Microbes, or organisms that are too small to be seen without using a microscope, have lived in deep-earth formations for only a billion years – at the lower end of previous scientific estimates – according to a new study published on Monday in the Proceedings of the National Academy of Sciences (PNAS) scientific journal.
The findings provide context for Earth’s deep biosphere – one of the least explored and understood environments on the planet, which hosts the majority of microbial life on Earth. Deep-earth rocks host microbial lineages, which are critical for understanding the origin and evolution of life on Earth.
Biosignatures and the thermal history of some of the oldest rocks were analyzed in order to conduct the study. Prof. Henrik Drake of Linnaeus University in Sweden and Prof. Peter Reiners of the University of Arizona co-authored the study, which was based on an estimated chronological timeline on the habitability of some of Earth’s oldest rocks through time.
The study ultimately suggested that the longest record of continuous habitability is not longer than one billion years, placing constraints on the possibilities of microbe evolution. The researchers studied rocks formed about four billion years ago, finding that the conditions of the terrain were not suitable as a host for microbes until the rock formations cooled off billions of years later.
“The cratonic rocks formed billions of years ago, deep in the crust, at temperatures too high for any life,” Drake explained. "It was only much later, following erosion, that the currently exposed rocks reached levels in the crust where temperatures were habitable."