Why does <insert name of body part here> look like that?
or
Why does <insert name of body part here> work like that?
These tend to come up when students learn about some feature of the human body that doesn't seem very sensical to them; favorite examples include the electron transport chain in aerobic respiration and urine production in the nephron. When students ask these questions, my answer generally looks something like this:
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| Neil deGrasse Tyson said it best |
... because, really, I don't know exactly why things look/work the way they do. In these situations, I turn to evolutionary biology.
If folks think of the human body as a structure that was designed from the ground up, with goals in mind, features may indeed seem pretty nonsensical. However, evidence suggests this was not a ground-up, goal-directed design process at all. Evolution is more a "repurposing" - taking a feature that already exists and modifying it to perform another function.
Even that is a not-entirely-accurate, wildly oversimplified way of explaining evolution, as those modifications are thought to be randomly-occurring. In the words of Lynn Sagan in 1967, "... evolution is opportunistic and not foresighted." What is already present may, with a few random mutations, become modified such that it performs a new function. And that new function may increase the organism's likelihood of surviving and reproducing. And that feature may not be the most elegant or streamlined way of performing that function, but if it works, it doesn't have to be pretty. In the end, this process may produce some systems that resemble Rube Goldberg machines.
As I mentioned, my students often find the mechanics of the electron transport chain (ETC) to be crazy-making. The part that confounds many is what they perceive as design flaws - "Why the heck would anyone set it up that way???" And my answer is, "They probably wouldn't." If we think about building a system with the goal of the ETC in mind (ultimately making a boatload of ATP), the current system looks positively nuts. If, however, we think about the ETC as a repurposing of already-existing elements in a way that ultimately permitted this function, it may come off a bit different.
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| Repurposing. |
Even that is a not-entirely-accurate, wildly oversimplified way of explaining evolution, as those modifications are thought to be randomly-occurring. In the words of Lynn Sagan in 1967, "... evolution is opportunistic and not foresighted." What is already present may, with a few random mutations, become modified such that it performs a new function. And that new function may increase the organism's likelihood of surviving and reproducing. And that feature may not be the most elegant or streamlined way of performing that function, but if it works, it doesn't have to be pretty. In the end, this process may produce some systems that resemble Rube Goldberg machines.
As I mentioned, my students often find the mechanics of the electron transport chain (ETC) to be crazy-making. The part that confounds many is what they perceive as design flaws - "Why the heck would anyone set it up that way???" And my answer is, "They probably wouldn't." If we think about building a system with the goal of the ETC in mind (ultimately making a boatload of ATP), the current system looks positively nuts. If, however, we think about the ETC as a repurposing of already-existing elements in a way that ultimately permitted this function, it may come off a bit different.
Scientific American published an article that speaks to this point of design vs. evolution in 2003. It discusses how humans might look and function differently were we designed from the ground up.
Sagan, L. (1967) On the origin of mitosing cells. J Theoret Biol. 14: 225-274. http://www.gps.caltech.edu/classes/ge246/endosymbiotictheory_marguli.pdf
References
Alberts, B., et. al. (2002) "The Evolution of Electron-Transport Chains." In Molecular Biology of the Cell, 4th edition. New York: Garland Science. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK26849/Sagan, L. (1967) On the origin of mitosing cells. J Theoret Biol. 14: 225-274. http://www.gps.caltech.edu/classes/ge246/endosymbiotictheory_marguli.pdf
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