New research suggests may be more ancient - and more common - than thought.
We never will know if life on Earth is unique, according to conventional wisdom, unless we come into contact with alien forms, sentient or not. Even the discovery earlier this year of minuscule structures that may be microorganisms from Mars is unlikely to challenge this notion. Not only do scientists remain skeptical, many also contend that Earth and Mars - close neighbors that they are - may have shared life's precursors.
The absence of little green people notwithstanding, there are other clues to this enduring mystery. New evidence suggests that life-forming processes are so tenacious they must be universal: Sustaining life, not starting it, may be the problem. Scientists are employing sophisticated new diagnostic techniques, both geologic and genetic, that are pushing the date of life's origins further into the past, close to the birth of the planet itself.
An international team of geologists, for example, has used a device cared a high-resolution ion microprobe to analyze sedimentary rocks from the Itsaq Gneiss Complex, a formation on Akilia Island in western Greenland some 3.85 billion years old.
According to Stephen J. Mojzsis of the Scripps Institution of Oceanography, the rocks were found to contain tiny "plugs" of a light carbon isotope within grains of apatite, a form of calcium phosphate. Apatite is associated with microfossils, and there is no known nonbiological process that can explain the presence of the isotope.
In other words, microbes were busily metabolizing about 300 million years earlier than previously believed - within 700 million years of Earths origin. They either were photosynthesizing sunlight or "feeding" on the chemical energy generated by hot hydrothermal vents on the ocean floor (see Science, Dec. 2).
The discovery has extended life's past "to the very bottom of our planets sedimentary pile," according to John M. Hayes of the Woods Hole Oceanographic Institution in Massachusetts. Biological activity so early in Earth's history would have occurred during a period of heavy bombardment of the planet by solar debris - debris which in later epochs would lead to mass extinctions. At the very least, the carbon residue in the Itsaq Gneiss rocks "derives from biochemical processes that developed with breathtaking rapidity after the last large impact," Hayes says - perhaps a product of a biota that was wiped out."
But the chronological proximity of the Itsaq Gneiss residue with solar debris may be more than coincidence. There is a persistent theory among scientists that life did not originate on Earth. Rather, it was seeded by the bombardment - Eden may have arrived by asteroid.
Either way, the discovery suggests that life-forming processes are common. …