It seems that this retracing of steps to escape the dead ends of the maze was repeated at each decisive evolutionary turning point. I have mentioned the evolution of the vertebrates from a larval form of some primitive echinoderm. Insects have in all likelihood emerged from a millipede-like ancestor -- not, however, from adult millipedes, whose structure is too specialised, but from its larval forms. The conquest of the dry land was initiated by amphibians whose ancestry goes back to the most primitive type of lung-breathing fish; whereas the apparently more successful later lines of highly specialised gill-breathing fishes all came to a dead end. The same story was repeated at the next major step, the reptiles, who derive from early, primitive amphibians -- not from any of the later forms that we know.
And lastly, we come to the most striking case of paedomorphosis, the evolution of our own species. It is now generally recognised that the human adult resembles more the embryo of an ape than an adult ape. In both simian embryo and human adult, the ratio of the weight of the brain to total body weight is disproportionately high. In both, the closing of the sutures between the bones of the skull is retarded to permit the brain to expand. The back-to-front axis through man's head -- i.e., the direction of his line of sight -- is at right angles to his spinal column: a condition which, in apes and other mammals, is found only in the embryonic, not in the adult stage. The same applies to the angle between backbone and uro-genital canal -- which accounts for the singularity of the human way of copulating face to face. Other embryonic -- or, to use Bolk's term, foetalised -- features in adult man are: the absence of brow-ridges; the scantness and late appearance of body hair; pallor of the skin; retarded growth of the teeth, and a number of other features -- including 'the rosy lips of man which were probably evolved in the young as an adaptation to prolonged suckling and have persisted in the adult, possibly under the influence of sexual selection' (de Beer).........
It is as if the stream of life had momentarily reversed its course, flowing uphill for a while, then opened up a new stream-bed. I shall try to show that this reculer pour mieux sauter -- of drawing back to leap, of undoing and re-doing -- is a favourite gambit in the grand strategy of the evolutionary process; and that it also plays an important part in the progress of science and art.
Monday 16 September 2019
The Dance of the Bats and Arthur Koestler
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