Proteinoid

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Proteinoids, also called thermal proteins, are protein-like molecules that form abiotically—from amino acids—without any life involved. Sidney W. Fox suggested they might be early precursors to living cells, or protocells. The term was once used for very short peptides found in hydrolyzed proteins, but that usage isn’t common today.

In the 1950s and 1960s Fox studied how peptide structures could form under conditions that might have existed on early Earth. He showed that amino acids could spontaneously link together to make small peptide chains. In one experiment, he let amino acids dry in a warm, dry spot. As the water evaporated, the amino acids joined into long, cross‑linked thread-like structures that Fox named proteinoid microspheres.

It was once thought that amino acids could polymerize into proteins only at temperatures above 140°C. Fox and colleagues found that phosphoric acid could speed up this reaction, allowing protein-like chains to form at about 70°C from a mix of 18 amino acids. Fox later found naturally occurring proteinoids in lava and volcanic ash from Hawaiian vents, where the heat from escaping gases helped the amino acids polymerize into proteinoids.

Other catalysts were discovered as well; for example, amidinium carbodiimide can form in primitive Earth-like conditions and works in dilute water. In water, proteinoids can assemble into small microspheres because some amino acids are more water-repellent than others, so the particles cluster like oil droplets in water.

These microspheres show a few cell-like properties: they can concentrate organic molecules and provide a sheltered environment during chemical evolution. Today, proteinoid microspheres are being explored as microscopic, biodegradable capsules for delivering drugs orally.

Fox also conducted volcanic experiments, placing amino acids near lava. He found temperatures above 100°C just beneath the surface and suggested such environments could have helped life begin, with molecules forming and then washing into the sea.

Although these protobionts were not real cells (they lack true proteins, proper bonds, and cell compartments), Fox’s work helped spark ideas about how life might have started. His experiments encouraged other theories of abiogenesis, such as RNA world, iron–sulfur world, and protocell ideas, even though proteinoids themselves are not considered direct precursors to living cells.