When you think about the basic ingredients for life to thrive on Earth, no doubt water and oxygen pop to mind. But there was a time on our planet when our atmosphere only had one-one thousandth of one percent of the amount of oxygen it has now, yet there were plenty of life forms around then too, although proof of them has been scarce. A recent discovery of fossilized bacteria dating to about 2.5 billion years ago provides long-sought-for evidence that the Earth was crawling with life even though it lacked much oxygen during a phase in our planet's development known as the Neoarchean Eon.
"These fossils represent the oldest known organisms that lived in a very dark, deep-water environment," says Andrew Czaja, the assistant professor of geology at the University of Cincinnati in the US who made the discovery. "These bacteria existed two billion years before plants and trees, which evolved about 450 million years ago. We discovered these microfossils preserved in a layer of hard silica-rich rock called chert located within the Kaapvaal craton of South Africa."