Charles Darwin was one of the first scientists to theorize on the function of the appendix, which in his day had been identified only in humans and other great apes. He hypothesized that the distant ancestors of these animals survived on a diet of leaves, and so they required a large cecum, a portion of the gut that houses bacteria that can break down stubborn plant tissue. Later, he speculated, these ancestors shifted to a largely fruit-based diet that was easier to digest. A large cecum was no longer necessary, and it began to shrink...Darwin thought the appendix, which juts off of the cecum, is one of its former folds that shriveled up as the cecum shrank. Consequently, he thought it carried no function.
I recently read a source which states the above. It made me wonder that if there is some other organ/part in our body which may shrink as further evolution progresses and it's use diminishes, just like our appendix. I'd like to know if this 'theory of shrinking' applies to any organ that may become useless.