Early life on land and the first terrestrial ecosystems

Abstract

Terrestrial ecosystems have been largely regarded as plant-dominated land surfaces, with the earliest records appearing in the early Phanerozoic (<550 Ma). Yet the presence of biological components in pre-Phanerozoic rocks, in habitats as different as soils, peats, ponds, lakes, streams, and dune fields, implies a much earlier type of terrestrial ecosystems. Microbes were abundant by~3,500 Ma ago and surely adapted to live in subaerial conditions in coastal and inland environments, as they do today. This implies enormous capacities for rapid adaptations to changing conditions, which is supported by a suggestive fossil record. Yet, evidence of "terrestrial" microbes is rare and indirect in comparison with fossils from shallow or deeper marine environments, and its record has been largely overlooked. Consequently, the notion that microbial communities may have formed the earliest land ecosystems has not been widely accepted nor integrated into our general knowledge. Currently, an ample record of shallow marine and lacustrine biota in~3,500 Ma-old deposits, together with evidence of microbial colonization of coastal environments~3,450 Ma ago and indirect geochemical evidence that suggests biological activity in >3,400 Ma-old paleosols endorses the idea that life on land perhaps occurred in parallel with aquatic life back in the Paleoarchean. The rapid adaptations seen in modern terrestrial microbes, their outstanding tolerance to extreme and fluctuating conditions, their early and rapid diversification, and their old fossil record collectively suggest that they constituted the earliest terrestrial ecosystems, at least since the Neoarchean, further succeeding on land and forming a biomass-rich cover with mature soils where plant-dominated ecosystems later evolved. Understanding how life diversified and adapted to non-aquatic conditions from the actualistic and paleontological perspective is critical to understanding the impact of life on the Earth's systems over thousands of millions of years.

Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits

“…The Dresser Formation terrestrial hot spring facies include geyserite, siliceous sinter terracettes and the mineralized remnants of hot spring pools. These findings extend the geological record of inhabited terrestrial hot springs by ∼3 billion years, the occurrence of an exposed land surface by up to ∼130 million years3637 and evidence of life on land by ∼580 million years21.…”

Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

“…If the latter is true, and multiple sources of organic carbon contributed to the sedimentary organic matter preserved in the deposit, as was recently shown to be the case for the Ediacaran Shuram Formation in Oman 60 , then the model presented here could conceivably explain covarying trends in paired d 13 C carb and d 13 C org records from the ancient geological record. Although higher level terrestrial plants were not present until the late Palaeozoic, the presence of terrestrial life in earlier time periods, including photosynthetic cyanobacteria, fungi and algae [61][62][63][64][65][66] , supports the possibility that ancient sedimentary organic carbon could have been composed of mixtures of marine and terrestrial organic carbon, in a situation analogous to the model of subaerial exposure proposed for the Neogene. In fact, the range in d 13 C org values of Precambrian sedimentary organic matter is the largest for any time period in Earth history 67 .…”

Aerobic Degradation of Petroleum Components by Microbial Consortia

“…The appearance of cyanobacterial akinetes (for N 2 fixation) in the Paleoproterozoic [184] attests to this early adaptation. The limiting nutrients such as P, can be supplied for organisms on land by dust deposition [185,186], which may be an alternative process for replenishment of nutrient loss by runoff and leaching in such environments [168,187]; S can also be acquired from minerals, aerosols, and as gaseous sources, likely present in the early atmosphere [188]. Thus, the nutritional requirements for oxygenic, photoautotrophic, primary producers seem not to have been a limiting factor for the colonization of the land.…”

“…A conceptualization of the functioning of the ancient terrestrial biosphere necessarily requires a general understanding of modern, analog microbial communities to evaluate their living requirements, diversity, physiology, and environmental impact, and to characterize any potential biosignature that could be used to recognize them in the rocks [168,169]. Modern terrestrial microbial communities are found worldwide and in a great variety of local conditions, in surface (solid rock, regolith) and subsurface (caves, groundwater, deep ground) environments.…”