Scientists have found what they think is probably the oldest fossil on Earth, a remnant of life from 3.7 billion years ago when Earth’s skies were orange and its oceans green.

In a newly melted part of Greenland, Australian scientists found the leftover structure from a community of microbes（微生物）that lived on an ancient seafloor. Based on their analysis of the fossils, the researchers determined that they are 220 million years older than those discovered in Western Australia, which were 3.48 billion years old.（66题）

The discovery shows life may have formed quicker and more easily than once thought, about half a billion years after Earth formed. And that may also give hope for life forming elsewhere, such as Mars, said study co-author Martin VanKranendonk of the University of New South Wales and director of the Australian Center for Astrobiology. “It gives us an idea how our planet evolved and how life gained a foothold,” VanKranendonk said.（63题）

Scientists had thought it would take at least half a billion years for life to form after the molten Earth started to cool a bit, but this shows it could have happened quicker, he said. That’s because the newly found fossil is far too complex to have developed soon after the planet’s first life forms, he said.（64题）

In an outcrop of rocks that used to be covered with ice and snow which melted after an exceptionally warm spring, the Australian team found stromatolites（叠层石）, which are layered structures that are often produced by a community of microbes. The stromatolites were about 1 to 4 centimeters high.

“It is like the house left behind made by the microbes,” VanKranendonk said. “Scientists used the layers of ash from volcanoes and tiny zircon（锆石） with uranium and estimated that they date back to 3.7 billion years ago.”

The dating seems about right, said Abigail Allwood, a NASA astrobiologist who found the previous oldest fossil, from 3.48 billion years ago, in Australia. But Allwood said she is not completely convinced that what VanKranendonk’s team found once was alive. She said the evidence wasn’t conclusive enough that it was life and not a geologic quirk（地质巧合）.（65题）“It would be nice to have more evidence, but in these rocks that’s a lot to ask,” Allwood said in an email.

【文章导读】此文是一篇说明文，介绍了化石的由来，科学家的新发现以及化石的主要种类。