June 4, 2013

Scientists resolve a 3.5 billion-year-old mystery of life

Most astrobiologists believe that life in some form is likely to exist away from Earth. But new research demonstrates that life as we know it on Earth might never have come to exist at all if not for a key element delivered to the planet by meteorites billions of years ago.

Scientists at the University of Washington and the University of South Florida found that during the Hadean and Archean eons – the first two of the four principal eons of the Earth’s earliest history – the heavy bombardment by meteorites provided reactive phosphorus essential for creating the earliest life on Earth.

Artist conception of a hypothetical planetp

Photo illustration by NASA

This artist’s conception shows a young, hypothetical planet around a cool star. A soupy mix of potentially life-forming chemicals can be seen pooling around the base of the jagged rocks.

When released in water, that reactive phosphorus could be incorporated into prebiotic molecules, and the researchers documented its presence in early Archean limestone, showing it was abundant some 3.5 billion years ago.

“The importance of this finding is that it provides the missing ingredient in the origin-of-life recipe: a form of phosphorus that can be readily incorporated into essential biological molecules like nucleic acids and cell-membrane lipids,” said Roger Buick, a UW professor of Earth and space sciences.

Buick is a co-author of a paper explaining the findings, published the week of June 3 in the early online edition of the Proceedings of the National Academy of Sciences. The lead author is Matthew Pasek, an assistant professor of geology at the University of South Florida.

The scientists concluded that the meteorites delivered phosphorus in minerals that are not now seen on the surface of the Earth, and these minerals corroded in water to release phosphite, a form of phosphorus seen only on the early Earth.

“Meteorite phosphorus may have been a fuel that provided the energy and phosphorus necessary for the onset of life,” said Pasek. “If this meteoritic phosphorus is added to simple organic compounds, it can generate phosphorus biomolecules identical to those seen in life today.”

He said the research provides a plausible answer for why we don’t see new life forms on Earth today: The conditions under which life arose billions of years ago are no longer present.

“The present research shows that this is indeed the case: Phosphorus chemistry on the early Earth was substantially different billions of years ago than it is today,” he said.

The findings are based on examination of samples from Australia, Zimbabwe, West Virginia, Wyoming and Florida. The presence of phosphite was detected only in the oldest samples, from surface materials and drill cores from the early Archean in Australia.

Previous research showed that the earliest biological forms might have evolved from RNA alone, before the modern DNA-RNA-protein life emerged. But scientists didn’t know how those early RNA–based proto-organisms incorporated environmental phosphorus, which in its current form, phosphate, is relatively insoluble and unreactive.

Meteorites would have provided reactive phosphorus in the form of the iron–nickel phosphide mineral schreibersite, which when placed in water released soluble and reactive phosphite. Phosphite is the salt scientists believe could have been incorporated into prebiotic molecules.

Though there could be other sources of phosphite, no other terrestrial sources could have produced the quantities needed to be dissolved in early Earth oceans that gave rise to life, the researchers concluded. The meteoritic phosphite would have been abundant enough to dominate the chemistry of the oceans, with its chemical signature then becoming trapped and preserved in marine carbonate.

“This finding opens the way for a lot more prebiotic chemical experimentation and may even allow us to produce a catalytic replicating RNA molecule in a test-tube, mimicking what might have naturally happened during the origin of life,” Buick said.

Other co-authors are Jelte Harnmeijer of the UW and the Edinburgh Center for Carbon Innovation in Scotland, and Maheen Gull and Zachary Atlas of the University of South Florida. The work was supported by grants from the National Science Foundation, the Agouron Institute and NASA.

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For more information, contact Buick at 206-543-1913 or buick@ess.washington.edu.

This story is based in part on an article by Vickie Chachere of the University of South Florida.