  The goal of finding and characterizing nearby Earth-like planets is driving
many NASA high-contrast flagship mission concepts, the latest of which is known
as the Advanced Technology Large-Aperture Space Telescope (ATLAST). In this
article, we calculate the optimal spectral resolution $R=\lambda/\delta\lambda$
and minimum signal-to-noise ratio per spectral bin (SNR), two central design
requirements for a high-contrast space mission, in order to detect signatures
of water, oxygen, and chlorophyll on an Earth twin. We first develop a
minimally parametric model and demonstrate its ability to fit synthetic and
observed Earth spectra; this allows us to measure the statistical evidence for
each component's presence. We find that water is the easiest to detect,
requiring a resolution $R \gtrsim 20$, while the optimal resolution for oxygen
is likely to be closer to $R = 150$, somewhat higher than the canonical value
in the literature. At these resolutions, detecting oxygen will require $\sim$2
times the SNR as water. Chlorophyll requires $\sim$6 times the SNR as oxygen
for an Earth twin, only falling to oxygen-like levels of detectability for a
low cloud cover and/or a large vegetation covering fraction. This suggests
designing a mission for sensitivity to oxygen and adopting a multi-tiered
observing strategy, first targeting water, then oxygen on the more favorable
planets, and finally chlorophyll on only the most promising worlds.
