Their fossilised remains have been found in the rock, which was blasted out of Mars 16 million years ago as the solar system was forming.
The meteorite, called Allen Hills 84001, made headlines in 1996 after fossils were found in it. Scientists believed they were bacteria from Earth that contaminated the rock while it lay in the frozen wastes.
But a Nasa report now says there is strong evidence they originated on Mars, according to The Sun.
Dr Emily Baldwin, deputy editor of the UK’s Astronomy Now magazine, said: “Many scientists argued that what looked like fossils in the meteorite were really caused by the explosive event, such as an asteroid impact, that blasted the rock out of Mars in the first place.
“But the Nasa team is now saying they have proved that they could not have been produced by the blast itself.
“If the features turn out to have an extraterrestrial, biological origin and were not formed during the 13,000 years the meteorite spent lying on Earth, this will have profound implications for our understanding of how life evolved in the solar system.”
Prof Colin Pillinger, of the Open University, who was behind Britain’s ill-fated Beagle 2 probe to the planet that was lost on Christmas Day 2003, said: “This is good quality work and more compelling evidence to add to the mix. These guys have been plugging away at this for years. It is a very careful study by very reputable people.”
The Nasa study, led by Kathie Thomas-Keprta, found carbonate discs and tiny magnetite crystals inside the space rock. Scientists were able to use high resolution electron microscopes that were not available 13 years ago.
They concluded “unusual chemical and physical properties” in the meteorite were “intimately associated within and throughout these carbonate disks”. That, they said, was evidence of interaction with water on Mars more than 3.5 billion years ago.
Nasa is expected to announce the findings, from its Johnson Space Centre in Houston, Texas, later this week.