And of the six essential elements of life--carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (aka CHNOPS)--phosphorus can be difficult to find in an available form.
"There's something fundamental about understanding the flexibility of life." Grind up any known organism and you get CHNOPS, she says, but all those organisms are also thought to come from a single ancestor.
An estimation of the possible number of covalently bonded compounds of CHNOPS
with a molecular weight less than 300 Da, conforming to the maximal size of the core components, yields a number at least in the millions and probably several orders of magnitude higher (Fink et al, 2005).