A relatively simple protein model could answer one of the most important questions in science: how did life emerge from non-life? If so, it would not only settle a question that has troubled humanity for thousands of years, leading to so many creation stories, but it could also aid the search for life on other planets.
All living things on Earth rely on amino acids, often referred to as the “building blocks of life”. The presence of amino acids on meteorites indicates they were likely abundant on Earth when life began. However, the original spark of life also required energy to assemble these building blocks, which would have required a catalyst.
A team searching for the first life’s crucial ingredients report finding what they consider a likely candidate in a new paper.
"Scientists believe that sometime between 3.5 and 3.8 billion years ago there was a tipping point, something that kickstarted the change from prebiotic chemistry – molecules before life – to living, biological systems," said study author Professor Vikas Nanda of Rutgers University in a statement. "We believe the change was sparked by a few small precursor proteins that performed key steps in an ancient metabolic reaction. And we think we've found one of these 'pioneer peptides.'"
The molecule in question is composed of two nickel atoms bonded by an amino acid backbone, leading Nanda and co-authors to name it nickelback. In this form, the nickel becomes a powerful catalyst, putting protons and electrons together to make hydrogen that would have been an energy source then, as it is today.
Many of the molecules that underlie modern life are only possible because of the assembly process life allows. Life’s initiators must have been simpler, the researchers reasoned, yet had the chemistry to transmute energy to power biochemical reactions.
However, existing proteins are generally too complex to fit the bill so the team sought to strip them back to something simpler. Nickelback is the most promising example they have found yet. Composed of 13 amino acids, along with the two distinctive nickel atoms, it would still have been a feat to assemble at the time – but the authors found it stable across the temperatures and acidities likely to have been present at the time. Consequently, once formed, it could have performed its role over and over again.
The abundance of nickel in the oceans of the early Earth, combined with the likely presence of amino acids make nickelback’s existence at the time plausible.
The team wants to conduct further research to explore whether nickelback can catalyze other reactions important at the time – such as reducing carbon-based molecules – or if it was just the headliner on a line-up requiring other peptides. The same team discovered ambidoxin five years ago, and consider it a possible collaborator, facilitating other reactions that prepared the way for life.
"This is important because, while there are many theories about the origins of life, there are very few actual laboratory tests of these ideas," Nanda said. "This work shows that, not only are simple protein metabolic enzymes possible, but that they are very stable and very active – making them a plausible starting point for life."
Astrobiologists are gearing up to search for molecules that indicate the presence of life, known as biosignatures, in the atmospheres of potentially habitable planets. The capacity to find biosignatures in planets orbiting other stars is currently at the very edge of our technology, but as more powerful or more focused telescopes are deployed it will become increasingly realistic.
Finding biosignatures on worlds as rich in life as modern Earth may be relatively easy once those instruments come online. However, nearby suitable worlds may be much less advanced. The presence of peptides like nickelback might indicate that even if life has not begun, it is likely soon.
The paper is published in Science Advances