All the molecules of life on Earth are orientated the same way, but the reason why that’s the case is one of the biggest enigmas facing evolutionary biologists investigating the origins of life.
A NASA study has only served to deepen the mystery by revealing there’s no reason why one orientation would be favoured over another.
The ingredients for life
Earth life is based around proteins. These biomolecules are built from 20 amino acids which combine in different orders to create everything from enzymes to muscle and bone cells.
Some of these amino acids can exist in two forms, each a mirror of the other, which are referred to as being right- or left-handed.
Scientists call this handedness a molecule’s ‘chirality’.
For some reason, life on Earth exclusively uses left-handed amino acids.
Solving the mystery
One theory is that the molecules found on Earth before life emerged may have favoured one chiral structure over the other.
To investigate this, a team of researchers recreated the conditions that may have been present on early Earth in a lab.
They incubated solutions of ribozymes – an RNA molecule that might have been a precursor to DNA – and amino acids, to see whether they favoured one chirality over another.
"The experiment demonstrated that ribozymes can favour either left- or right-handed amino acids," says Irene Chen, from the University of California, Los Angeles, one of the study’s authors.
Such versions of early Earth "would not necessarily have a strong bias for the form of amino acids we observe in biology now."
It could be later evolutionary pressure that favoured one handedness over another.
Unfortunately, our planet’s plate tectonics have long since recycled the fossil record of the time, meaning it’s impossible to determine exactly when the preference asserted itself.
Instead, planetary geologists are searching for clues by looking at comets and asteroids.
These are thought to have been one of the main ways that the building blocks of life, including amino acids, were originally delivered to Earth.
"Understanding the chemical properties of life helps us know what to look for in our search for life across the Solar System," says Jason Dworkin from NASA’s Goddard Space Flight Center, project scientist on asteroid sample-return mission OSIRIS-REx.
"We are analysing OSIRIS-REx samples for the chirality of individual amino acids,” says Dworkin, “and in the future, samples from Mars will also be tested in laboratories for evidence of life, including ribozymes and proteins."
