Abstract: Ashes of ancient meteors recovered from a 2.7-billion-year-old lake bed imply that the upper atmosphere was rich in oxygen at a time when all other evidence implies that the atmosphere was oxygen-free. See Letter p.235 Most evidence in support of low atmospheric oxygen levels on early Earth relates to the composition of the lower Archean atmosphere. This paper identifies fossil micrometeorites — the oldest so far found — preserved in 2.7-billion-year-old limestone rocks in Australia. These micromeorites preserve a record of chemical interaction with the Archean upper atmosphere, since their composition suggests that they became oxidized as they melted on entering an oxygen-rich atmosphere.