Paleobiological Perspectives on Early Microbial Evolution

Abstract

Microfossils, stromatolites, and chemical biosignatures indicate that Earth became a biological planet more than 3.5 billion years ago, making most of life's history microbial. Proterozoic rocks preserve a rich record of cyanobacteria, including derived forms that differentiate multiple cell types. Stromatolites, in turn, show that microbial communities covered the seafloor from tidal flats to the base of the photic zone. The Archean record is more challenging to interpret, particularly on the question of cyanobacterial antiquity, which remains to be resolved. In the late Neoproterozoic Era, increasing oxygen and radiating eukaryotes altered the biosphere, with planktonic algae gaining ecological prominence in the water column, whereas seaweeds and, eventually, animals spread across shallow seafloors. From a microbial perspective, however, animals, algae, and, later, plants simply provided new opportunities for diversification, and, to this day, microbial metabolisms remain the only essential components of biogeochemical cycles.

Figure 1.

Proterozoic fossils interpreted as cyanobacteria. (A–C) Coccoidal microfossils, some showing evidence of binary fission, 750- to 800-million-year-old (Ma) Draken Formation, Spitsbergen. (D) Hyella-like endolithic microfossil penetrating an ooid, 750 to 800 Ma Upper Eleonore Bay Group, Greenland; upper surface of ooid arcs just above the fossil. (E) Cyanostylon-like stalk-forming cyanobacterium, Draken Formation. (F,G) Interpreted akinete and associated germling-like short trichome, ca. 1500 Ma Kotuikan Formation, Siberia. (H) Phosphatized helical Spirulina-like filament, basal Cambrian, Siberia. (I) Gloeocapsa-like cell quartet, 1100 to 1200 Ma Angmaat Formation, Baffin Island. (J) Entophysalis-like mat-building cyanobacteria, ca. 2000 Ma Belcher Supergroup, Hudson Bay, Canada. (K) Silicified population of mat-building filaments, showing alternating vertical and horizontal filament orientations, 750- to 800 Ma Backlundtoppen Formation, Spitsbergen. (M,N) Chroococcus-like dyads from the Bitter Springs (M) and Angmaat (N) formations. Scale bars, 10 µm (A–C,E,M,N); 75 µm (D); 15 µm (F,G); 20 µm (H,J,L); 25 µm (K).

Figure 2.

Stromatolites. (A) Modern stromatolites accreting subtidally in Shark Bay, Western Australia. (B) Columnar stromatolitic bioherm, ca. 800 Ma Inzer Formation, Russia. (C) Domal stromatolite from a coastal environment, ca. 800 Ma Grusdievbreen Formation, Spitsbergen. (D) Early Archean stromatolites, ca. 3400 Ma Warrawoona Group, Western Australia; note conoidal form in right-center. Scale bars, 20 cm (A); 12 cm (B,C); 8 cm (D).

Figure 3.

A summary of early microbial evolution (see the text for references).

Figure 4.

Iron formations and their paleobiology. (A) Filamentous and coccoidal microfossils in stromatolitic iron formation of the 1900 to 1850 Ma Gunflint Formation, Ontario. (B) Unusual microfossil in basinal chert of the Gunflint Formation. (C) Banded iron formation, 2480 to 2495 Ma Brockman Iron Formation, Western Australia; reddish and whitish bands reflect iron- and silica-rich layers, respectively. Scale bars, 25 µm (A); 10 µm (B); 3 cm (C).