Four billion years ago, the solar system was still young. Almost fully formed, its planets were starting to experience asteroid strikes a little less frequently. Our own planet could have become habitable as long as 3.9 billion years ago, but its primitive biosphere was much different than it is today. Life had not yet invented photosynthesis, which some 500 million years later would become its main source of energy. The primordial microbes—the common ancestors to all current life forms on Earth—in our planet's oceans therefore had to survive on another source of energy. They consumed chemicals released from inside the planet through its hydrothermal systems and volcanoes, which built up as gas in the atmosphere.