Findings by Harvard researchers and colleagues narrow the range of possible dates for a critical change in the Earth’s atmosphere. Scientists had previously believed oxygen first appeared sometime between 2.45 billion and 2.22 billion years ago, a span of about 230 million years. The new findings narrow that window dramatically – to about 130 million years. “It’s a fascinating transition, from a physical, chemical, and biological point of view,” said Heinrich D. Holland, Harry C. Dudley Research Professor of Economic Geology, who, with postdoctoral fellow Andrey Bekker and colleagues from other institutions, conducted the research. The increase in oxygen allowed the development of early oxygen-using creatures and sowed the seeds for the eventual development of large land animals, an event scientists believe occurred after a second large increase in oxygen more than a billion years later. Whatever the cause, Bekker, Holland, and their colleagues, writing in the Jan. 7, 2004, issue of Nature, detailed their investigation of a black shale, a sedimentary rock thought to have been deposited 2.32 billion years ago in a shallow marine river delta. Researchers examined rock cores drilled by mining companies exploring South Africa’s rich mineral resources.