Billions of years ago, an unknown location on the primordial Earth became a cauldron of complex organic molecule from which the first cells emerged. Origin-of-life researchers have proposed a lot of different ideas about how that happened. The most difficult to account for are the proteins, which are made by living cells. How did the first protein form?
Scientists have been looking at Earth. A new discovery suggests that the answer could be found inside dark clouds.
A group of Astrobiologists showed last month in Nature Astronomy that there is a way to form a molecule on the frozen particles of the universe. Some of the starting materials for life could have traveled inside comets and meteorites to other worlds.
Serge Krasnokutski, the lead author on the new paper and a researcher, said that the new space-based mechanism for forming peptides is more promising than the chemical processes that could have occurred on a lifeless Earth. He said that the simplicity suggests that the first molecule involved in the evolutionary process leading to life was the proteins.
Researchers say they have found a way to make a simpler chemical pathway that makes sense to the theory that the earliest stages of life were the creation of proteins.
It's very much an open question whether those peptides could have contributed meaningfully to the origin of life. The chemistry demonstrated in the new paper is very cool, but it doesn't bridge the phenomenal, said Paul Falkowski, a professor at the School of Environmental and Biological Sciences at Rutgers University.
The finding by Krasnokutski and his colleagues shows that there is more to the universe than scientists thought.
Cells make it easy to make something. They manufacture both peptides and proteins extravagantly, because of the environments rich in useful molecules like amino acids and their own stockpiles of genetic instructions and catalytic enzymes.
There wasn't an easy way to do it on Earth before cells existed. The production of peptides is an inefficient two-step process that involves first making and then removing water, and then linking up the two chains in a process called polymerization. Both steps have a high energy barrier, so they only occur if large amounts of energy are available to kick-start the reaction.
Because of these requirements, most theories about the origin of proteins have focused on scenarios in extreme environments, such as near the ocean floor, or assumed the presence of molecules with catalytic properties that could lower the energy barrier enough to push the reactions forward. The most popular origin-of-life theory proposes that the first molecule in life wasRNA. Krasnokutski says that special conditions would need to be met to concentrate the amino acids. It isn't clear how and where those conditions could have arisen on the primordial Earth.
Researchers say they've found a way to use a simpler chemical pathway to get around the theory that there was a lot of proteins early in the creation of life.
Krasnokutski predicted in Low Temperature Physics last year that a more direct way to make peptides could be found inside the dense and frigid clouds of dust and gas that linger between the stars. Some of the most abundant of which are carbon monoxide, atomic carbon and ammonia can be found in the clouds of new stars and solar systems.
Krasnokutski and his colleagues showed in their new paper that the condensation of carbon onto dust particles and the formation of small molecule called aminoketenes would likely happen in the gas clouds. Polyglycine is a very simplepeptide formed by the link up of these aminoketenes. Without the need for energy from the environment, reactions could proceed spontaneously.
To test their claim, the researchers did a simulation of the conditions in the clouds. They deposited carbon monoxide and ammonia onto plates that were chilled to minus 263 degrees Celsius inside an ultrahigh vacuum chamber. They put carbon atoms on top of the ice layer to make it look like they were condensation inside the clouds. Various forms of polyglycines, up to chains 10 or 11 subunits long, were produced by the vacuum simulation.
The researchers theorize that billions of years ago, as the dust stuck together and formed asteroids and comets, simple peptides on the dust could have traveled to Earth in meteorites and other impactors. They may have done the same on many other worlds.
Daniel Glavin said that the delivery of peptides to Earth and other planets would give a head start to forming life. I think there is a big jump to life on Earth from the chemistry of interstellar ice dust.
The perils of their journey through the universe, from radiation to water exposure inside asteroids, would have to be overcome first. They would have to survive the impact of hitting a planet. Glavin said that they would have to go through a lot of chemical evolution to get large enough to fold into a useful biological chemistry molecule.
Is there any evidence that this has happened? One study from 2002 found that two meteorites held small, simple peptides made from two amino acids. Glavin said that researchers have yet to discover other convincing evidence for the presence of such peptides and proteins in meteorites or samples returned from asteroids or comets. It's not clear if the nearly total absence of even relatively small peptides in space rocks means that they don't exist or if we haven't detected them yet.
Glavin said that Krasnokutski's work could encourage more scientists to look for more complex molecule in extraterrestrial materials. Glavin and his team plan to look for some of these types of molecule when NASA brings back samples from the asteroid Bennu next year.
The researchers want to see if different types of peptides can form in the clouds. Krasnokutski said that the formation of larger and more complex molecule might be triggered by other chemicals and energetic photons in the interstellar medium. Through their unique laboratory window, they hope to see peptides getting longer and longer, and one day folding, like natural origami, into beautiful proteins that burst with potential.
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