Content area
Full Text
GEOLOGY
When did life first arise on Earth? Analysis of ancient rocks in Greenland that contain structures interpreted as bacterial in origin suggest that Earth might have been an abode for life much earlier than previously thought. See Letter p.535
Did life on this planet begin only after a relatively long planetary evolution, until suitable environments emerged that allowed life to gain a toehold, or was the cradle of life ready and rocking when Earth itself was but an infant? An answer may come from a paper on page 535 by Nutman et al.1 that analysed 3.7-billion-year-old rocks in the Isua Greenstone Belt in Greenland. These are not the kinds of rocks that palaeobiologists would consider a good prospect for signs of life, because they are not sedimentary like those that host most of Earth's fossil record. Rather, they are metamorphic, which means that they have been extensively deformed and altered by heat and pressure during deep burial and are no longer the sedimentary rocks they once were.
However, Nutman and colleagues came across a rarity in the Isua Greenstone Belt. In a small area newly revealed by snow melt, they found relatively well preserved rocks that have survived geological time with some of their original sedimentary attributes intact. In this tiny window into the deep past are subtle geochemical and textural clues to an ancient surface environment that is surprisingly familiar as a habitat for life. Within the rocks can be seen ancient ripple marks and piles of rock fragments deposited during an ancient storm. Combined with seawater-derived mineral chemistry, these features all point to a shallow marine, carbonate, mineral-depositing environment similar to those that have hosted abundant biota throughout Earths history.
In the middle of it all are structures resembling...