In 1848 Louis Pasteur discovered that some molecules can exist in two mirror-image forms, termed right or left-handed. Curiously, in living things the molecules tend to be one of these types and not a mixture of both.

Amino acids, the molecular building blocks of life are all left-handed where as sugars, including deoxyribose an important component of DNA, are always right handed.

By contrast, when these molecules are made in the laboratory equal numbers of the right and left-handed versions are made.

The Anglo-Australian Observatory

Why life on the molecular level is like this is a mystery but now an international team of astronomers may have discovered why life on planet Earth is left-handed.

The astronomers believe that this left-hand bias began long before our Sun was even born. At the time when our star was forming out of a vast cloud of gas and dust more than five billion years ago.

Comets, mountain-sized dirty snowballs of dust and frozen gasses, were formed in this Sun's birth-cloud. The left-handed molecules were incorporated into comets and found their way to Earth as they struck the young planet.

Using the Anglo Australian Telescope to study a stellar nursery scientists may have discovered how the left-hand effect came about.

Professor James Hough of the University of Hertfordshire, England, said: "We detected high degrees of circularly polarised light in a region of the Great Nebula in Orion where new stars are being formed.

"This region may well be similar to the region in which our own solar system formed."

The importance of the detection of polarised light in the stellar nursery is that such light is known to destroy one of the types of mirror molecules leaving an imbalance.

This is a profound discovery. Many scientists believe that life could only have developed on Earth because of the bias towards left-handed molecules.

If this is so then one of the reasons we are here is because of the behaviour of light in the vast cloud of gas and dust which gave birth to our Sun billions of years ago.