Pharyngula

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I throw away a lot of creationist email; most of it is ranty and weird, or pious and dull, so it isn’t worth dealing with. Every once in a while (but sadly, not that often) one is polite and asks a simple question, and then I feel compelled to reply. If it’s short and sweet, I’ll just fire off a one-liner—for instance, when I was asked why I reject Intelligent Design creationism, I could simply say that I haven’t seen any evidence for it.

Some are a little more persistent, requiring a little more effort to answer, so they get posted here. I’ll answer this one to some degree online, tell the person where to find it, and let the commenters chew on it some more. Be nice and pretend this fellow is sincere, OK?

Here’s his question:

Thank you for your reply that there is no evidence for design. I am trying
to figure out as an impartial person why scientists say there is no
evidence for design.

I think species should have evolved first with only one eye. After
realizing that one eye cannot create depth perception, nature would have
generated another eye following thousands of years of evolution. We know
this is not true. Someone or something already knew that one eye would not
be enough.

Please tell me what is wrong with my theory?

I’ve seen this question before.

That’s right, it’s a Pinkoskiism!

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This is precisely where some knowledge of development informs our understanding of evolution. Eyes (as the discrete, homologous organs with which we are most familiar) didn’t come first; the patterning information that establishes planes of symmetry—anterior vs. posterior, dorsal vs. ventral, and left vs. right—is generated before the eye field is established. We form eyes at the confluence of sets of interacting genes that turn on the Pax6 domain, and those genes have mirror symmetric patterns of expression. It takes additional, left-right specific patterns of gene activity to modify those domains selectively to turn off eye development on one side—in other words, it is easier to make two symmetrical eyes than one asymmetric eye.

His assumption is incorrect. The bilaterian ancestor would have evolved first with two eyes.

Comments

  1. #1 BMurray
    March 29, 2006

    Dawkins’ discussion of symmetry in Climbing Mount Improbable is the most effective answer to this question I think.

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