Science nerds are still giddy from last week’s discovery of seven earth-sized planets orbiting a nearby star, in part because it brings researchers closer to the most tantalizing question It would help if we had some idea of how life started on earth. A geological breakthrough announced on Wednesday in Nature makes that question slightly easier to answer. Scientists examining the chemical makeup of some of the Earth’s oldest rocks, located in northwestern Canada, may have found the oldest-known chemical fossils of bacteria.