Reconstructions of 4 billion-year-old proteins have provided insights into the habitat and origins of life on Earth, Spanish and U.S. researchers say.

Writing in the journal Structure, the researchers report the reconstructed proteins can survive in the extreme environments of high acidity and temperature that would have existed on the early Earth and, possibly, also on Mars.

The ancient proteins' properties suggest they may have been adapted to that environment, they said, sharing features with "extremophiles," bacteria living today in hot springs and deep within Earth's crustal rocks.

"So far, attempts to understand protein structure evolution have been based on the comparison between structures of modern proteins," senior study author Jose Sanchez-Ruiz of the University of Granada said. "This is equivalent to trying to understand the evolution of birds by comparing several living birds."

"But it is most useful to study fossils so that changes over evolutionary time are apparent," he said. "Our approach comes as close as possible to 'digging up' fossil protein structures."

An intriguing possibility suggested by the protein study, the researchers said, is that the ancient protein came to Earth in meteorites, having formed at an earlier time on another planet -- like Mars.

"Four billion years ago Mars was a much a safer place than Earth," Sanchez-Ruiz said. "Maybe we have resurrected Martian proteins. Maybe the last universal common ancestor (the first life) formed on Mars and transferred to Earth."