The complexity of molecules can clearly determine whether or not they were made by living organisms, researchers say – a discovery that could have profound implications for the search for extraterrestrial life.
Lee Cronin’s team at the University of Glasgow, UK, has developed a measure of molecular complexity, known as the Molecular Assembly Index (MA), and uses it to examine a large number of samples, including one from a meteorite, the in the 1960s.
Their research suggests that complex molecules produced by biological organisms have an MA of 15 or higher and can be distinguished from molecules produced by non-living processes such as volcanoes or atmospheric chemistry, which typically have an MA of 12 or higher have lower. The method could be used to analyze samples of material from the solar system or the light from distant exoplanets to clearly tell what evidence of biosignatures – chemicals produced by living organisms – contain, regardless of the chemistry on which they are based .
The MA index is based on the smallest number of chemical steps required to artificially create a molecule. Simple molecules like carbon dioxide only have a single type of carbon-oxygen bond and therefore have an MA of 1, but complex molecules with many different bonds and chemical groups have much higher MAs. One example is taxol, a chemotherapy drug extracted from the bark of the Pacific yew tree and chemical formula C. Has47H51NO14th, a molecular weight of 853.9 and an MA of 30.
The study uses mass spectrometry to determine the MA of biological samples – such as yeast cultures, algae, and home-brewed beer – and inorganic samples such as quartz, sandstone, and granite. The researchers also analyzed dead but once living substances like coal and blinded samples of unknown origin, including one from the carbonaceous chondrite meteorite that fell near the Australian city of Murchison in 1969. The Murchison meteorite sample contained millions of molecules, but none showed an MA greater than 15. “Murchison is dead and has always been dead,” says Cronin.
MA is independent of the method used to determine it – mass spectrometry, nuclear magnetic resonance, infrared spectroscopy or others. It could be used on spacecraft looking for life in the solar system, on spectroscopic data on light from distant planets, or by laboratory researchers looking for evidence that they created synthetic life, says Cronin.
In the case of the phosphine detected on Venus, which has been suggested as a sign of microbial life in the atmosphere, the simple molecule has the lowest possible MA of 1. This means that it can never be a unique biosignature, and indeed the researchers involved have large ones Efforts are being made to rule out non-biological causes, says Cronin.
“One of the unique properties of life is that it forms complex and otherwise improbable molecules,” says Caleb Scharf, director of astrobiology at Columbia University in the United States. “The trick is to evaluate this improbability, and this paper describes a robust and well-motivated approach that uses the well-known rules of chemical assembly routes.”
“By developing a metric for molecular complexity and demonstrating that it is a robust indicator of past or present biological activity in environmental samples, [researchers] have developed a powerful tool for astrobiology, “says planetary scientist Ian Crawford of Birkbeck, University of London, UK.” Given that we do not yet know anything about extraterrestrial biology, it is especially important that the technology is based on it, too realizing what life is doing – that is, building molecular complexity – and not having to rely on a definition of what life actually is. “