Origin of flight
One of the more remarkable facts of evolution is that the ability to fly occurred independently and separately four times among different kinds of creatures.
1 Insects Winged insects appeared suddenly and plentifully alongside wingless insects in the Carboniferous period, some 300 million years; ago. Before that there are no fossil insects at all, flying or otherwise, so any suggestion as to how they evolved the capacity of flight is sheer guesswork.
2 Dinosaurs Flying pterosaurs (e.g. pterodactyls and pteranodons) were abundant from about 180 million years ago until they were wiped out in the general extinction of dinosaurs sixty-five million years ago. Again, the first known fossils show them fully capable of flight, and although the earliest ones were rather less specialized for flight than the later ones, there is absolutely no sign of earlier intermediate stages.
3 Birds As described in the first chapter, there is no agreed origin of birds. They arrived plentifully in the fossil record about sixty million years ago, simultaneously equipped with feathers, hollow bones, a new digestive system, air sacs) and a number of other novelties that distinguished them from their reptilian ancestors. Academically, the debate has centred on whether dinosaurs, pterosaurs and birds all had a common ancestor, or whether some early dinosaurs were transformed into birds.
4 Bats The last group of creatures to gain the capacity of flight was the bats, fifty million years ago. Again there is no clue as to what transitional stages there may have been. It is presumed they evolved from some earlier, insect-eating, shrew-like animals that climbed trees. Today, there are a few species of animals that can glide from branches of trees. But they are anatomically unlike bats in almost every other respect, and nobody regards them as providing a model for the origin of bat flight.
In particular, it is difficult to imagine intermediate stages for bats. A half-developed forelimb would mean the creature could neither fly nor walk properly. Also, a bat's pelvic girdle has rotated 180° compared with other mammals, enabling it to fly; what possible selective advantage could there be in a 90o turn? The milk teeth of baby bats are turned inwards so that they can cling on to the mother's hair as she hangs high from the ground. If this faculty had not appeared fully operable, would it not have been fatal both for individual baby bats and thus for the entire species?
Looking for explanations of evolutionary novelties, huge in number as they are, it is perhaps not surprising to find that textbooks in general are reluctant to explore these tricky questions, Sir Julian Huxley's classic work Evolution: The Modern Synhesis, first published in 1940 and still today, after many editions, a key statement of neo-Darwinian theory, does not mention the evolution of the eye at all. Nor does Paul Amos Moody's Introduction to Evolution, one of the most widely approved textbooks of the last twenty-five years; nor Grizmek's Encyclopedia of Evolution; nor the Encyclopaedia Britannica.
Colin Patterson, in his recent book Evolution, published by the British Museum of Natural History, also steers clear of the eye problem, except to admit that a lens in the eye is no use unless it works, and that a distorting lens might be worse than no lens at all.
(He also raises the question of another evolutionary 'How can the segmentation of an animal like an earthworm or a centipede arise bit by bit?' Again he drops the subject abruptly apart from confessing: 'an animal is either segmented or it is not’.2)
Darwin worried about the eye for four and a half pages in Origin, and in the end took comfort in noting the large number of different kinds of eyes that had emerged in various creatures, living and extinct, ranging from the primitive to the near-perfect. This line has been followed by his disciples ever since (panel 16), but is open to severe doubt. Eyes from different species can be arranged neatly on the printed page in a graded series, but none of them necessarily has anything to do with how the human eye evolved. It shows, of course, that nature has taken several different paths in creating the ability to see, but this in a way merely aggravates the problem, for the process of evolution in none of them is clear.
The abracadabra approach used to explain away the fossil gaps is equally in evidence when it comes to evolutionary novelties. With a wave of the wand, difficulties and complexities are minimized. Thus the truly vast anatomical steeplechase that has to be run to turn a small hoofed animal (presumably) into a whale becomes:
The major differences between the whales and the early mammals are attributable to adoptions for the swimming life. The forelimbs have become paddles. The rear limbs have been lost altogether, though there are a few small bones buried deep in the whale's body to prove that the whale's ancestors really did, at one time, have back legs.3 Darwin himself offered a prime abracadabra on the subject: I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their habits, with larger and larger mouths, till a creature was produced as monstrous as a whaler.4
Panel 16
Many ways of seeing things
Darwin having had the shudders about the eye in January 1860, seems to have been happier by April. He wrote again to his friend Asa Gray: ‘I remember the time when the thought of the eye made me cold all over, but I have got over this stage of the complaint.’
His solution in Origin was that ‘natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class’.
Ernst Mayr approved this, saying that there was 'a correct nucleus in his claim', even thought it was 'somewhat oversimplified to put it all down to photosynthesis'.
George Gaylord Simpson wrote that one can observe in life on Earth today 'representative stages at every gradually different level, from diffuse photosensitivity of the whole body through scattered photosensitive cells to cell plates, basins, basins and vesicles plus lenses, and so on to the fully developed image- forming eye with lens, iris, and its other complexities. These photoreceptors function splendidly at every level and do not wait to start working until the final stage is reached. They simply enlarge, refine, and to some extent change their functions, as they become more complex.’20
An illustration in Sir Gavin de Beer's Atlas of Evolution graphically showed the Darwinian explanation of eye evolution. But a critical reviewer aptly commented: 'This mere listing of eyes from various animals, which he neglects (or is unable) to show to be related can carry no conviction for the case for evolution. It would be equally stupid to place a candle, a torch and a searchlight side by side and proceed to advance to a genealogic relationship.’21
Sir Gavin de Beer's Atlas of Evolution showed (top) a one-celled organism with light-sensitive spot and a primitive lens, (centre) a section through the eye of a jellyfish showing light-sensitive cells forming a cup-shaped retina, and (bottom) a tadpole where the retina is formed from the lining of the brain cavity. Taken in sequence, the three diagrams give an impression of the evolution of the eye from simple to complex.
Cautious man as he was, Darwin must have had second thoughts about this, for the sentence was quietly dropped from later editions of Origin. However, Sir Gavin de Beer in his Atlas of Evolution was not deterred. Whale ancestors, he wrote, had dentitions enabling them to feed on large animals, but some took to preying on fish and rapidly evolved teeth like sharks . . . Next, some whales preyed on small cuttle fish and evolved a reduced dentition. Finally the whalebone whales, having taken to feeding on enormous numbers of small shrimps, also evolved rapidly.6
