The meteorite ALH84001 may be the most important rock in the world. This ancient fragment of the planet Mars may be the source of what NASA claims are the fossilized remnants of primitive Martian life-forms. Analysis of this extraterrestrial visitor has revealed a remarkable history. Carbon-dated at 4.5 billion years old, the rock was hurled from the Martian surface and into space by an enormous asteroid impact 16 million years ago. After drifting though space, it fell to Earth in Antarctica 13,000 years ago.

This potato-sized gray meteorite launched the controversy over whether there was once life on Mars. The rock probably originated beneath the Martian crust when Mars formed some 4.5 billion years ago. Its name encodes its discovery: ALH stands for Allan Hills, the Antarctic ice field where it was the first meteorite found 1984 by the National Science Foundation's Antarctic Meteorite Program.

On August 6,1996, NASA stunned the world. It announced that a research team at the Johnson Space Center in Houston, Texas, had evidence | that primitive life may once have existed on Mars. The team produced photographs of what are claimed to be miniscule fossilized bacteria from the Red Planet.

NASA's astonishing statement came after an intensive two-year-long analysis of ALH84001—a 4.2-pound chunk of glass-like Martian meteorite that was discovered in Antarctica in 1984. ALH84001 is very different from the 13 other Mars meteorites discovered up until then. Not only is it three times older, but cracks and fissures inside the rock contain tiny, disc-shaped globules of carbonates—compounds that are deposited from water and which can fossilize and preserve dead life-forms. The NASA team says that the globules were formed about 3.6 billion years ago, at a time when water is believed to have flowed on the surface of Mars. Within these carbonates, the team found a combination of features that led them to their startling conclusion— that microbes may have once lived and died in the rock.

The most striking of these features are tiny fossilized structures called microfossils. At just one-thousandth the thickness of a human hair, they are as small as any microfossils ever found on Earth. Some of these so-called nanobacteria are worm-shaped, others are egg-shaped. One type of microfossil appears to be segmented, as if it were a filament composed of many separate cells.

Other telltale deposits found in ALH84001 include organic compounds called PAHs (polycyclic aromatic hydrocarbons) and mineral deposits of iron oxides and iron sulphides. On Earth, these are often the only traces left behind by decomposed microorganisms.

But some scientists say that the evidence can be explained in other ways. They point out that NASA used scanning electron microscopes, which do not reveal either organic matter or structures such as cell walls— two signatures of life crucial to identifying microfossils. They also argue that the PAHs could have entered the rock while on Earth and that these carbon compounds are of terrestrial origin. But PAHs are most plentiful deep within the rock, contrary to what might be expected if the meteorite had been contaminated on Earth. Others skeptics argue that the fossil structures are too small to contain all the genetic material needed for life. In any case, they may not be microfossils at all—just odd-shaped crystals.

Recently, some members of the NASA team turned their attention to another source of Martian rock—a meteorite that struck the village of Nakhia, Egypt, in 1911. An electron microscope revealed what they claim is more microfossil evidence. One particular filament, they say, is reminiscent of the thread-like trails left behind by some bacteria on Earth. Unlike ALH84001, which lay on Earth for 13,000 years before its discovery, this Nakhia fragment was collected right after the impact, so contamination by terrestrial bacteria is doubtful. I The controversy over whether the meteorites contain Martian microfossils will not be easy to resolve. But it may be settled after 2008, when NASA will send an unmanned mission to Mars with a probe that will bring back fresh rocks. Until then, the jury is out.