The chemical elements that are needed to sustain life on earth are generally found near the earth’s surface.

Humans, for example, need iron to transport oxygen in our blood. We also need cobalt for that job too, and it has an important use in maintaining our nervous system.

We can get adequate intakes of iron and cobalt, as well as all other minerals and vitamins that our bodies require, by eating a balanced diet.

But if we get too much exposure to certain chemicals, then it can kill us. Please don’t add polonium to your weekly shopping basket.

In his book, A Short History of Nearly Everything, Bill Bryson explains that our tolerance to chemicals is related to their abundance on the earth’s surface.

We have evolved to utilise (or tolerate) the chemicals that we encounter most often.

Bryson gives the example of the different copper requirements of cattle and sheep.

“Modern cattle need quite a lot of copper because they evolved in parts of Europe and Africa where copper was abundant. Sheep, on the other hand, evolved in copper-poor areas of Asia,” he explains.

This could also explain why copper requirements vary across different breeds of the same livestock species.

For example, continental breeds of sheep, particularly Texel, are much more susceptible to copper poisoning than the likes of Suffolk, Cheviot and Blackface sheep, which originated in Britain.