Snake Threat May Have Spurred Evolution of Primate Eyes

Snake Threat May Have Spurred Evolution of Primate Eyes, National Geographic, Stefan Lovgren, August 10, 2006 ...

[Graphic: Coquerel's Sifaka lemur, Duke University Primate Center]

The ability to detect threatening snakes may have shaped the visual system of our primate ancestors, a new study says. In a sort of evolutionary arms race, primates kept improving their eyesight to help spot and avoid snakes as the snakes became more dangerous, suggests Lynne Isbell, a behavioral ecologist at the University of California, Davis. "The initial change in primate [eyes] ... occurred when they had to deal with constricting snakes, probably about 90 million years ago," Isbell said. "That ended up with primates that have forward-facing eyes, whereas other mammals tend to have eyes on the sides of their heads." Forward-facing eyes allow better depth perception. When poisonous snakes evolved about 60 million years ago, primates further specialized their visual systems. "That resulted in the anthropoid primates-which we are one of-which had better vision all around, compared to the earlier primates that only had to deal with constricting snakes," Isbell said. The study is published in the July issue of the Journal of Human Evolution. Modern mammals first appeared about a hundred million years ago. Snakes were probably their first major predator-species with mouths big enough to eat mammals show up in the fossil record at about this time. "Snakes evolved a variety of ways to get their food, including widening their mouths enough to eat some mammals," Isbell said. And venomous snakes raised the stakes even further, forcing primates to get better at detecting the predators by, for example, developing excellent color vision and pattern recognition. [See also LiveScience. This "Just-So" Story ("How the Primate Got its Eyes!") highlights the unfalsifiability of Darwinian evolutionary explanations. Of course it might indeed have been that those primate ancestors which had better vision to detect snakes were the ones who survived to reproduce and pass on their genes for better vision to the next generation, so driving the gradual improvement of primate eyes. But then again it might have been better vision to detect anything (including other predators, width of branches, fruit, etc). Or, if all else fails it might be explained as sexual selection for "forward-facing eyes"! Indeed, it might have been all of the above! As Walter ReMine observed, "Evolutionary theory is a smorgasbord: a vast buffet of disjointed and conflicting mechanisms waiting to be chosen by the theorist":

"The central illusion of evolution lies in making a wide array of contradictory mechanisms look like a seamless whole. There is no single evolutionary mechanism-there are countless. Evolutionary theory is a smorgasbord: a vast buffet of disjointed and conflicting mechanisms waiting to be chosen by the theorist. For any given question, the theorist invokes only those mechanisms that look most satisfying. Yet, the next question elicits a different response, with other mechanisms invoked and neglected. Evolutionary theory has no coherent structure. It is amorphous. It is malleable and can readily adjust to disparate patterns of data. Evolution accommodates data like fog accommodates landscape. In fact evolutionary theory fails to clearly predict anything about life that is actually true." (ReMine, W.J., "The Biotic Message: Evolution Versus Message Theory," St. Paul Science: Saint Paul MN, 1993, p.24)

But then the problem is, how could this ever be tested?

Another problem is explaining why primates "ended up with forward-facing eyes, whereas other mammals tend to have eyes on the sides of their heads"? The same natural selection should have applied to all mammals, which the article says snakes preyed upon. Therefore, since natural selection cancels out, the true explanation must be, as Darwin's contemporary Samuel Butler pointed out, the variations (or in modern terms mutations) which were then naturally selected. That is (without endorsing Butler's Lamarckian answer) he was correct in his claim that "the `Origin of Variation,' whatever it is, is the only true 'Origin of Species'" (my emphasis):

"The question is not concerning evolution, but as to the main cause which has led to evolution in such and such shapes. To me it seems that the `Origin of Variation,' whatever it is, is the only true 'Origin of Species,' and that this must, as Lamarck insisted, be looked for in the needs and experiences of the creatures varying. Unless we can explain the origin of variations, we are met by the unexplained at every step in the progress of a creature from its original homogeneous condition to its differentiation, we will say, as an elephant; so that to say that an elephant has become an elephant through the accumulation of a vast number of small, fortuitous, but unexplained, variations in some lower creatures, is really to say that it has become an elephant owing to a series of causes about which we know nothing, whatever, or, in other words, that one does not know how it came to be an elephant." (Butler, S., "Life and Habit," [1910], Wildwood House: London, 1981, pp.263-264. Emphasis original)

As Darwin himself admitted, unless there was "the occurrence of profitable variations. ... natural selection can do nothing" (my emphasis):

"We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the conditions have changed, and this would manifestly be favourable to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, natural selection can do nothing." (Darwin, C.R., "The Origin of Species By Means of Natural Selection," Sixth Edition, 1872, Senate: London, 1994, p.64)

So what Darwinism needs to explain, but doesn't, is why a series of mutations occurred in a line of primates towards forward-facing, better eyes, which could then be naturally selected?

In the absence of something better than just "random" because, to the atheists who dominate evolutionary biology there is nothing that they would accept, "that could guide mutation in directions that are non-random":

"There is a fifth respect in which mutation might have been nonrandom. We can imagine (just) a form of mutation that was systematically biased in the direction of improving the animal's adaptedness to its life. But although we can imagine it, nobody has ever come close to suggesting any means by which this bias could come about. It is only in this fifth respect, the 'mutationist' respect, that the true, real-life Darwinian insists that mutation is random. Mutation is not systematically biased in the direction of adaptive improvement, and no mechanism is known (to put the point mildly) that could guide mutation in directions that are non-random in this fifth sense. Mutation is random with respect to adaptive advantage, although it is non-random in all sorts of other respects. It is selection, and only selection, that directs evolution in directions that are nonrandom with respect to advantage." (Dawkins, R., "The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design," W.W Norton & Co: New York NY, 1986, p.312. Emphasis original)

a Christian theist (like me) can consider as the true explanation, the theory of Harvard Professor of Botany and fellow Christian theist Asa Gray, "that variation [i.e. mutations] has been led along certain beneficial lines":

"Wherefore, so long as gradatory, orderly, and adapted forms in Nature argue design, and at least while the physical cause of variation is utterly unknown and mysterious, we should advise Mr. Darwin to assume, in the philosophy of his hypothesis, that variation has been led along certain beneficial lines. Streams flowing over a sloping plain by gravitation (here the counterpart of natural selection) may have worn their actual channels as they flowed; yet their particular courses may have been assigned; and where we see them forming definite and useful lines of irrigation, after a manner unaccountable on the laws of gravitation and dynamics, we should believe that the distribution was designed." (Gray, A., "Natural Selection Not Inconsistent With Natural Theology," Atlantic Monthly, October 1860, in Dupree, A.H., ed., "Darwiniana: Essays and Reviews Pertaining to Darwinism," [1861], Belknap: Cambridge MA, 1963, pp.121-122)

Stephen E. Jones, BSc (Biol).
`Evolution Quotes Book'