A cluster of 3.4 billion-year-old fossilized cells
What's the News: Geologists have found fossils of microorganisms from 3.4 billion years ago, which may be the oldest fossils ever uncovered. Since these microbes date from a time when Earth's atmosphere was still oxygen-free, astrobiologists could look for similarly structured microbes when searching for extraterrestrial life. How the Heck:
These microfossils, as such single-celled fossils are called, were found in a sandstone deposit in Western Australia---an area that was likely a beach on one of Earth's earliest islands---nestled between ancient grains of quartz sand.
The researchers dated the rock formation to 3.4 billion years old. Back then, the atmosphere was thick with methane, not oxygen, and Earth's oceans were far warmer than they are today, likely about 110˚F.
With microfossils so tiny and so old, the big challenge for scientists is proving that what they've found really are fossils, rather than non-biological aberrations or patterns in the rock. Using a variety of methods, the research team gathered several types of evidence suggesting---albeit circumstantially---that they've found bacterial fossils.
Examining samples of the rock, the research time saw tiny rounded and elongated structures that had many of the properties of cells: they were hollow, had the same shape and size as bacterial cells, and some where glommed together in the midst of what looked like cell division. They also noticed that the cells tended to be very close to the same size, a more regular size distribution than you'd likely see from non-biological formations but about what you'd expect if looking at microbes of one species.
Analyzing the chemical composition of the samples, the team found carbon and nitrogen in the microfossil's cell walls, elements found in all cell walls of organisms alive today. The researchers also found sulfur-containing compounds, in particular fool's gold, which they say suggest that these microbes had a sulfur-based metabolism, getting their energy from sulfur rather than oxygen.
What's the Context:
Scientists have been slugging it out over which fossils are the world's oldest for the past decade, since it's so difficult to tell what's a fossilized bacterium versus a mineral artifact. One of the researchers who found these latest fossils was a vocal critic of an earlier find, from a site 20 miles away, of purported microfossils that were slightly older, at 3.465 billion years. Chemical signatures of life---though not any cellular structures---have been reported in 3.8 billion year old rock deposits in Greenland.
The Future Holds:
The methods of this study, the researchers hope, could be applied to astrobiology. Life on other planets, also lacking in oxygen, might have been along the same lines as these sulfur-metabolizing microbes. Using these same imaging techniques and chemical analyses could help researchers carefully search for evidence of life on Mars.
Reference: David Wacey, Matt R. Kilburn, Martin Saunders, John Cliff & Martin D. Brasier. "Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia." Nature Geoscience, published online August 21, 2011. DOI: 10.1038/ngeo1238
Image courtesy of David Wacey / Nature Geoscience