NASA astrobiologist Richard Hoover thinks he's found fossilized alien bacteria inside a meteorite. If he's right, it's world-shaking news. But that's a very, very big if.

"There are legitimate reasons to initially be skeptical of these findings," wrote University of Oklahoma geophysicist Michael Engel in a commentary published March 7 by the Journal of Cosmology, where Hoover's claims were announced March 4. Nevertheless, "I encourage people to keep an open mind when forming an opinion as to the significance of this work."

Hoover's study involves carbonaceous chondrites, a class of rare meteorite that formed early in the solar system's history and contain organic chemicals, ostensibly picked up on their passage through space.

When those chemicals are found on Earth, they're considered signs of life. When found in a meteorite, their origin is intriguing but undetermined – a likely sign of contamination by Earthly chemicals or microbes, but hypothetically evidence of extraterrestrial life.

According to Hoover, he hasn't just found suggestive chemical traces, but complex filament-like structures that could only come from bacteria. But almost no nitrogen was found in the meteorites, consistent with what's seen in ancient fossils in which nitrogen has degraded. The meteorites landed on Earth within the last few centuries; for their fossils to be nitrogen-free, they would have needed to form thousands of years before crashing here.

"This finding has direct implications to the distribution of life in the cosmos," wrote Hoover, with great understatement.

The claims have already set off an internet tempest of commentary, some of it noting Hoover's relatively unusual announcement venue. The Journal of Cosmology is produced by a community of colorful astrobiologists who are sometimes zealously evangelical about galactic panspermia, or the notion that Earth was seeded by life arriving from space.

Fifty years ago, that notion was crackpot. Nowadays, it's rather more plausible, if not so certain as some proponents insist. Life's building blocks have been found in comets, which could serve as spacefaring petri dishes. There are likely billions of Earth-like planets just in the fraction of space visible to the Hubble telescope. Given trillions of years and billions of opportunities, other examples of self-replicating chemicals don't seem improbable.

'I see no convincing evidence that these particles are of biological origin.'Beyond the venue, however, critiques of Hoover's claims become more substantive. "As a microbiologist who has looked at thousands of microbes through a microscope, and done some of my own electron microscopy, I see no convincing evidence that these particles are of biological origin," wrote SETI Institute astrobiologist Rocco Mancinelli in an e-mail to Wired.com.

"The main claim of similarity with modern earthly analogs is totally inadequate," said University of Croatia microbiologist Stjepko Golubic, who specializes in the sort of bacteria to which Hoover compared the alleged fossil remains. "It is important to note that the SEM [scanning electron microscope, used in this study] is an inadequate tool for identifying cyanobacteria, and this includes those images offered in this paper for comparison."

Mancinelli also criticized Hoover's sterilization techniques, which were not fully described in the paper. "It is unclear to me if the techniques used for the analyses were adequate. For example, the paper states that the implements used were flame-sterilized. Does that mean they were placed in a Bunsen burner where soot" – which could confound the results – "could get on them?" Mancinelli wrote.

NASA astrobiologist Chris McKay defended Hoover's practices, saying he is "a careful and accomplished microscopist so there is every reason to believe that the structures he sees are present and are not due to contamination."

'If these structures had been reported from sediments from a lake bottom there would be no question that they were classified correctly as biological remains.'"If these structures had been reported from sediments from a lake bottom there would be no question that they were classified correctly as biological remains," wrote McKay. [Not to be confused with NASA astrobiologist David McKay who lead a team that in 1996 reported evidence of Martian microbes in a meteorite. That claim remains inconclusive.]

Nevertheless, the structures could conceivably be random. They could also just be fossilized bugs from Earth, said Allan Treiman, planetary petrologist at the Lunar and Planetary Institute in Houston.

"The meteorites Dr. Hoover studied are rich in carbon compounds that formed off of the Earth. However, Earth microbes do not know the difference, will eat alien carbon with gusto," he said. "Because all of these meteorites have been on the Earth for many years, and have not been kept isolated from Earth microbes, it's certain that they have been exposed to Earth microbes."

Yet Engel is less willing to draw conclusions. Independent experts need to analyze Hoover's images, and it would be useful if there were better grounds for comparison than visual similarity with known bacteria, he wrote.

According to Engel, his own analyses of a meteorite analyzed by Hoover found no evidence of common amino acids, suggesting that Earthly microbes had not invaded them.

"Faced with the actual possibility of evidence for extraterrestrial life, we quite often feel more compelled to ignore it or refute it rather than embrace it," wrote Engel. "Perhaps this has something to do with our inherent fear of the unknown."

SETI research director Jill Tarter compared Hoover's findings to the controversial recent discovery of extremophile bacteria that may metabolize arsenic, something never before seen on Earth and suggestive of how extraterrestrial bugs could survive in supposedly inhospitable environments.

Like that claim, Hoover's "may turn out to be correct, but it has not yet been proven," said Tarter. "Incredible claim. Incredible evidence, not so much."

Correction 3/7/2011: The article originally stated that the Journal of Cosmology is not peer-reviewed. That is inaccurate. For more information, see the journal's editorial guidelines. We apologize for this mistake.

Dave Mosher contributed to this report.

*Image: *Journal of Cosmology

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Citation: "Fossils of Cyanobacteria in CI1 Carbonaceous Meteorites." By Richard B. Hoover. Journal of Cosmology, March 2011.