tokens: the latter is also unknown for putative extraterrestrial organisms, in principle a comparatively robust estimate is probably set by biophysical considerations: at least one photon must be incident on the area of the photosynthetic apparatus (e.g., photosystem I or II) over a single temporal cycle involving electron transfer. The minimum empirical bound of ∼6 × 10 11 photons cm −2 s −1 has been explained through charge recombination in photosystem II, protein turnover, and H + leakage (Raven et al. 2000 ). Therefore, combining these results, the critical PAR energy flux becomes Ψ 0 ∼ 2.1 erg cm −2 s −1 . Substituting this value into ( 5 ), and solving for the cutoff distance ( d P ) yields the extent of the “photosynthesis zone.” By doing so, d P is given byA couple of important points are worth highlighting regarding the above formula: (i) the net primary productivity of biospheres at these low light levels is potentially several orders of magnitude smaller with respect to modern Earth, and (ii) photoautotrophs require not only sufficient fluxes of photons at appropriate wavelengths, but also reactants (e.g., CO 2 ), nutrients (e.g., ), and electron donors (Lingam Loeb 2018a , 2019b ). The photosynthesis zone has been plotted as a function of M BH in Figure 1 . Substituting the parameters for the Milky Way, we obtain d P ≈ 0.34 kpc. This distance is fairly “large” because it approximately equals the scale height of the thin disk of our Galaxy. If we consider black holes with masses on the order of 10 9–10 M ⊙ , we find d P ≳ 10 kpc. In such instances, large swathes of galaxies would fall within the domain of the photosynthesis zone. As noted in Sections 2 and 4 , UCDs are characterized by very high stellar densities, indicating that the photosynthetic zone calculated above could also encompass most stars in these galaxies. 3.2. Timescales for the Evolution of Photosynthesis Finally, we must also examine the issue of timescales. Analyses of isotopic ratios on Earth favor the existence of anoxygenic photosynthesis by ∼3.5 Ga (Tice Lowe 2004 ; Olson 2006 ; Butterfield 2015 ), or possibly even earlier; recall that we posited life had originated by ≳3.7 Ga. The origin of oxygenic photosynthesis remains less tightly constrained, with some proxies (e.g., U–Th–Pb isotopic ratios) placing it as far back as ≳3.7 Ga (Rosing Frei 2004 ), although unambiguous evidence from microfossils appears to date back only to 1.9 Ga (Lyons et al. 2014 ; Fischer et al. 2016 ). Despite this uncertainty, there are multiple grounds for contending that oxygenic photosynthesis may have evolved by ≳2.7 Ga, based on isotopic ratios, phylogenetics, and microfossils (Buick 2008 ; Schirrmeister et al. 2016 ), thus amounting to its emergence by at least several 100 Myr prior to the Great Oxidation Event at ∼2.4 Ga; see also Knoll et al. ( 2016 ), Lingam Loeb ( 2019c ), and Cardona et al. ( 2019 ). In toto , it is not altogether unreasonable to surmise that both anoxygenic and
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