The Galactic Habitable Zone (GHZ) is a region believed hospitable for life. To further constrain the GHZ, observations have been conducted of the J = 2 → 1 transitions of methanol (CH3OH) at 97 GHz, toward 20 molecular clouds located in the outer Galaxy (R GC = 12.9–23.5 kpc), using the 12 m telescope of the Arizona Radio Observatory. Methanol was detected in 19 out of 20 observed clouds, including sources as far as R GC = 23.5 kpc. Identification was secured by the measurement of multiple asymmetry and torsional components in the J = 2 → 1 transition, which were resolved in the narrow line profiles observed (ΔV 1/2 ∼ 1–3 km s−1). From a radiative transfer analysis, column densities for these clouds of N tot = 0.1–1.5 × 1013 cm−2 were derived, corresponding to fractional abundances, relative to H2, of f (CH3OH) ∼ 0.2–4.9 × 10−9. The analysis also indicates that these clouds are cold (T K ∼ 10–25 K) and dense (n(H2) ∼ 106 cm−3), as found from previous H2CO observations. The methanol abundances in the outer Galaxy are comparable to those observed in colder molecular clouds in the solar neighborhood. The abundance of CH3OH therefore does not appear to decrease significantly with distances from the Galactic Center, even at R GC ∼ 20–23 kpc. Furthermore, the production of methanol is apparently not affected by the decline in metallicity with galactocentric distance. These observations suggest that organic chemistry is prevalent in the outer Galaxy, and methanol and other organic molecules may serve to assess the GHZ.

For billions of years, life has continuously adapted to dynamic physical conditions near the Earth’s surface. Fossils and other preserved biosignatures in the paleontological record are the most direct evidence for reconstructing the broad historical contours of this adaptive interplay. However, biosignatures dating to Earth’s earliest history are exceedingly rare. Here, we combine phylogenetic inference of primordial rhodopsin proteins with modeled spectral features of the Precambrian Earth environment to reconstruct the paleobiological history of this essential family of photoactive transmembrane proteins. Our results suggest that ancestral microbial rhodopsins likely acted as light-driven proton pumps and were spectrally tuned toward the absorption of green light, which would have enabled their hosts to occupy depths in a water column or biofilm where UV wavelengths were attenuated. Subsequent diversification of rhodopsin functions and peak absorption frequencies track the diversification of surface ecological niches induced by the accumulation of atmospheric oxygen. Inferred ancestors retain distinct associations between extant functions and peak absorption frequencies. Our findings suggest that novel information encoded by biomolecules can be used as paleosensors for conditions of ancient, inhabited niches of host organisms not represented elsewhere in the paleontological record. The coupling of functional diversification and spectral tuning of this pervasive protein family underscores the utility of rhodopsins as universal testbeds for inferring remotely detectable biosignatures on inhabited planetary bodies.