Microbes may have lived underground for only a billion years

Microbes, or organisms that are too small to be seen without using a microscope, have lived in deep earth formations for just a billion years, at the lower end of scientific estimates, according to a new study published Monday in the procedures of the National Academy of Sciences (PNAS) scientific journal.

The findings provide a context for Earth’s deep biosphere, one of the least explored and understood environments on Earth, and is home to most of the microbial life on Earth. Rocks deep in the Earth harbor microbial lineages, which are critical to understanding the origin and evolution of life on Earth.

The biosignatures and thermal history of some of the oldest rocks on Earth were analyzed to carry out the study. Professor Henrik Drake of Linnaeus University in Sweden and Professor Peter Reiners of the University of Arizona were co-authors of the study, which was based on an estimated chronological timeline of the habitability of some of the oldest rocks on Earth to the long of the time.

Ultimately, the study suggested that the longest record of continuous habitability does not exceed 1 billion years, limiting the possibilities for microbes to evolve. The researchers studied rocks formed about 4 billion years ago, and found that ground conditions weren’t suitable for harboring microbes until the rock formations cooled billions of years later.

Co-author Prof. Henrik Drake (credit: COURTESY OF HENRIK DRAKE)

“Cratonic rocks formed billions of years ago, deep in the crust, at temperatures too high for any life. It was only much later, after erosion, that the currently exposed rocks reached levels in the crust where temperatures were habitable, ”Drake explained.


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