Scientists have published what they claim is “conclusive evidence” that bacteria once lived on Mars. The study is a follow-up to the famous 1996 research that purported to show ancient bacterial forms in a meteorite from Mars. Critics at the time said the evidence was too thin to prove anything. Now, a Nasa-backed team from the US, Spain and Germany has published further work which it says strengthens the earlier claims. But some British experts, at least, are still sceptical, arguing the latest study falls short of absolute proof. The argument centres on tiny crystals found in a potato-sized meteorite picked up in Antarctica in 1984. The international team says the crystals are the same as those deposited by earthly microorganisms known as magnetotactic bacteria. Since the meteorite is from Mars, it can only be assumed that the same bacteria must have lived on Mars as well, the team says.