It's life, but not as the scientists of Nasa know it

It was one of Nasa's most intriguing messages: an invitation to a briefing where it would "discuss an astrobiology finding that will impact the search for extraterrestrial life". The event held on 2 December at Nasa's headquarters in Washington led to headlines around the world, including The Independent's "Science grapples with the concept that alien life may be among us".

However, a growing body of dissent in the scientific blogosphere has cast serious doubts on the findings and called into question the ponderous nature of the classic peer-review system on which science is based.

The findings emerged after researchers funded by Nasa discovered a unique form of life in the shape of a bacterium that could apparently use an arsenic-based compound as a vital building block of its DNA – the first organism on Earth capable of living off arsenic.

The bacterium, known as GFAJ-1, was discovered in Mono Lake in California, which has naturally high levels of arsenic. When the scientists fed the bacteria in the lab on a diet rich in arsenate, which is chemically similar to phosphate, they found that it could incorporate the compound into the structural "backbone" of its DNA.

The lead scientist, Felisa Wolfe-Simon of the Nasa Astrobiology Institute and the US Geological Survey, said that the discovery of an organism on Earth that does something so radically different to all other known forms of life opens the door to what is possible for life elsewhere in the Universe.

In other words, the concept of what is required for life to exist was suddenly changed. Some scientists even suggested that perhaps GFAJ-1 is evidence that alien lifeforms carried to Earth on meteorites may be living among us undetected because until now we have not been looking for them in the right places.

The study, published in the peer-reviewed journal Science, appeared to tick all the requisite boxes in terms of scientific validity. But within days of the findings being published, other experts in the field of astrobiology and microbiology began to punch holes in the work, publishing their doubts in their blogs. The first serious critique came from Rosie Redfield, a microbiologist a the University of British Columbia in Canada, who said that if a PhD student had come to her with these results they would have been sent straight back to the lab bench.

Dr Redfield was one of many who began to think that Nasa had made the same mistake it made in 1996, when it prematurely announced that it had discovered signs of fossilised life in a Martian meteorite called ALH8400, which had fallen to Earth in 1984.

"Nasa's shameful analysis of the alleged bacteria in the Mars meteorite made me very suspicious of their microbiology, an attitude that's only strengthened by my reading of this paper. Basically, it doesn't present any convincing evidence that arsenic has been incorporated into DNA (or any other biological molecule)," Dr Redfield said.

Other scientific bloggers waded in, suggesting that Nasa and Science had not done enough cross-checking. One suggestion was that the findings were the result of a contamination, with arsenic compounds sticking to the bacterium's DNA rather like mud to a shoe.

Alex Bradley, a microbiologist at Harvard University, even suggested that the Nasa-funded scientists had unwittingly demonstrated a flaw in their own research because in their study they had dipped the bacterial DNA into water and because all arsenic compounds fall apart quickly in water this would have resulted in a lot of fragmented genes, which was not the case.

Nature, a rival publication to Science, waded into the debate claiming that the same Nasa scientists who had previously been happy to promote their findings had now retreated "behind the walls of peer review".

"You may have seen claims scientists at Nasa have discovered a bacterium that can replace the phosphorus in its DNA with arsenic. You may have heard that this could help the hunt for aliens. You may even have heard that 'arsenic bacterium' is itself an alien," sniffed Nature. "What you have not seen or heard is a detailed response from Nasa and the scientists involved to online criticism of their work. In the face of worldwide attention on their paper, which Nasa and the team deliberately courted, the researchers have stuck their head in the digital sand."

It was after this scathing editorial in Nature, which said that it strongly encouraged online discussions of peer-reviewed work, that Dr Wolfe-Simon finally posted a response to some of her critics. But now this response has since been criticised by her opponents.