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In the human
embryo, eyes
develop from
bulges in the
brain's neural
tube that
pinch in to
form cavities.
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Inside-out
development |
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| Evolution,
which works by repeatedly modifying preexisting structures,
can explain the inside-out nature of our eyes quite simply.
The vertebrate retina evolved as a modification of the
outer layer of the brain. Over time, evolution progressively
modified this part of the brain for light sensitivity.
Although the layer of light-sensitive cells gradually
assumed a retina-like shape, it retained its original
orientation, including a series of nerve connections on
its surface. Conversely, mollusk eyes are wired optimally
because rather than evolving from brain cells, which have
wiring on the surface, they evolved from skin cells, which
retained their original orientation with the wiring below
the surface. |
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The living
world is filled with examples of many other organs and
structures that clearly have their roots in the opportunistic
modification of a preexisting structure rather than the
clean elegance of design. This does not, despite the
fears of "intelligent design" advocates, amount to evidence
against the existence of a Deity. Properly understood,
as Darwin himself pointed out, it only deepens our respect
for the power and subtlety of the Creator's remarkable
ways.  |
Mollusk eyes
don't share the
"design
problem" that
human eyes
have. |
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