Paleoarchean Era
Oldest indirect evidence of life
The Paleoarchean is the second era in the Archean Eon, occurring after the Eoarchean Era, and before the Mesoarchean Era.
Above: Group of 3.4 billion year old tubular fossil forms (David Wacey/UWA)
What happened during this time?
Biological
The oldest indirect evidence of fossil bacteria is found in Western Australian Paleoarchean rocks
Cyanobacteria are probably present
3.465 billion years ago: "11 specimens of five taxa of prokaryotic filamentous kerogenous cellular microfossils permineralized in a petrographic thin section of the Apex chert of northwestern Western Australia" (Schopf et al., 2017)
Microbially induced sedimentary structures (MISS) from Dresser Formation in Pilbara, Australia (3.48 Ga)
There is evidence indicating that "the black cherts formed during precipitation from silica-rich, carbon-bearing hydrothermal fluids in vein systems and vent-proximal seafloor sediments. Given the volcanic setting and lack of organic-rich sediments, we speculate that the vent-mound systems contain carbon derived from rock-powered organic synthesis in the underlying mafic-ultramafic lavas, providing a glimpse of a prebiotic world awash in terrestrial organic compounds" (Rasmussen & Muhling, 2023)
It is probable that these structures were abiotic in origin and not evidence of early bacterial life
Ten-micrometer tubular microfossils found in between sand grains in a 3.4 billion-year-old sandstone.
The oldest stromatolites date back to about 3.43 billion years old (Wacey et al., 2011).
These fossil stromatolites have layers similar to those seen in living stromatolites
Evidence of structures 3.46 Ga are inorganic and not bacteria (Brasier et al., 2014): elongated filaments were created by minerals forming in hydrothermal systems.
Secondary structures interpreted as simple filamentous microfossils have been recovered from the layers.
Archaea were probably present during this time
South African fossils preserve fossils of archaea from a hydrothermal vent dating to 3.42 Ga (Cavalazzi et al., 2021)
Possible evidence of non-oxygen-generating photosynthesis (Czaja, 2013)
Purple bacteria probably developed photosynthesis first
The last universal common ancestor (LUCA) of all life on Earth existed during this era
Geophysical
The S2 Crater in South Africa is the remnants of a meteorite 50-200x bigger than the K-Pg impactor (Drabon et al., 2024)
The impact caused a tsunami, partial ocean evaporation, and darkness that likely harmed shallow-water photosynthetic microbes in the short term. In contrast, life in the deeper oceans and hyperthermophiles was less impacted.
The impact also released phosphorus into the environment, and the tsunami brought iron-rich deep water to the surface.
As a consequence, there was a temporary bloom of iron-cycling microbes. Giant impacts were not just agents of destruction but also conferred transient benefits on early life.
Continent formation begins, with increasingly larger land masses emerging from the oceans
Possible first super-continent, Vaalbara formed around 3.2 Ga
Evidence from similar sedimentary sequences on the South African Kaapvaal craton and the West Australian Pilbara craton
Right: Fig. 8 from Drabon et al., 2024
Additional Resources
Giant meteorite impact 3.26 billion years ago may have aided early life (Phys.org 21Oct2024)
└Drabon et al. (2024) Effect of a giant meteorite impact on Paleoarchean surface environments and life
All life on Earth comes from a single common ancestor (The Brighter Side 16Jul2024)
└Moody et al. (2024) The nature of the last universal common ancestor and its impact on the early Earth system.
3.42-billion-year-old fossil threads may be the oldest known archaea microbes (Science News 26Jul2021)
└Cavalazzi et al. (2021) Cellular remains in a ~3.42-billion-year-old subseafloor hydrothermal environment.
Did Life Evolve More Than Once? (The Conversation 18May2023)