Deriving human size from fundamental constants In this week's paper, William Press argues that intelligent life requires: (i) complex chemistry (ii) a substantial atmosphere (iii) bodies strong enough to withstand gravity From these premises, he derives the density of organisms, the environmental temperature, the mass and radius of their planets, and ultimately the characteristic size and lifespan of intelligent beings. The only free parameter is the effective bond energy, ε ≈ 0.003 Ry (hydrogen bonds). Creature size depends only weakly on ε (∝ ε^1/4), but lifespan depends very strongly (∝ ε^–2.75). His conclusion: intelligent beings should be roughly Earth-sized and inhabit Earth-like planets, yet their timescales for metabolism and lifespan could vary drastically with local chemistry and stellar conditions.