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This paper presents the proportional evolutionary time hypothesis, whichposits that the mean time required for the evolution of complex life is afunction of stellar mass. The "biological available window" is defined as theregion of a stellar spectrum between 200 to 1200 nm that generates free energyfor life. Over the $\sim$4 Gyr history of Earth, the total energy incident atthe top of the atmosphere and within the biological available window is$\sim$10$^{34}$ J. The hypothesis assumes that the rate of evolution from theorigin of life to complex life is proportional to this total energy, whichwould suggest that planets orbiting other stars should not show signs ofcomplex life if the total energy incident on the planet is below this energythreshold. The proportional evolutionary time hypothesis predicts that late K-and M-dwarf stars (M < 0.7 M$_{\odot}$) are too young to host any complex lifeat the present age of the universe. F-, G-, and early K-dwarf stars (M > 0.7M$_{\odot}$) represent the best targets for the next generation of spacetelescopes to search for spectroscopic biosignatures indicative of complexlife.

Does the evolution of complex life depend on the stellar spectral energy distribution?