. However, there was an ancient class of fish, now represented only by the coelacanths and lungfish, which possessed bony paddles where modern fish have fins. It was these fish which first crawled onto land, and their paddles became feet - each with five digits. This is why all land vertebrates are four-footed. I suppose it is theoretically possible that, on another world, a particularly long type of fish would have a middle pair of paddles as well, from which would evolve a whole tribe of six-legged land vertebrates. But I consider it unlikely that more than six legs would be possible.
MutantsThe 1950s and 1960s, when we lived under the threat of nuclear war, and the effects of radiation were not well known to the general public, were also the heyday of the pulp science fiction novels. The result was a lot of misleading depictions of the "mutants" resulting from such a conflict.
Radiation can cause mutations. The biggest, the most visible ones, such as cleft palate and cystic fibrosis, are nearly always bad, because they result from the failure of a gene to carry out its allotted task. However, most physical and mental characteristics are the result of many genes, and most genes have multiple effects. That means that beneficial mutations are nearly always small in scope, the result of tweaking the rate of growth of a body part, or the effect of a hormone, for example. What you cannot get are what used to appear in those pulp scifi stories: an extra set of eyes, a set of horns, and such, for they would require a combination of a large number of changes in a large number of genes.
The other point to remember is that you get two sets of each gene, one from each parent. If the gene is dominant, it requires only one in order to manifest itself. If it requires two, it is recessive. Thus, the gene for albinism is recessive because it simply does nothing: it does not produce melanin. If you get a normal gene from one parent and the albinism gene from the other, you will be all right, because the single normal gene can produce enough melanin for your body. Only if you receive an albinism gene from both your parents will you suffer from albinism. That is why most damaging mutations are recessive.
It has been estimated that we all carry an average of three lethal genes. The reason why we don't die is that we possess only a single copy of each defective gene, rather than two. The reason our children don't die is that our partners in life possess three different lethal genes. Only if both parents are unlucky enough to carry the same mutation is there a chance that their children will be affected. The corollary is that even if increased radiation results in increased mutations, they will not show up for several generations, until the victims carrying the same mutation intermarry. That is why there was no increased incidence of birth defects following the atomic bomb attacks on Hiroshima and Nagasaki.
My next article will be on Man-Eating Plants. Click here, however, if you wish to return to the Index.