| The fossil record shows that life was well established on land by the early Paleozoic (less than 545 million years ago), but entries before that time are few and controversial. Nevertheless, there is no reason to believe that microbes, abundant in aquatic environments since the Archean (more than 2500 million years ago), could not have colonized terrestrial environments as well. Modern cyanobacteria, a widespread group of oxygenic phototrophs, well-adapted to UV radiation exposure and desiccation conditions, are well known pioneers in colonizing terrestrial environments today. Their biosignatures are also known from Archean rocks. I contend that cyanobacterial communities formed major terrestrial ecosystems on land before the advent of plants. Aiming to provide evidence for this hypothesis, chemical and morphological biosignatures, necessary to recognize microbes in ancient rocks, were characterized in cyanobacteria-dominated communities that grow on unvegetated soils from desert environments. By comparing biological soil crusts to non-crusted soil from the same environment, I found that microbial communities deplete their surroundings of transition metals and enrich them with biogenic elements (mainly C and N), which could be used as a fossil biosignature. I also found that the formation of particular sedimentary structures, such as roll-ups, gas domes and vesicles, reticulated surfaces, sand folds, sand ridges, and wrinkle marks, were closely associated with terrestrial cyanobacterial communities. Among those, I determined that roll-up structures are key morphological biosignatures. I used these biosignatures to look for evidence of terrestrial communities in Precambrian rocks of the Apache Group of Arizona, that were deposited in terrestrial environments circa 1200 million years ago. Although no conclusive chemical biosignatures were found in these rocks, an array of sedimentary biostructures and microfossils analogous to those found in modern arid-land microbial communities constitute sufficient evidence of life's presence on these ancient terrestrial environments. This implies that fully developed microbial communities were already contributing to Earth's terrestrial biogeochemistry long before plants evolved. |
