Let us reason about this problem by answering a
few questions.
- Which part of the eye is most
essential? The
answer is obvious: the retina. If light cannot be detected, the
remainder of the eye is useless.
- Can a retina, on its own,
without the rest of the eye, play a useful role? Obviously, yes. Many groups of not
too complex animals have ocelli, photo-receptor cells that just react to
the presence of light, but cannot form images. Of course, perceiving the
presence of light offers advantage against being totally blind. The proof
of this: ocelli have appeared independently in at least 40 different
animal groups.
- What is the next step? We also have
traces among current animals. The Planaria is a Platyhelminth
(flatworm) whose ocelli are located at the bottom of a concavity in its
body. Thanks to this, the Planaria not only detects the
presence of light, but also, to some extent, the direction it comes from.
It is also obvious that being able to perceive the direction of light
provides advantage to those who can, against those who cannot.
- And the next step? We can see it in the Nautilus,
a very old cephalopod that appeared in the Cambrian period and has barely
changed in 500 million years. At present there are only six species,
although they were quite abundant in the Paleozoic and the Mesozoic. It
happens that the eye of the Nautilus is equivalent to what is called a pinhole camera, a lens-less camera that
focuses the image by making light pass through a narrow hole, equivalent
to a pupil. In the history of mankind, the pinhole camera was the first
technological development after the invention of the camera obscura, and
dates back to the fifth century b.C.e. in China. Both in the history of
human technology and in the history of life, it plays the role of an intermediate
step in evolution, since its structure is similar to the vertebrate eye,
although it lacks the lens.
- And the lens? It seems logical to assume that
this was the last step in the formation of the eye, and that in turn it
went through two successive stages: first a fixed focus lens
would appear, which would just focus at a certain distance. Even so, it
would provide advantages, since natural selection would adapt the focal
length to the needs of each species. For long distance predators, it would
focus infinity, while those that captured small prey at close range would
focus much closer. In the second stage, the lens would have a variable
focal length, the same as our crystalline lens.
Charles Darwin |
The adaptive importance of vision is so evident, that it is not surprising that the
problem has been solved in many different ways. Among animals, there are few
totally blind, almost all have developed some form of vision. A few of them are
spectacular, as the compound eyes of insects, or the
eyes of scallops. The vertebrate eye stands out among all of them. As Darwin
pointed out, we cannot follow its evolution in detail, because even the most
primitive fish had eyes essentially identical to ours. Its evolution during the
Cambrian period must have been quite fast, in primitive chordates. Probably the
presence of such a perfect eye was one of the causes why vertebrates quickly
reached dominance in the evolution of the animal kingdom.
But there is another group of animals with eyes
very similar to ours: the cephalopods. And in them, as we have seen,
intermediate stages are preserved. The fact that the structure of our eyes
arose twice independently in the history of life, lets us deduce that its
evolution was not so difficult as the supporters of intelligent
design would have us believe.
And now one last question: Why has the structure of the
vertebrate eye remained stable for 500 million years? The history of
technology gives us the answer. The configuration space of inventions is not
regular, there are niches that can be represented as dips in said space. The
bottom of the dip can be reached by many different paths, but once there, it is
not possible to leave it, for any change in design will always be negative,
causing a loss in efficiency. Then we say that a stable equilibrium has
been reached. It is evident that the eye of vertebrates and cephalopods is in
that situation.
Does this tell us anything on whether evolution
is a consequence of pure chance, or a design tool? We’ll get into that in the
next post.
Manuel Alfonseca
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