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A Clue To The Origin Of Life

Anglo-Australian Observatory
PO Box 296, Epping
NSW 2121 Australia

Friday, 31 July 1998

A Clue to the Origin of Life

Astronomers using the Anglo-Australian Telescope have found a possible
explanation for why life on earth almost exclusively uses left-handed amino
acids and right-handed sugars as the building blocks of proteins and nucleic
acids -- a mystery that has puzzled scientists for 150 years.

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.

Further information:

Dr Jeremy Bailey
Anglo-Australian Observatory
phone 61 2 9372 4823
email jab@aaoepp.aao.gov.au

                     - End -

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.


[NOTE: Images supporting this release are available at
http://www.aao.gov.au/press/life.html]

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