How life emerged on Earth from an assortment of non-living molecules is a stubbornly enduring mystery. Experiments can show us how key steps might have happened, but for every leap forward there are confounding dead ends.
Water, for instance, seems like an essential component of life from the very start. Yet the process of growing some of life’s most vital components has a frustrating aversion to getting wet.
“We know amino acids are the building blocks of proteins and proteins are essential for life,” says University of Wisconsin–Madison biochemical engineer John Yin.
“In prebiotic chemistry, it’s long been a question of how we could get these things to form bonds and strings in a manner that might eventually lead to a living cell. The question is hard because the particular chemistry involved is one that tends to fail in the presence of water.”
The prevailing theory since the time of Charles Darwin is that life emerged from a decidedly wet ‘primordial soup‘, making it hard to reconcile the precise role water might play in the origins of those first sustained, self-replicating reactions.
So University of Wisconsin–Madison chemical engineer Hayley Boigenzahn led a study into a simulated changing environment – one that altered between wet and dry conditions that are easily replicated in nature with tidal and day/night cycles, as well as changing weather.
Boigenzahn’s team combined a selection of amino acids which have proven quite easy to produce naturally. As the building-blocks of proteins – units that can perform the mechanical work of living processes – the resulting structures are a sound bet for playing a major role in early forms of biology.
Unfortunately, getting those units to link together into longer chains is something of a challenge. In this case, the researchers used the amino acid glycine.
Then they added trimetaphosphate into their soup, a molecule naturally produced in volcanoes.
Finally, the soup was spiced with sodium hydroxide (NaOH) to increase its pH.
Lo and behold, during the first hour of the experiment, glycine coupled up to make a two unit molecule called a dimer. This reaction releases protons that in turn neutralize the pH required for the dimerization to take place, effectively putting the brakes on the whole process.
As found in previous research, as the solution’s pH became more neutral the dimers slowly started to link with each other into slightly longer chains. As the solution dried out, however, the reaction rate increased, possibly due to concentrations of the molecules crowding closer together, the team suspects.
“What we’re showing here is that it doesn’t necessarily have to be the same environment throughout all the reactions,” says Boigenzahn. “They can occur in different environments, provided that the reactions that are occurring help create an environment that’s beneficial for the next steps.”
A cycle of transitions between wet and dry conditions could grow the molecule into more complex proteins, of which some might promote other chemical reactions involved in life.
“The fact that these reaction mechanisms have been known for many years and there has been limited appreciation for the link between them suggests that it may be worthwhile to pay greater attention to the effects of proposed prebiotic reactions on their environment, in addition to the effects of the environment on the reactions,” Boigenzahn and team note.
This is not the first clue that the origins of life may have occurred at the edge of wetness either. Earlier this year chemists found free floating amino acids were more reactive at the air-water boundary of tiny droplets. What’s more, these reactions happened in normal environmental conditions without the need of other chemicals or radiation.
There’s still a long way to go before understanding all that’s involved, but making sense of the processes behind the creation of life could also open the door to new, more powerful chemistry-based technologies.
“Eventually you might create chemical systems that are able to store information, adapt and evolve,” says Yin.
“DNA stores information at thousands of times the density of a computer chip can. If we could get systems that do this without necessarily being living cells, then you start to think about all sorts of new functions and processes occurring at the molecular level.”
This research was published in Origins of Life and Evolution of Biospheres.