Seeking the key to early life on Earth

A team of researchers at ILL have used neutron scattering to uncover a key step in the development of life on Earth.

Our researchers, along with scientists from the University of Lyon and University Grenoble Alpes, used neutrons to investigate the impact of molecules called alkanes, which could have allowed the first life to emerge around deep-sea hydrothermal vents.

“This is the very first time that a model of cell protomembranes that incorporates alkane molecules has been studied – it’s never been done before,” explains Loreto Misuraca, a PhD student at our laboratory. According to Misuraca, a puzzle to researchers is how early cells formed their membranes, and how these could survive the intense heat and pressures around deep-ocean vents.  “Membranes could only be formed by simple molecules that were available at that time, ” he says. “But these proto-living systems also needed energy, so the membranes must have endured the environment around hydrothermal vents to get to that energy.”

Today, species that live around hydrothermal vents have complex adaptations to their environment. So, the question is how the precursors to early life forms were able to survive the high temperature and pressure while lacking the complexity that we see today.

“We investigated whether the incorporation of alkanes could provide an advantage for the supposed protomembranes,” Misuraca explains. Alkanes are simple molecules, which are present in large quantities in meteorites. They are believed to have arrived on the early Earth via asteroid collisions. Misuraca and his colleagues wanted to know if alkanes could help create a membrane that would remain stable under high pressures and temperatures as high as 100°C, which is found around deep-sea vents.

“Since alkanes are very simple molecules, they could have easily been inserted into the membranes, providing stability under high temperatures, pressures or both,” he says.

He describes the protomembrane models used for the experiments as resembling an onion, with many concentric shells. Although different in structure and complexity from modern cell membranes, he says they can be studied to uncover a step towards the creation of life. “Once you have a stable membrane, you can encapsulate other molecules, and little-by-little increase complexity towards an actual living system,” he says.

The protomembranes were made of capric acid (a fatty acid) and decanol (a type of alcohol) – both simple molecules that were also available on the early Earth along with the alkanes. The team examined the impact of incorporating alkanes into the protomembranes using small-angle neutron/X-ray scattering (SANS/SAXS) and elastic-incoherent neutron scattering (EINS). These techniques, according to Misuraca, allowed them to examine the protomembranes structure and the mobility of the molecules forming them.

“They assemble into an onion-like shape, and between every shell (membranes), there is water,” he says, explaining that SANS and SAXS allow them to follow the protomembrane shrinking or swelling due to changes in the environment.

EINS, meanwhile, allowed the team to visualise how the ‘tails’ of the fatty acid molecules wiggle and move within the membranes. As Misuraca points out, the ‘tails’ tend to move more under higher temperatures and the extent of this motion can eventually lead to membrane disruption. By bombarding the shells with neutrons, they were able to build up a kind of image that contains information about the molecule dynamics. Profiting from the properties of neutrons and by using heavy water (where the hydrogen is replaced by deuterium), they could produce higher-contrast pictures and only see the motion of the molecules belonging to the membranes.

The team discovered that alkanes helped the protomembranes stay stable under conditions similar to those around a hydrothermal vent. “We’ve shown there’s a possible strategy that can explain how they might have survived under those conditions – that wasn’t known before,” he said. He adds: “The alkanes make the protomembranes significantly more stable, and this is one of the steps that could have helped the appearance of life.” The study, he says, can be expanded to cover life beyond Earth.

The team hopes to take their work forwards by looking at other types of alkanes and check whether different types can lead to new membrane properties. They also intend to see if  the protomembranes can trap molecules and retain them at high temperature and pressure, allowing them to work biologically as a precursor to modern cells.