Running with the theme of the previous post: the brain. For millennia the brain was a complete mystery. Some of the earliest writings of philosophers noted the brains purpose as a radiator of heat, that would spring a leak (runny nose) when overheated (fever). Thoughts and emotions were thought to reside in other organs of the body - hypochondriasis (beneath the breast-bone) and melancholia (residing in the liver) to name a few.
There are several advantages that we have today that our predecessors did not in determining the function of the brain. We have machines that can peer into the functional brain, evaluating it's macroscopic structure and large-scale metabolic function. Still, so much fine detail resides at the microscopic level, in the neurons and the connections they have with each other.
The link below is to a presentation given at a TED conference by Allan Jones about his work to map the brain in extraordinary detail, and also reveals some of the implications of his work.
Allan Jones: A map of the brain