I see wikipedia has an article "Assembly theory" invented by Cronin.,(reply>), The assembly number seems to also be related to entropy, although maybe not. Life leaves free oxygen in the atmosphere, which is unstable over time, so proof life is here, even if O2 itself has a low assembly number.,(reply>), And searching for extra terrestrial life looks for spectra for certain unlikely molecules, but still probably pretty simple. I suppose we can also look at spectra of dust clouds in space and see what the most complex molecules we can find, as assembly theory measures. Looking for PH3 on Venus probably isn't a great molecule marker for life, simple enough, right?!,(reply>), It seems like an "assembly number" is a sort of heuristic measure, not quantitatively meaningful, but qualitatively useful to give a sense of complexity, and differentiating between life-generated and nonlife processes.<br>It also seems something about life moved towards increasing complexity, which assembly theory can help explain.