Friday, 31 July 1998

A Clue to the Origin of Life

They believe the asymmetry was imprinted in organic molecules in interstellar space before the formation of the Solar System. These molecules then found their way onto the Earth via the impacts of comets and meteorites to provide the starting material for the origin of life. This was revealed today in a paper in the international journal 'Science' by Dr Jeremy Bailey, from the Anglo-Australian Observatory, and his colleagues.

In 1848, Louis Pasteur discovered that some molecules can exist in two mirror image forms, right-handed or left handed. In living organisms, molecules tend to be all one form, not a mixture of both. Amino acids for example, the building blocks of protein, are always left-handed, where as sugars (including deoxyribose, an important component of DNA) are always right-handed. When these molecules are synthesised in a laboratory, equal numbers of right and left are formed. The reason for the imbalance puzzled scientists for decades.

In 1930, scientists discovered a way of destroying molecules of one- handedness, providing a partial solution to the problem. They used circularly polarised light.* But this was only part of the story. When life began on earth, there was no source of circularly polarised light.

Last year, scientists at Arizona State University discovered an excess of left-handed amino acids in the Murchison meteorite. (The Murchison metorite fell in 1969 near Murchison in Victoria, Australia and has been found to contain an extraordinary variety of organic molecules.) This remarkable discovery shows that the asymmetry already existed before life began on Earth, and may well have been present in the material from which the Solar System formed.

Dr Bailey and his colleagues used the Anglo-Australian Telescope at Siding Spring Mountain near Coonabarabran to show how the asymmetry might have been generated.

"We detected circularly polarized light (below right) in a region of the Great Nebula in Orion called Orion Molecular Cloud 1 (OMC-1, pictured left). We know that new stars are being formed here, and we also know that organic molecules are present," Dr Bailey said.

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

The circularly polarized light in such a region could imprint a preferred handedness on any organic molecules in the region, including those in a cloud beginning to collapse to form a star and its planets.

"We know that ultraviolet circularly polarised light is needed to select handedness in molecules such as amino acids, but unfortunately thick dust clouds prohibited observations at these wavelengths," Dr Bailey said. "So we made the observations at infrared wavelengths. Our calculations however, show that circular polarisation is present at all wavelengths, from infrared to ultraviolet," he added.

Many scientists believe that a preferred handedness in molecules must have been present in order for the origin of life to be possible. These results therefore suggest that the suitability of our planet for life may be as much a consequence of the environment in which our solar system formed as of the local conditions on the early Earth.

Footnote:

* Light is an electromagnetic wave consisting of oscillating electric and magnetic fields. The direction of the electric field determines the 'polarization' of the light. Many light sources such as the sun and most artificial lights produce unpolarized light. The fields are randomly oriented with no preferred direction.

In 'linearly polarized' light, the electric field oscillates along a fixed line. A familiar example is the light from the daylight sky. Polarizing filters such as those used in some sunglasses, or for mounting on cameras, filter out linearly polarized light and will cause the sky to darken if rotated to the appropriate angle.

In 'circularly polarized' light, the electric field direction rotates in a circle rather than oscillating from side to side. Circularly polarized light is rarely encountered in natural situations. Depending on the direction of rotation (either clockwise or anticlockwise) circular polarization can be either left-handed or right-handed.

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