Our bodies contain proteins that are made of smaller
molecules that can be either left- or right-handed, depending upon their
structure. Regardless of which hand we use to write, however, all human
beings are ‘left-handed’ at the molecular level. Life on Earth uses the
left-handed variety and no one knows how this preference crept into
living systems. In 2012, ESA’s Rosetta lander will land on a comet to
investigate, among other things, if the origin of this preference lies in the
stars.
Living cells use tiny organic molecules (called amino
acids) to build proteins in the same way as children build
things out of Lego bricks. Most amino acids come in two mirror-image
varieties, right- and left-handed. The arrangement of the thumb and
four fingers on a left hand is the mirror image of the arrangement on the
right. In amino acids, the arrangement of the atoms determines whether
the molecule is left- or right-handed.
Uwe Meierhenrich, at the University of Bremen, Germany, thinks that
the Earth’s early supply of amino acids came from space, carried by
comets. He is part of a European team who reproduced the way organic
molecules form in space, to try to understand what the Rosetta lander
might find on Comet Wirtanen in 2012.
At a laboratory in Leiden, The Netherlands, they lowered the
temperature of a chamber to -261°C, pumped out the air, and injected a
rarefied mixture of molecules known to exist in space: water, ammonia,
and simple carbon molecules. These molecules froze onto artificial dust
grains inside the chamber. They then shone an ultraviolet lamp onto the
samples, to simulate starlight. “Our aim was to simulate interstellar
conditions as accurately as possible. We did not adapt the conditions to
produce amino acids,” says Meierhenrich.
Using a version of Rosetta’s Cometary Sampling and Composition
(COSAC) experiment, they found newly formed right- and left-handed
amino acids in equal quantities. Earthly life, however, uses only
left-handed amino acids. Experiments during the 1950s showed that
adding right-handed amino acids to proteins stopped the proteins from
growing. Was Earth supplied with more left-handed amino acids than
right-handed ones in the beginning, allowing life to begin? One famous
study has suggested this. Meierhenrich says, “The excess (of left-handed
amino acids) found in the Murcheson meteorite is really small and some
of the analyses are controversial. Rosetta will give us a much clearer
picture.”
Some scientists think the key to creating an excess of left over right is in
the type of ultraviolet light that shines on the amino acids. When
ultraviolet starlight strikes dust grains, it can begin to twist – either
clockwise or anticlockwise. Depending on the direction in which it is
twisting, it destroys one handedness of amino acid more than the other. In
1998, an international group of astronomers discovered large amounts of
‘twisty’ light occurring naturally in the dusty cocoons of some young
stars, where planets might be forming. “I think this is the most probable
origin of the excesses,” says Meierhenrich who is now using a Paris
Laboratory where scientists can twist ultraviolet light to recreate these
conditions.
We may well find that Rosetta’s findings on Comet Wirtanen reliably
indicate that our molecular left-handedness is indeed a legacy of the
stars.
