Soon after the Big Bang, the universe contained essentially only hydrogen and helium (in the ratio 75% to 25%). There were small amounts of lithium, and even smaller traces of beryllium and boron, but that was all. To an astronomer, then, the universe consists of hydrogen, helium and everything else; all the elements heavier than hydrogen and helium—the “everything else”—are called metals. Now, the biochemistry of terrestrial organisms, and the biochemistry of any extraterrestrial organisms we can plausibly imagine, depends crucially on six elements: hydrogen (H), sulfur (S), phosphorus (P),
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