Thursday, March 05, 2009

"Why Do We Have The Senses That We Have?"

A common debating tactic used by creationists and ID supporters is to ask, "why do we have the senses that we have, and not some other ones"? The answer they usually provide is something like, "because that's the way the Intelligent Designer intended them to be".

The question of why we have the senses that we do is a very interesting one. As just one example, consider the sense of sight. As a type G2V yellow dwarf star, the sun gives off a relatively narrow range of electromagnetic radiation, including (from longest wavelength to shortest) radio waves, infrared radiation, “visible (red, orange, yellow, green, blue, and purple) light", and ultraviolet light. Almost all of these wavelengths of electromagnetic radiation can penetrate the Earth’s atmosphere (although the shorter wavelengths of ultraviolet light are somewhat attenuated by absorption by ozone/O3 in the upper atmosphere).

So, which wavelengths of electromagnetic radiation can we perceive? The answer depends on who you define as “we". Vertebrates have visual pigments in the cone cells of the retina that can absorb only three of these wavelengths: red (absorbed by the rhodopsin protein erythrolabe, which absorbs sunlight in the range of 564–580 nanometers), green (absorbed by the rhodopsin protein chlorolabe, which absorbs sunlight in the range of 534–545 nanometers), and blue (absorbed by the rhodopsin protein cyanolabe, which absorbs sunlight in the range of 420–440 nanometers). So, we vertebrates can only directly perceive red, green, and blue light (that’s why color computer monitors generate only red, green, and blue pixels).

However, most insects (including honey bees) have different visual pigments, and so see very different colors than we do. They do not have a visual pigment that corresponds to vertebrate erythrolabe, and so cannot perceive the color we call “red". However, they have a visual pigment vertebrates do not have, which can absorb light in the near ultraviolet range. Hence, insects can see colors (including ultraviolet) that we cannot see, and so the world appears very different to them.

So far, no organism on Earth has been discovered that can perceive the radio waves given off by the sun. This is probably because to do so would require absorptive structures several meters in length (the wavelength of most radio waves).

So, one answer to the creationist's question is that, taken as a whole, living organisms can perceive (or at least absorb) a range of light from the far infrared to the near ultraviolet, but lack receptors for most of the electromagnetic spectrum (such as radio waves, gamma radiation, etc). In other words, the range of electromagnetic radiation that can be perceived by living organisms matches quite closely the range of electromagnetic radiation given off by the sun and transmitted through the Earth’s atmosphere (with the exception of radio waves, which are too long to by absorbed by any known biological molecule).

That this is the case is exactly what one would expect to have evolved by natural selection, which can only work with what is available. Furthermore, it illustrates one of the basic ideas of evolutionary descent with modification: that the solutions to evolutionary problems vary from group to group as the result of historical contingency. Vertebrates see red, green, and blue, while insects see green, blue, and ultraviolet because two of our visual pigments (chlorolabe and cyanolabe) evolved before the divergence of insects and vertebrates from our common ancestor, while the third visual pigment evolved independently in the two groups, resulting in two different sets of perceived colors.

Compare this to the answer that ID provides: vision is the way it is because that’s how the Intelligent Designer intended it to be. Which of these answers to the creationist's query involves detailed empirical scientific investigation, and which simply relies on unsupported assertions?

As always, comments, criticisms, and suggestions are warmly welcomed!


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