Scientists have achieved a significant breakthrough on the way to solving how life arose on Earth and whether it still exists somewhere in the universe.
How life arose from non-living, or abiotic, chemicals is a mystery. For the first time ever, researchers at Purdue University have shown that peptides, which are chains of amino acids that are the most important building blocks of life, can form spontaneously in water droplets during the rapid reactions that occur when water meets the atmosphere — for example, when a wave hits a rock and throws up a misty spray. This could have happened under conditions similar to those that existed on Earth about 4 billion years ago, when life first appeared on our planet.
According to a study published in the journal Proceedings of the National Academy of Sciences, this discovery “a plausible route for the formation of the first biopolymers” which are complex structures produced by living organisms. The team says the discovery could even speed up the development of new drugs and treatments by providing a new environment to accelerate fast chemical reactions.
“There are a very large number of studies showing peptide formation, but they all use catalysts or modified amino acids to make species unlikely to exist naturally,” said R. Graham Cooks, who serves as the Distinguished Professor of Analytical Chemistry and is a senior author of the study.
Cooks and his colleagues showed that peptides are readily formed in chemical systems that existed on ancient Earth, such as sea spray from our planet’s primordial oceans or fresh water dribbling down slopes.
The most interesting implication is that similar chemistry explains other essential biological polymers, not just peptides,” he noted, adding that his team plans to publish more on this topic soon.
Simply put, the new study has opened a rare window into the early years of our planet, when non-living compounds somehow assembled into living organisms: a still-unexplained transformation known as abiogenesis. The formation of peptides is an important step in abiogenesis because these structures form the basis of biomolecules — such as proteins — that can carry out the self-replicating mechanisms necessary for life.
The team was able to reconstruct the possible formation of these peptides by running “droplet fusion” experiments that simulate how water droplets collide in the air, which Cooks described as “like two kids with garden hoses spraying each other.”
These experiments show that the surface of droplets, where water meets air, is a region that can be extremely productive for making peptides from the types of amino acids that have been delivered to Earth by meteorites over billions of years. Experiments offer a possible solution to the so-called “water paradox” – a problem that has puzzled scientists in the field of abiogenesis for years.
“The water paradox is the contradiction between (i) the very considerable evidence that the chemical reactions leading to life occurred in the prebiotic ocean and (ii) the thermodynamic constraint against exactly these (water loss) reactions occurring in water,” Cooks explained. “Proteins are formed from amino acids by loss of water” and “loss of water in water will not occur because the process will be reversed by the water (thermodynamically forbidden).”
That is, peptides require a certain level of dehydration to form, but this is very difficult to achieve in a hydrated environment such as a water droplet. For more than a decade, Cooks and colleagues have shown that microdroplets have many unique characteristics, including accelerated reactivity on their surface. These air-water interfaces are like a reverse oasis — that is, a dry refuge in the watery world of the droplet — that enables the water-loss reactions necessary to build peptides from amino acids.
The discovery has implications beyond Earth. The study suggests that sea spray droplets on ancient Earth and other air-water interfaces may have been peptide factories that enriched the environment with the ingredients for life. The same processes may be common on other planets with similar conditions, which is useful information as scientists continue to search for life in our own solar system and beyond.
Indeed, Cooks said the results “reinforce the NASA mantra ‘follow the water,’” referring to the priority that researchers in the search for alien life place on planets that may contain liquid water. He proposed an additional mantra – “seek rough oceans” – based on the results, particularly the potential role of sea spray as a key factor in peptide formation.