The secret ingredient is a type of nitrogen found in the ancient rock samples. Early life forms may have been able to live without oxygen — which didn't appear in Earth's atmosphere until what scientists call a "great oxygenation event" 2.3 billion years ago — but they required nitrogen to build genes and for other essential life processes. And unfortunately for the planet's ancient organisms, the kind of nitrogen in Earth's atmosphere exists in tightly bonded pairs that are useless when it comes to chemical reactions.

Non-biological processes, such as lightning discharge, may have converted bonded atmospheric nitrogen in small quantities, but not frequently enough to sustain large populations of living cells. For that, life on Earth needed to find a way to acquire its own nitrogen — an enzyme that could pull the compound from the atmosphere and convert it to its "fixed" or usable form.

Evidence of such an enzyme is what researchers found in their samples, which were sourced from some of Earth's oldest rock in South Africa and Australia and range from about 2.8 billion to 3.2 billion years old. The rocks contain a chemical signature of the nitrogen-fixing process, offering "hard evidence" that the conditions for life to flourish have existed 50 percent longer than scientists once believed, according to co-author Roger Buick.

"People always had the idea that the really ancient biosphere was just tenuously clinging on to this inhospitable planet, and it wasn't until the emergence of nitrogen fixation that suddenly the biosphere become large and robust and diverse," Buick, a professor at the University of Washington, said in a university press release. "Our work shows that there was no nitrogen crisis on the early Earth, and therefore it could have supported a fairly large and diverse biosphere."

Buick said these early organisms could have crawled out of the ocean and lived on land in a single layer of cells, coating the planet's rocks with a thin film of slime and quietly exhaling small amounts of oxygen. The presence of the chemical signature is indirect evidence of this hypothesis, but it's firm — the kind of chemical reaction preserved in the rocks can only happen in the presence of life.