A meteorite that plunged from space into a frozen lake in British Columbia is the best-preserved sample yet found of rock left over after the solar system was formed. The extraterrestrial visitor, which is believed to be a chunk of a distant and primitive asteroid, is giving scientists clues about the early chemistry of our solar neighbourhood.

The Tagish Lake meteorite closely matches an ancient class of asteroids that orbit the sun half a billion miles from Earth. These asteroids were formed before the planets and are rich in carbon, the basic element of life.

"It contains pre-solar grains which came from the sun before the planets were formed," explains Dr Takahiro Hiroi of Brown University, in Rhode Island, who wrote a report on the meteorite in the journal Science. The complex chemistry present in the rock has been preserved in the deep chill of space for five billion years. "It has never been heated to more than room temperature," says Hiroi.

The chemicals found were from a narrow range of light compounds, including carboxylic acids, aromatic hydrocarbons and fullerenes. Most meteorites are of similar age, but this one is the most primitive, making it the oldest object yet found.

The meteorite was recovered from the lake last year by a local man after a spectacular fireball was witnessed across northern Canada as it fell to Earth. Fortunately for the scientists, this fragment landed on soft snow and ice after the larger meteor broke up in Earth's atmosphere. "It's so fragile and very porous. If it hadn't landed on soft snow it would not have survived. It is preserved in pristine condition," says Hiroi.

Other groups have studied the chemistry of the meteorite, but no one was sure of its exact origins other than that it had probably come from the asteroid belt. Hiroi and his co-workers were the first to match this meteorite to a particular class of carbon-rich asteroid and even identify the exact asteroid, called 773 Irmantraud, from which it was probably ejected.

Hiroi probed the Tagish Lake rock with light, using a wide range of wavelengths. The amount of light reflected was measured and compared with measurements made by astronomers observing asteroids by telescope.

"Before my work, nobody knew where the meteorite came from," says Hiroi. "I measured its optical properties from the visible to the near infra-red, which gave me a fingerprint. I compared the fingerprint with all the data available from astronomical measurements and found it to match a 'D-type' asteroid, the first sample to be found."

Asteroids orbit the sun in a wide belt between Mars and Jupiter. The Tagish Lake meteorite is thought to have broken from an asteroid in the outer half of the belt, nearest to the giant and destructive gravitational field of Jupiter. As well as the optical fingerprint measurements, Hiroi used information about the time and trajectory of the meteor to narrow down its point of origin.

Theories exist that life on Earth was "seeded" by falling comets and meteorites that introduced the important carbon-based chemicals that are the building blocks of life. The early material of Earth, had it originally contained complex chemicals, would have been sterilised by the high temperatures during planetary formation.

"The question of how life started on Earth is for the biologists to answer," says Hiroi, "but meteorites are a good way to get fresh organic material. It's possible that they contributed to the early organic materials that led to life."

Many researchers believe contamination of meteorites is inevitable once they have landed. This makes it difficult to distinguish between traces of extraterrestrial organic compounds and Earthly contaminants. The Tagish Lake meteorite is a prize catch for the astrobiologists who are studying the possibility of life originating from material in space. It fell into a frozen, near-sterile environment and has been expertly handled and stored since.

Dr Peter Foote is a research scientist at BAE Systems in Bristol. He is participating in the Media Fellow programme run by the British Association for the Advancement of Science