| Summary: | Two recent papers [1, 2] report geological, geochemical, and geochronological data from the early Archaean of West Greenland to suggest that life existed on Earth by greater than or equal to 3850 Myr ago. The crucial evidence is based on zircon U-Pb age measurements of magmatic rocks (metamorphosed to orthogneisses) regarded to be younger than adjacent metasedimentary rocks containing components with a carbon isotope signature characteristic of a biological origin [1]. This new lower age limit for the existence of life on Earth overlaps the period at which massive, destructive impacts were occurring on the moon's surface (and presumably on Earth as well), probably terminating close to 3.8 Gyr ago [3]. A critical re-examination and re-interpretation of the relevant geochronological evidence in West Greenland suggests that the rocks of possibly biological significance are some 120-190 Myr younger than claimed in the cited papers. By this time, major cataclysmic impacts would long have been over, and more congenial conditions for earliest life established.
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