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Name: Phebe
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: Winter 2012-2913


Question:
What is the appendix homologous to in other mammals? What do homologous structures indicate?



Replies:
Phebe,

Traditionally, evolutionary biologists have speculated that the appendix is a vestigial structure (meaning a little used anatomical remnant) from an expanded cecum, which is part of the intestines. The argument was that the enlarged cecum in other mammals used to digest cellulose (a major plant fiber) was largely discarded by evolution. This theory still has its attractions, but recent evolutionary data suggests that the appendix has actually evolved several times independently, suggesting it has more function than merely a remnant of a functional structure. Separately, recent studies have suggested a role for the appendix in the immune system, as it provides a reservoir for beneficial bacteria in our intestines. It is unclear, then, whether the appendix is indeed a remnant of a larger cecum or not.

Nevertheless, there are many anatomical structure that are homologous. For example, the bones in a marine mammal's flippers (such as a dolphin or whale) are largely the same as our own hand bones, although their shape and size is quite different. It seems odd that these sea creatures would need such a large and sophisticated set of bones in their flippers until you realize that they evolved from land mammals that needed more than just an undifferentiated flipper. Even more strikingly, these mammals have vestigial remnants of hindlimb bones that have no known practical use. These homologous structures give us clues to evolutionary relationships between different species, and helps us trace how anatomical structures are modified and discarded with time in response to selection pressures. Two species who share homologous structures probably had a common ancestor which exhibited these basic anatomical structures, which were then separately modified through evolution.

A fascinating article on this subject was written in 1978 by Stephen Jay Gould, titled "The Panda's Peculiar Thumb". In it, he discusses a case of the panda's thumb which is NOT a homologous structure to a human thumb, despite their apparent similarities (these are called analogous structures). You may find it worthwhile reading; it is written for a largely non-scientific audience.


S. Unterman Ph.D.


Hi Phebe,

Thanks for the question. I am not sure what structure the appendix is homologous to in other mammals. Homologous structures indicate the descent from a common ancestor. I would recommend reading the entry about homology on Wikipedia. I think it will be of benefit to you.

I hope this helps. Please let me know if you have more questions. Thanks Jeff


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