Nature: Forget the primordial soup: according to the latest theory, we are all descended from ET

Life on Earth may have been seeded by comets and meteorites, according to new research. Charles Arthur, Science Editor, says that means we're all extra-terrestrials - and that life has existed longer than the solar system's five billion years.

Chemistry students studying the structure of compounds soon learn that many structures can have a "left-handed" and a "right-handed" formation. The molecules are composed of the same elements, linked in the same order, but their physical structures are mirror images - like gloves. Amino acids, the small molecules which make up proteins, can also show this "handedness".

Now, scientists at the University of Oklahoma and the University of Virginia have analysed the amino acids in the "Murchison meteorite", and found that the amino acids inside it are mainly left-handed. And it is probably no coincidence that amino acids in living things on Earth are the "left- handed" form.

Increasingly, scientists reckon that the early life on Earth did not start in a "primeval soup" powered by lightning bolts, but instead was seeded by chemicals from comet dust and meteor impacts. That idea has been boosted by discoveries of complex molecules in the tails and icy cores of comets.

The news vindicates the scientists Fred Hoyle and Chandra Wickramsinghe, who have maintained that life could and did originate in cometary material, which then brought it to Earth. Their views were frequently dismissed. But recently more scientists have come around to agree with them.

Early theories suggested that life on Earth formed in a "primeval soup". Chance would suggest that it would have had equal amounts of left- and right-handed amino acids, and hence proteins. But it does not. Living proteins are built on left-handed amino acids.

The Murchison meteorite, which struck Earth in 1969, also has a surplus of left-handed amino acids. Critics have repeatedly suggested that that was because the meteorite was contaminated by terrestrial sources. But the latest studies, reported today in the science journal Nature, have shown that this is not the case: amino acids deep inside the meteorite had the wrong proportions of different isotopes of nitrogen to have been formed on Earth.

That means extra-terrestrial life - and its precursors - is left-handed, just like life on Earth. But many cosmic objects are far older than the Solar System, which formed about 5 billion years ago.