If you look across different species of mammals, everything but humans seem to get about a billion heart beats, and so in some ways that is a measure of what a life is – it is a certain number of heart beats and that’s like a unit of time. If you’re a shrew you live a really quick life, everything passes fleetingly, and if you’re a whale you take slow beats, but it really does literally measure us. As modern humans we get an extra life’s worth of heartbeats – we get 2 billion – which is a pretty special modern situation.
Brain surgery is thousands of years old, but it wasn't till the late 19th century that someone first undertook a heart operation, at least for medical purposes.
Biologist and writer Robert Dunn has written a history of heart surgery which reads like a fast-paced, and occasionally macabre, thriller. It's called The Man Who Touched His Own Heart.