A Nasa scientist has stirred up fresh debate over life elsewhere in the cosmos after claiming to have found tiny fossils of alien bugs inside meteorites that landed on Earth.

Richard Hoover, an astrobiologist at the US space agency's Marshall space flight centre in Alabama, said filaments and other structures in rare meteorites appear to be microscopic fossils of extraterrestrial beings that resemble algae known as cyanobacteria.

Some of the features look similar to a giant bacterium called Titanospirillum velox, which has been collected from the Ebro delta waterway in Spain, according to a report on the findings.

Laboratory tests on the rocky filaments found no evidence to suggest they were remnants of Earth-based organisms that contaminated the meteorites after they landed, Hoover said. He discovered the features after inspecting the freshly cleaved surfaces of three meteorites that are believed to be among the oldest in the solar system.

Hoover, an expert on life in extreme environments, has reported similar structures in meteorites several times before. So far, none has been confirmed as the ancient remains of alien life.

But writing in the Journal of Cosmology, Hoover claims that the lack of nitrogen in the samples, which is essential for life on Earth, indicates they are "the remains of extraterrestrial life forms that grew on the parent bodies of the meteorites when liquid water was present, long before the meteorites entered the Earth's atmosphere."

Rudy Schild, a scientist at the Harvard-Smithsonian Centre for Astrophysics and editor of the journal, said: "The implications are that life is everywhere, and that life on Earth may have come from other planets."

In a note posted alongside the paper, Schild said he had invited 100 scientists to comment on the research. Their responses will be published on the journal's website from Monday. "In this way, the paper will have received a thorough vetting, and all points of view can be presented," Schild wrote.

Proof that alien microbes hitched across the cosmos inside meteors, or by clinging to their surfaces, would bolster a theory known as panspermia, in which life is spread from planet to planet by hurtling space rocks. To many scientists, Hoover's work recalls the adage that extraordinary claims require extraordinary evidence.

Hoover is not the only researcher to claim a discovery of alien life inside meteorites. In 1996, David McKay, another Nasa researcher, said he had found what appeared to be traces of Martian life inside a meteorite recovered from Allan Hills in Antarctica in 1984.