Did terrestrial life emerge later than geochemists think?
March 2003 A lot hangs on the notion that life can make it from abiogenic chemistry very quickly once a world has watery seas. Evidence from oxygen isotopes in the oldest known terrestrial zircons suggests that liquid water was around on Earth by about 4400 Ma (see Pushing back the “vestige of a beginning” in Earth Pages News of February 2001, and The Hadean was cool June 2002). It lies behind the search for signs of life on Mars and the fiasco surrounding the premature announcement of bacterial fossils in a meteorite reputedly from the Red Planet. Right here, controversy has been raging over the once-living status of tiny patterns in 3500 Ma cherts from Western Australia (see Doubt cast on earliest bacterial fossils in Earth Pages News, April 2002), and on the true significance of isotopically light carbon trapped in apatite crystals in the 3800 ma Akilia metasediments of West Greenland. Both have been claimed as signs of early, well-organised life, but the evidence is circumstantial.
