Why Nature Chose Potassium

The presence of most of the atoms involved in the building up of living cells can be explained by their intrinsic physico-chemical properties. Yet, the involvement of the alkali metal potassium cation (K+) is somewhat of a mystery for most scenarios of origins of life, as this element is less abundant than its sodium counterpart in sea water, the original medium bathing the majority of proposed sites as the cradle of life. Potassium is involved in key processes that could as well have been fulfilled by sodium (such as maintenance of an electrochemical potential or homeostatic osmolarity). However, K+ is also required for the setup of a functional translation machinery, as well as for a fairly enigmatic metabolic pathway involving the usually toxic metabolite methylglyoxal. Here we discuss the possibility that potassium has been selected because of some of its idiosyncratic properties or whether it is just the outcome of the accidental place where life was born. Specific physico-chemical properties of the K+ ion would argue in favour of positive selection in the course of life's evolution. By contrast, the latter explanation would require that life originated on potassium-rich environments, possibly continental but yet of unknown location, making K+ presence just a frozen accident of evolution.