Adam's serum
Adam's serum was a unique substance which could increase a living being's lifespan by a large, but finite, amount.
The serum, which was first tested on a microbe, could multiply a living being's lifespan by a factor of ten, freezing the ageing process until the span of ten times the regular lifetime had mostly elapsed. In a human, this would mean a lifespan of about a thousand years; having first taken the formula in what appeared to be his thirties, Adam began ageing again around the time he turned 930.
The serum's creator, Adam, was a scientist, a member of a comparatively advanced human civilisation which eventually disappeared off the face of the Earth when most of its members left Earth behind to colonise another, distant planet, millennia prior to the 20th century. Adam only ever created a single dose, and it would have been impossible to create enough for the whole population. As such, the fellow scientists to whom he disclosed his discovery urged him to destroy it, lest it become an object of bitter conflict between would-be immortals. Unable to bring himself to destroy his miraculous discovery, however, Adam impulsively swallowed it himself. (COMIC: Dr. Who's Time Tales)
