For seven years, Yuichiro Ueno made an annual pilgrimage to a hot and arid region of Australia, where he would spend up to three months collecting and looking at rocks. “The nearest town was 100 kilometres away,” he recalls. “Every two weeks we would go there to buy supplies.” His visits to town also gave him the opportunity to check in with his lab at the Tokyo Institute of Technology.

Ueno and his colleagues were in the Pilbara area of northwest Australia studying some of the world's oldest sedimentary rock formations. “My personal focus was trying to find traces of early life,” says Ueno, who began the project as an undergraduate student and pursued it for his PhD under Shigenori Maruyama.

Although it took several years, Ueno found what he was after. The paper on page 516 of this issue describes evidence for one of the most primitive organisms known — one that may have helped to regulate Earth's climate during the Archaean era, more than 2.5 billion years ago.

In 2001, Ueno and Maruyama found organic matter in their Australian rock samples. By analysing the carbon within this matter, they realized that it had come from a living organism. In fact, the samples showed the signature of single-celled organisms that had been alive some 3.5 billion years ago.

But what were the organisms? Ueno suspected that they were a type of microbe known as methanogens, which produce the greenhouse gas methane. To prove his idea, Ueno began a very tricky experiment. “I thought the best way to show the existence of methanogens is to find methane in the rocks,” he explains.

By examining his rocks under the microscope, Ueno found that they contained tiny liquid puddles with gas bubbles inside them. He decided to extract the contents of these bubbles to see whether they contained methane. No mean feat: it was a “very difficult task”, Ueno says. It was particularly challenging to keep the methane in the surrounding air from contaminating his samples. “It took me half a year to reduce the chances of introducing methane artificially into the system,” says Ueno. “I had to develop procedures for cleaning the samples, crushing the rocks and extracting the gas.”

Having perfected his technique, Ueno took his methane samples to Naohiro Yoshida, also at the Tokyo Institute of Technology. Yoshida had developed a technique that would reveal whether the gas had been produced by microbes. The answer was a resounding ‘yes’. Because of where the gas was found, methane-producing microbes — distant relatives of those that exist today — must have been alive on Earth 3.5 billion years ago.

“If methanogens existed at that time, there is a very strong possibility that they provided sufficient greenhouse gas to keep the surface temperature of Earth above freezing,” explains Ueno.