h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This is the title of an essay by the paleontologist Stephen Jay Gould, which shows how the said body part links Darwin to US President Andrew Jackson.

Canning had been Lord Castlereigh's opponent in a pistol duel -- he was hit on the left buttock, Castlereigh was saved by a tunic button. Had Castlereigh been killed, he would not have been able to intervene at Ghent in the British-American treaty to end the War of 1812. The war had gone badly for the Americans, and the British were making heavy demands. Castlereigh persuaded them to be more moderate, and a peace was signed, essentially restoring the status quo. Before news of it reached America, Jackson won a crushing victory at New Orleans. This made him a national hero, and opened the way to the White House. Had the treaty been harsher, his triumph would probably have been viewed as a bad joke.

Also, if Castlereigh had been killed, he obviously wouldn't have been able to commit suicide. Fitzroy had the same morose temperament as his uncle, and, according to Gould, this, coupled with the lonliness of command, drove him to despair. This is why he paid to have the civillian Darwin on board, as somebody he could dine with and talk to. Had Castlereigh not committed suicide, it is unlikely that his nephew would have fully realised the seriousness of his own situation, and would not have felt the need to invite Darwin on board the Beagle.

And that, says Gould, is why Andrew Jackson and Charles Darwin owed it all to George Canning's left buttock.

(You can find the original in Gould's book 'Bully for Brotosaurus')