Researchers studying rocks from Greenland announced today that they've uncovered evidence to show when life on earth began.
Analysis shows the rocks may have been host to our earliest ancestors: single-celled organisms that lived 3.85 billion years ago.
If the dating is accurate, the rocks push back the biological record of life on earth by about 450 million years.
Scientists from the University of Chicago reported in today's issue of the journal Science that the rocks may have once been in a prehistoric ocean, but they were cooked at high temperature under pressure, which drastically modified their chemistry.
But by using mass spectrometry, researchers said, they found atomic signatures in the rocks indicating that they're sedimentary - the type of rock that would form along rivers, in lakes and in oceans - and could host bacteria or some other microscopic form of early life.
"These are the oldest sediments on earth, so anything they have to tell us is important," said Nicolas Dauphas, the study's lead author, who is an assistant professor at the university and a researcher at Chicago's Field Museum.
Dauphas said his team was able to identify the rocks as sedimentary by measuring subatomic variations, or isotopes, in the composition of the iron they contained.
Sedimentary rocks leave a more complicated isotopic signature than the earth's other type of prehistoric rocks - igneous rocks cooled from a once-molten state. But only sedimentary rocks are capable of preserving evidence of life.
Dauphas said the iron isotope signature also is consistent with the presence of photosynthesis, a chemical process that would signal the presence of bacteria.
Other experts dispute the researcher's findings.
"Is it an important piece of work, yes. But is it in any way a definitive answer to the larger questions of some of these issues? I don't think so," said Christopher Fedo, a researcher at George Washington University who has studied other rocks in Greenland.
The rocks used in the study were collected over the years from the southwest coast of Greenland and Akilia Island, a remote outpost that turned into a research hotspot in the mid-1990s. That's when researchers discovered a peculiar green and white layer of rocks, known as the Banded Iron Formation.
Rocks in the formation - believed to have been formed shortly after the last major asteroid impact 3.8 billion years ago - have been intensely studied over the past decade to see if they contain evidence of life.
Among geologists, it's a contentious issue.
"There's a lot of arguments about these rocks going back and forth," said Meenakshi Wadhwa, a curator at Chicago's Field Museum and a co-author of the report.
The earliest known life forms on earth are microfossils of cyanobacteria, a blue-green algae, believed to have lived in Australia more than 3.4 billion years ago.
Dauphas' study did not find actual evidence of life in the rocks and won't end arguments about the origins of life. But he believes there's "about an 85 percent chance" that such evidence will eventually be found in the rocks.
"This is the way that science goes. When everything makes a consistent picture, you can believe in the picture until someone comes up with evidence to disprove it," Dauphas said.
Fedo said it's an open question whether the rocks are 3.8 billion years old and whether they were ever capable of hosting microscopic life. Like most rocks, the granulates analyzed by Dauphas have changed over time, he said.
They were subjected to extreme temperature and pressure changes over billions of years, as water and other fluids flowed through them, gradually changing their chemistry. He said such changes make it difficult to determine whether the iron that formed the isotopes was part of the original rock or added later.
"Over 2 billion years of time, these rocks have repeatedly gone through the ringer," he said.