Earth Life Appeared on Land 1.5 billion Years Earlier Than Previously Thought

Astrobiology Evidence of life on Earth has been found as far back as 3.8 billion years. Given that the intense bombardment that accompanied the birth of the solar system only subsided 3.85 billion years ago, life seems to have appeared on Earth as soon as it was possible for it to. The appearance of life on land took much longer. Until recently, the most ancient evidence pointing to life on solid ground came from 1.2 billion year old microfossils found in Arizona. As such it was thought that life was confined to the oceans for more than 2.6 billion years.

Scientists from Penn State University, a member of NASA's Astrobiology Institute, have published a paper in the 30 November 2000 issue of Nature magazine wherein they report evidence of land-based life 2.6 to 2.7 billion years ago. Not only does this push back the origin of land life by more than a 1.5 billion years, suggesting that life's ability to adapt is more robust than previously known, it also suggests that we may find it easier to detect life on planets circling other stars by virtue of what life does to the atmosphere of the planet it lives on.

According to Penn State "examining samples taken from Mpumalanga Province, South Africa, using a variety of geochemical methods, the researchers report that a paleosol (a layer of ancient soil) dating to between 2.6 and 2.7 billion years ago contains organic carbon that was neither created by high temperature fluids nor is the remnant of later petroleum migration, but is in-situ biological in origin. "

There are a number of processes not involving the presence of living organisms that could conceivably account for the presence of this Carbon. The researchers document how they methodically eliminated three potential non-biological sources: graphite crystals created when the underlying serpentine formed under high temperatures; that the carbon might have resulted from liquid hydrocarbons introduced after the soil formation ended; or that the organic carbon represents remnants of biomats developed on the soil surface (of more recent origin).

According to Penn State "the researchers are also certain that the mats formed on land, not in the oceans, because the carbon isotope values for the carbon in the paleosol are distinctly different from the organic carbon found in marine sedimentary rock. "

According to NASA "This significant discovery presents the strongest evidence to date that life on land occurred at a much earlier stage in Earth's history than was previously believed by most scientists. It also suggests that an ozone shield and an oxygen-rich atmosphere existed on Earth 2.6 billion years ago, both necessary conditions for life on land to emerge. "

The implications of these findings are not confined to life on Earth. Various proposals are being considered for how to go about looking for what Carl Sagan called "Pale Blue Dots" - planets circling other stars that might support life. Telescopes are being designed that could resolve planets circling other stars. Moreover they could also be used to examine the components of these planet's atmospheres. Some atmospheric components (so-called "biomarkers") are thought to be indicative of the presence of life on a planet.

Planets with out life are thought to have atmospheres that are maintained in equilibrium with a planet's surface. Worlds such as Earth, carpeted with life, have an atmosphere that is maintained in a state of disequilbrium wherein some components (higher Oxygen and Ozone, and lower Carbon Dioxide being the most prominent examples) are maintained in quantities that would not occur on worlds without some active process of pulling Carbon Dioxide out and putting Oxygen into the atmosphere. Ozone is formed when atmospheric Oxygen interacts with solar radiation. Once formed, Ozone serves to provide critical shielding for life forms from solar UV radiation.

According to Dr. Michael Meyer, astrobiology discipline scientist at NASA Headquarters, Washington, D.C. "The suggestion that an ozone shield existed as early as 2.6 billion years ago boosts our chances in the search for life on planets orbiting other stars. Ozone would be easily detectable by the Terrestrial Planet Finder, a planned interferometer mission in NASA's 'Origins' program."

Related Links

° 30 November 2000: Geochemical evidence for terrestrial ecosystems 2.6 billion years ago, (abstract), Nature [ abstract] [Full Article]
° 29 November 2000: Ancient South African soils point to early terrestrial life, Penn State
° 29 November 2000: Astrobiologists Find Evidence of Early Life on Land, NAS ARC

Background Information

° Astrobiology Web
° Astrobiology at NASA ARC
° NASA Astrobiology Institute
° Penn State AstrobiologyResearch Center
° Pale Blue Dot II Workshop, (1999), NASA ARC
° Pale Blue Dot Workshop: Detecting Other Habitable Worlds (1996), NASA ARC


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