Where were my body’s atoms a million years ago?

Four elements account for 96 per cent of your mass: oxygen, carbon, hydrogen and nitrogen. Oxygen makes up over 61 per cent of your mass, compared with hydrogen at just 10 per cent, but hydrogen is a much lighter element, so around 63 per cent of your atoms are hydrogen atoms, 24 per cent oxygen and 12 per cent carbon.

Most of your hydrogen atoms come from the water you drink, and if this is tap water from a reservoir, then 100 years ago these atoms could have been in any of the world’s oceans. Groundwater aquifers exchange water more slowly though, over timescales of several thousand…