A reassessment of the "hard-steps" model for the evolution of intelligent life

Abstract

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.

Fig. 1.. The temporal distribution of our candidate hard steps.

The vertical colored bars represent the earliest unequivocal evidence for each candidate hard step in the geologic record with widths spanning the upper and lower age constraints (bar lengths are arbitrary). While there exist more contentious geochemical and molecular clock estimates for these steps that would place them each farther back in time, we have chosen the least controversial evidence to produce the most conservative timeline possible. Therefore, each candidate hard step necessarily preceded, but occurred no later than, their displayed dates, and the incorporation of other lines of evidence would necessarily shift the origin of each step back in time to varying degrees. The time intervals separating adjacent steps were calculated using the minimum age constraints only and are displayed in bold and expressed in billions of years (Gyr). With respect to the eukaryotic fossil record, there is ongoing uncertainty concerning when the LECA evolved (66), which marks the completion of “eukaryogenesis” (199). Specifically, it remains unclear whether LECA emerged hundreds of millions of years before the oldest eukaryotic-grade fossils (1.63 to 1.67 Ga), or hundreds of millions of years after (to use the two end-member scenarios) (66). In order to explore both scenarios, we display (i) the oldest fossil evidence for recognizable crown-eukaryotes (1.06 to 1.03 Ga), which designates all eukaryotes, extant and extinct, descended from LECA, and (ii) the oldest fossil evidence for total-eukaryotes (1.67 to 1.63 Ga), which comprises both crown-eukaryotes and now-extinct eukaryote lineages that diverged before LECA (stem-eukaryotes). A cladogram depicting the concepts of total, stem, and crown groups is displayed on the left, with “✝” designating extinct stem-lineages. Data sources: oldest evidence for life (–203); oxygenic photosynthesis (204, 205); total-eukaryotes (206); crown-eukaryotes (56); crown-metazoa (207); and Homo sapiens (208).

Fig. 2.. Phylogenetic comparisons between a single origin versus multiple origins of an evolutionary innovation.

(A) The phylogenetic pattern reconstructed when a given evolutionary innovation is constrained to a single living clade (monophyletic group), the result of a single origin (designated by the red dot). (B) The phylogenetic pattern reconstructed when a given evolutionary innovation is found in two different living clades, the result of two independent origins. (C) The phylogenetic pattern reconstructed when an evolutionary innovation is constrained to a single living clade, but as the result of the extinction of lineages that had independently evolved the innovation.

Fig. 3.. Unidirectional changes in Earth’s surface environment over geologic time.

(A) Increasing solar luminosity (expressed as percentage of modern values) since the origin of Earth (209). (B) Increasing primary productivity (expressed as the ratio between ancient and modern levels) over Earth history, using values from figure 1A from Crockford et al. (159). (C) Increasing atmospheric O₂ (expressed as the percentage of present atmospheric levels) over geologic time, taken from figure 3 of Mills et al. (140). (D) Evolution of the geosphere (crustal abundance, supercontinental cycles) over geologic time, taken from figure 1 from Crockford et al. (210) and references therein. Overall figure design and content was inspired by figure 1 from Crockford et al. (210). Phan = Phanerozoic eon, 539 to 0 million years ago.

Fig. 4.. The total lifespan of the biosphere.

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) (–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) (–125). The atmospheric O₂ curve (green) was modified from Ozaki and Reinhard 2021 (211). pO₂ = partial pressure of atmospheric O₂; PAL = present atmospheric levels.