A Reassessment of the “Hard-steps” Model For The Evolution Of Intelligent Life
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 last eukaryote common ancestor (LECA) evolved (55), which marks the completion of the ‘eukaryogenesis’ process (51). Specifically, it remains unclear whether the LECA emerged hundreds of millions of years before the oldest eukaryotic-grade fossils (1.63-1.67 billion years ago, or Ga), or hundreds of millions of years after (to use the two end-member scenarios) (55).
In order to explore the implications of both scenarios, we display both: 1) the oldest fossil evidence for recognizable crown-group eukaryotes (1.06-1.03 Ga), which designates all eukaryotes, extant and extinct, descended from the LECA, and 2) the oldest fossil evidence for total-group eukaryotes (1.67-1.63 Ga), which comprises both crown-group eukaryotes and the now-extinct eukaryote lineages that diverged prior to the LECA (stem-group eukaryotes). A cladogram depicting the concepts of total, stem, and crown groups is displayed on the left, with † designating extinct stem-group lineages. Data sources: oldest evidence for life (56–59); oxygenic photosynthesis (60, 61); total-group eukaryotes (62); crown-group eukaryotes (63); crown-group animals (64); and Homo sapiens (65). — q-bio.PE
According to the “hard-steps” model, the origin of humanity required “successful passage through a number of intermediate steps” (so-called “hard” or “critical” steps) that were intrinsically improbable with respect to the total 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 the core assumptions of the hard-steps model in light of recent advances in the Earth and life sciences. Specifically, we advance a potential alternative model where there are no hard steps, and evolutionary novelties (or 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 in human evolution (e.g., the origin of eukaryotic cells, multicellular animals), then the “delay” in the appearance of humans can be best explained through the sequential opening of new global environmental windows of habitability over Earth history, with humanity arising relatively quickly once the right conditions were established.
In this co-evolutionary (or geobiological) scenario, humans did not evolve “early” or “late” with respect to the total lifespan of the biosphere, but “on time.”
Daniel B. Mills, Jennifer L. Macalady, Adam Frank, Jason T. Wright
Subjects: Populations and Evolution (q-bio.PE); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2408.10293 [q-bio.PE] (or arXiv:2408.10293v1 [q-bio.PE] for this version)
https://doi.org/10.48550/arXiv.2408.10293
Focus to learn more
Submission history
From: Daniel Mills
[v1] Mon, 19 Aug 2024 16:30:06 UTC (1,829 KB)
https://arxiv.org/abs/2408.102933
Astrobiology,
