The Critical Path of Evolution

The evolution of life is treated as the traversal of N ~ 10¹⁴ small, high-probability steps of increasing complexity, with mean step time μ — a critical path from the thermal gate of the cosmic microwave background to a technological civilization over a time T ≈ 13.8 Gyr. When the step times are independent, the concentration of their sum compresses the relative spread of exit times as 1/√N; within the model this yields a dispersion σ = √(μT). We show that the observed absence of "loud" civilizations (cosmic silence) gives an upper bound on the interplanetary dispersion of exit times σ ≈ 800–1400 yr — of the order of the horizon of the transition to detectability; from this σ follows μ ≈ 25–75 min, which coincides with the generation time of microorganisms. The same silence places an upper bound on the Galactic population at n ~ 10⁴–10⁵ civilization-bearing planets, in agreement with astrophysical estimates of the number of Earth-like habitats. Two properties of the critical path — its progression under a local surplus of resources and the kinematic uniqueness of the fastest route — make its parameters approximately universal and ensure a synchronous tempo for the development of life; cosmic silence indicates that this route is realized through the carbon–organic chemistry of life of our type. A logical consequence of the same picture is the convergence of civilization form: the carriers of technological civilizations are expectedly close in functional plan to humanoids, although the details of morphology may differ.