## Scientists may have misread a 2-billion-year-old clue about Earth Around 2.5 to 2 billion years ago, Earth underwent the largest chemical transformation ever recorded at its surface. Oxygen began accumulating in the atmosphere, setting in motion changes that eventually helped make possible the rise of complex organisms such as plants and animals about half a billion years ago. As oxygen levels increased for the first time, enormous amounts of microbial material were buried beneath the seafloor. That burial trapped carbon in rocks and left behind an unusual isotopic signature. For decades,…