By Michael Marshall
Blobs of simple carbon-based compounds could have been the precursors to the first living cells. A new study suggests that such droplets could have formed quickly and easily on the young Earth.
“We were able to find these interesting microdroplet structures that could be synthesised from prebiotically available resources,” says Tony Jia of the Tokyo Institute of Technology in Japan. “Maybe they weren’t the direct precursors to modern cells, but perhaps they could have had some effect or had a role in the emergence of initial life.”
All modern cells are surrounded by an outer wall called a membrane, which is made of long chain-like molecules called lipids. Given the ubiquity of these membranes, many researchers studying how life began have made simple membrane-lined spheres, which they say could mimic the first simple cells.
The droplets Jia and his colleagues made are different. “They don’t have an outer layer,” says Jia. “In that sense they’re membrane-less.”
The first cells?
The team made them from simple chemicals called alpha-hydroxy acids. These are made by the same processes that create amino acids, suggesting they were present on the early Earth, says team member Kuhan Chandru of the National University of Malaysia. “You can find them in meteorites as well.” He showed in 2018 that alpha-hydroxy acids link up to form complex molecules at a wide range of temperatures.
In the new study, the team simply dissolved the acids in water, then left them to dry out at 80 °C for a week – mimicking the conditions near a hot volcanic pond.
The acids turned into a thick jelly, because they had again formed complex molecules. When the researchers added water, the jelly formed hundreds of droplets a few micrometres across. Further experiments showed that crucial biological molecules, including protein and RNA, could enter the droplets and still perform their functions.
Cells without walls
Membrane-less droplets were a key element of the first popular hypothesis for life’s origin, which was set out by Russian biologist Alexander Oparin in the 1920s. However, the idea fell out of favour when it emerged that all cells have membranes.
The idea is now being re-assessed, says Kate Adamala of the University of Minnesota in Minneapolis. She suspects that life went through a “membrane-less stage” and that membranes only arose later.
Both droplets and membrane-based cells are a container for life’s components. This is crucial, says Adamala, because it keeps all the parts together, creating an individual organism from what would otherwise be a mess of chemicals.
But membranes are such good barriers that the first cells would have struggled to get food in and waste out, Adamala argues. So at the very beginning, membrane-less droplets would be better. “You don’t have to be shut off from the environment, because those droplets are permeable and you can have things diffusing in and out of them.”
Journal reference: PNAS, DOI: 10.1073/pnas.1902336116