  This paper presents the proportional evolutionary time hypothesis, which
posits that the mean time required for the evolution of complex life is a
function of stellar mass. The "biological available window" is defined as the
region of a stellar spectrum between 200 to 1200 nm that generates free energy
for life. Over the $\sim$4 Gyr history of Earth, the total energy incident at
the top of the atmosphere and within the biological available window is
$\sim$10$^{34}$ J. The hypothesis assumes that the rate of evolution from the
origin of life to complex life is proportional to this total energy, which
would suggest that planets orbiting other stars should not show signs of
complex life if the total energy incident on the planet is below this energy
threshold. 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 life
at the present age of the universe. F-, G-, and early K-dwarf stars (M > 0.7
M$_{\odot}$) represent the best targets for the next generation of space
telescopes to search for spectroscopic biosignatures indicative of complex
life.
