The timetable of evolution

Abstract

The integration of fossils, phylogeny, and geochronology has resulted in an increasingly well-resolved timetable of evolution. Life appears to have taken root before the earliest known minimally metamorphosed sedimentary rocks were deposited, but for a billion years or more, evolution played out beneath an essentially anoxic atmosphere. Oxygen concentrations in the atmosphere and surface oceans first rose in the Great Oxygenation Event (GOE) 2.4 billion years ago, and a second increase beginning in the later Neoproterozoic Era [Neoproterozoic Oxygenation Event (NOE)] established the redox profile of modern oceans. The GOE facilitated the emergence of eukaryotes, whereas the NOE is associated with large and complex multicellular organisms. Thus, the GOE and NOE are fundamental pacemakers for evolution. On the time scale of Earth's entire 4 billion-year history, the evolutionary dynamics of the planet's biosphere appears to be fast, and the pace of evolution is largely determined by physical changes of the planet. However, in Phanerozoic ecosystems, interactions between new functions enabled by the accumulation of characters in a complex regulatory environment and changing biological components of effective environments appear to have an important influence on the timing of evolutionary innovations. On the much shorter time scale of transient environmental perturbations, such as those associated with mass extinctions, rates of genetic accommodation may have been limiting for life.

Keywords: Earth history; evolution; evolutionary theory; geochronology.

Fig. 1. The evolutionary timetable, showing the course of evolution as inferred from fossils, environmental proxies, and high-resolution geochronology.

Phanero, Phanerozoic; Prot, Proterozoic; Ceno, Cenozoic; E, Ediacaran; Cam, Cambrian; O, Ordovician; S, Silurian; D, Devonian; Car, Carboniferous; Per, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; Pal, Paleogene; Neo, Neogene. Crosses indicate times of major mass extinctions.

Fig. 2. The geologic history of Fe in seawater and O₂ in the atmosphere and surface ocean.

Fossil images from left to right show biogenic stromatolites, accreted by microbial mat communities (2700 Ma; Fortescue Group, Australia), an early eukaryotic microorganism (1400 to 1500 Ma; Roper Group, Australia), and an Ediacaran metazoan (543 Ma; Nama Group, Namibia). PAL, present atmospheric level.

Fig. 3. The regeneration process.

Gene duplication (A) or recombination (B) generates a starting condition for the search process, at rate w. (C) From the starting condition, we require k mutational steps, each at rate u, to reach the target sequence, which encodes a new function. At each step, there is the possibility to receive inactivating mutations, at rate v, which destroy the search. The frequency of the wild type is denoted by x₀. The frequencies of the intermediate steps in the search process are denoted by x_i. At steady state and assuming neutrality, we have the following frequencies: ${\mathit{x}}_0=\frac{\mathit{v}}{\mathit{v}+\mathit{w}}$ and ${\mathit{x}}_{\mathit{i}}=\left(\frac{\mathit{w}}{\mathit{u}}\right)\left(\frac{\mathit{v}}{\mathit{v}+\mathit{w}}\right){\left(\frac{\mathit{u}}{\mathit{v}+\mathit{u}}\right)}^{\mathit{i}}$. Let us consider a numerical example: w = 10⁻⁷, u = 10⁻⁹, v = 10⁻⁷ per cell division. Then, cells that have made as many as 10 steps toward the target have a frequency of about 5 × 10⁻¹⁹ and are present on a planetary scale with a total cell number of the order on 10³⁰.