The lifespan of the biosphere is necessarily constrained between the onset of Earth’s habitability (the “habitability window,” constrained between ~4.5 and ~3.9 Ga) (147) and its end (the extinction of all life, constrained to ~1.0 ± 0.5 Gyr into the future) (40–42). The temporal distributions of the five extant clades corresponding to each of our candidate hard steps (Fig. 1) are displayed by the horizontal bars, with dashed segments representing uncertainties surrounding the timing of the origin (left) and eventual extinction (right) of each group. The timing of extinction for each group is purely schematic, following the general prediction that declining pO2 in the future (as well as other factors not displayed here, such as rising sea surface temperatures) will drive these groups to go extinct in the reverse order of their appearance (42). The “window of human habitability [pO2],” represented by the blue vertical bar, approximates the interval of Earth’s total history (past and future) where pO2 exceeds the threshold necessary to support long-term human habitation (53 to 59% PAL O2) (123–125). The atmospheric O2 curve (green) was modified from Ozaki and Reinhard 2021 (211). pO2 = partial pressure of atmospheric O2; PAL = present atmospheric levels. — Science Direct via PubMed
According to the “hard-steps” model, the origin of humanity required “successful passage through a number of intermediate steps” (so-called “hard steps”) that were intrinsically improbable in the time available for biological evolution on Earth.
This model similarly predicts that technological life analogous to human life on Earth is “exceedingly rare” in the Universe. Here, we critically reevaluate core assumptions of the hard-steps model through the lens of historical geobiology.
Specifically, we propose an alternative model where there are no hard steps, and evolutionary singularities required for human origins can be explained via mechanisms outside of intrinsic improbability.
Furthermore, if Earth’s surface environment was initially inhospitable not only to human life, but also to certain key intermediate steps required for human existence, then the timing of human origins was controlled by the sequential opening of new global environmental windows of habitability over Earth history.
Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻
