Microorganisms moved from being exclusively marine species to living also on land much earlier than thought-about 2.75 billion instead of the widely accepted 1.2 billion years ago, according to Birger Rasmussen from the Curtin University of Technology in Bentley, Australia, and his collaborators. Their revised time frame is based on analysis of sedimentary and volcanic rocks from the Hardey Formation in the Pilbara region of western Australia. It suggests that cavities in and between sandstone and volcanic rocks were shelters, providing the first land habitats for microbial life on Earth. Details appear in the May issue of Geology (37:423-426).  

"The Hardey Formation is part of a sequence of sedimentary and volcanic rocks that is remarkably well preserved given their antiquity," Rasmussen says. "Hence, these rocks are a prime source of information about the nature and diversity of ancient life and its habitat." The Australian team found centimeter-sized columns of pendant deposits of microbial origin in sandstone sediment cavities that also contain nonmarine stromatolites, which are finely layered, mound-like structures of limestone built by mats of photosynthetic bacteria or algae living in shallow marine or lake environments. "Although the pendant structures and stromatolites occur in the same rocks, they are not associated," he says. 

The pendant deposits appear to derive from microbial biofilms that grew downward from the ceilings of centimeter- sized cavities that developed in near-surface sediments as gases seeped upwards but were trapped beneath impermeable microbial mats, according to Rasmussen and his collaborators. Well after they formed, the biofilms were fossilized by silica, a process that kept those structures intact. Despite resembling stromatolites, the pendant deposits differ because they did not follow the upward, light-seeking columnar growth pattern of stromatolites, which are fossilized remains of photosynthetic microorganisms, whereas the pendant columns apparently grew downward and were not phototrophs. 

"We didn't find any evidence for free oxygen in these rocks," Rasmussen says. "We seem to be dealing with an essentially anaerobic environment at 2.75 billion years. The extremely 13C-depleted composition of organic matter in our samples suggests that methane-producing and-cycling microbes (methanogens and methanotrophs) probably inhabited the sediments."  

"If their interpretation is correct, then the land environment during the Precambrian was not only habitable but inhabited," says Ariel Anbar, Professor of Chemistry and Biochemistry at Arizona State University, Tempe. "We have long assumed it was habitable for microbial life, but the evidence of habitation has been skimpy and contentious." 

"What is new and intriguing about [Rasmussen's findings] is the extension of life in cavities to rocks of this age," says Timothy Lyons, Professor of Biochemistry at the University of California, Riverside. "This is key because the microbial life must have made a living by something other than photosynthesis, just as cave/cavitydwellers do today. Instead of relying on light, these microbes must have been chemosynthesizers, and the geochemical data point to methanotrophy or sulfide oxidation as the dominant chemosynthetic pathways. This is an important extrapolation of recent ecologies to the very deep past and might also make good targets for life exploration beyond Earth." 

The cavity-dwelling pendant structures likely were next to a low-energy lake, which was subject to episodic volcanic eruptions, Rasmussen surmises. There are indications of raindrop imprints and mud cracks in the vicinity, which without plants was rocky and barren. Life was exclusively microbial 2.75 billion years ago, but included a variety of stromatolites, microbial mats, and cavities with pendant microbial deposits. "The lack of glacial deposits of this age suggests that it was probably quite warm," he says. "In the absence of oxygen, there was no ozone shield in the upper atmosphere, so damaging radiation would have bombarded the Earth's surface. Also the length of the average day was significantly shorter as the Earth rotated around its axis much faster than today. The pendant deposits also represent a viable and highpriority target in the search for ancient life in Martian rocks." 

Barry E. DiGregorio 
Barry E. DiGregorio is a freelance writer in Middleport, N.Y.

Comments (0)

Subscribe to this article's comment feed

Show/Hide comment form

Name

Email

Website

Title

Comment

smaller | bigger

Subscribe via email (Registered users only)

I have read and agree to the Terms of Usage.

Add Comment