How life began on earth over three billion years ago remains a mystery, but thanks to a study published in the journal Structure, we might have gotten a little closer to solving it. A team of researchers lead by José Manuel Sanchez Ruíz, a physical chemist at the University of Granada in Spain, reconstructed 4-billion-year-old proteins, potentially offering new insight into the origin of life.

Scientists offer different theories as to what initiated the precursors for cellular life: lightning strike to the primordial ooze, chemical reactions in deep-sea hydrothermal vents and the more well-known space rocks dropping the life-creating components to earth. However, none of these events can be, or so far have been, recreated, and attempts to understand protein evolution have been based on  comparisons between structures of modern proteins. “This is equivalent to trying to understand the evolution of birds by comparing several living birds,” Sanchez Ruíz said.

Ruíz and his team decided that since studying fossils was the best way to understand how life evolved, scientists would need to create fossil proteins to learn how life began. For that venture, they chose to recreate thioredoxins, a class of proteins that exists in all of the world’s organisms. They also perform many functions in organisms in the three domains of life—archaea, bacteria and eukaryota—suggesting that thioredoxins may be descendants of the earliest proteins. The team analyzed the differences between proteins in organisms from each domain and then mapped the differences to the dates when the organisms were thought to have separated. From those results, the team then determined the likely amino acid sequence of the early thioredoxin proteins. Finally, they recreated the primitive protein.

The “fossil” protein performed well. It bound to different chemicals, was very stable and functioned well in a highly acidic environment, which was what scientists believed the environment was four billion years ago. “Many people think that the temperature was high, and the oceans were acidic,” Sanchez Ruíz said. It also performed well under heat, suggesting that the earliest life existed in a hot environment.

Because the proteins are recreations, it is hard to determine how closely they match the ancient originals. “There is no way to make absolutely certain unless we invent some kind of time machine,” Sanchez Ruiz said. “But we know that the properties we measure for these proteins are consistent with what we would expect of 4-billion-year-old proteins.”