The genesis of life on our planet is still a mystery, but scientists are slowly figuring out the steps involved and the necessary ingredients. Scientists think life began in a primordial soup of organic chemicals and biomolecules on the early Earth, eventually leading to actual organisms.
There has been a long-held idea that some of these ingredients may have come from space. Now, a new study published in Science Advances shows that a special group of molecules, known as peptides, can form more easily under the conditions of space than those found on Earth. That means they could have been delivered to the early Earth by meteorites or comets – and life may be able to form elsewhere too.
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The functions of life are carried out in our cells (and those of all living beings) by large, complex carbon-based (organic) molecules called proteins. How to make the large variety of proteins we need to stay alive is coded in our DNA, which is itself a large and complex organic molecule.
However, these complex molecules are assembled from a variety of small and simple molecules such as amino acids – the so-called building blocks of life.
To explain the origin of life, we need to understand how and where these building blocks form and under what conditions they spontaneously assemble themselves into more complex structures. Finally, we need to understand the step that enables them to become a confined, self-replicating system – a living organism.
Role of Water
The infrared (IR) absorption spectra were procured by placing the reactants on substrates at extremely cold temperatures of 10K and then gradually heating them to 300K. Both samples were created under identical conditions. The only variable was the inclusion of water in the gas mixtures used to generate ice for one sample. As observed, the presence of water has a relatively minor impact on the absorption spectra. The primary distinction is that the peptide absorption bands for the sample with water are approximately twice as weak. A decrease in intensities indicates a lesser quantity of peptide bonds formed when water was introduced.
As evidenced in the data, the formation of various peptides is noted, which aligns with the IR spectroscopy measurements. The key difference, however, is that the main products are not NH2-Glyn-NH2 peptides but more straightforward polymers of aminoketene with only an additional hydrogen atom. The peptides formed here have NH2 and COH terminals. Furthermore, the equilibrium is tilted towards the formation of short peptides. In this experiment involving water, we observe the predominant formation of dimers with a significantly reduced number of trimers, while in the experiments devoid of water, the peak for glycine trimers with amino terminals on both ends was the most intense one. Hence, we deduce that ammonia, as initially proposed, was indeed crucial for the polymerization of aminoketene molecules.
When water molecules are present in the ice, the rise in temperature results in the evaporation of ammonia before water. Therefore, during the evaporation of ammonia, aminoketene molecules would be ensnared in the water ice matrix, devoid of the ability to move freely. The aminoketene molecules will only be able to encounter each other during the evaporation of water when the majority of the ammonia has already evaporated. This substantiates the significant catalytic role of ammonia in the polymerization of aminoketene and concurs with the previous discovery that ammonia catalyzes the polymerization of H2CO to polyoxymethylene, which is the first detected polymer in space. Our findings also align with the predicted low rate of the H2O + C → H2CO reaction and efficient diffusion of the C atoms on the surface of water ice since the quantity of H2CO molecules produced in current experiments with water was below the reliable detection limit.
Journey to Life
DNA is composed of approximately 20 distinct amino acids. Similar to the characters of an alphabet, these are arranged in various combinations within the double helix structure of DNA to encode our genetic blueprint.
Peptides, too, are an assembly of amino acids, structured in a chain-like formation. Peptides can consist of as few as two amino acids, but can also extend to hundreds of amino acids.
The assembly of amino acids into peptides is a crucial step, as peptides perform functions such as ‘catalyzing’, or accelerating, reactions that are vital for sustaining life. They are also potential molecules that could have been further assembled into early versions of membranes, encapsulating functional molecules within cell-like structures.
However, despite their potential significance in the origin of life, the spontaneous formation of peptides under the environmental conditions of the early Earth was not a straightforward process. In fact, the scientists behind the current study had previously demonstrated that the frigid conditions of space are actually more conducive to the formation of peptides.
In the extremely low density of molecular clouds and dust particles in a region of space known as the interstellar medium, individual carbon atoms can adhere to the surface of dust grains along with carbon monoxide and ammonia molecules. They then react to form molecules similar to amino acids. When such a cloud becomes denser and dust particles begin to coalesce, these molecules can assemble into peptides.
In their new study, the scientists examine the dense environment of dusty disks, from which a new solar system with a star and planets eventually emerges. Such disks form when clouds abruptly collapse under the force of gravity. In this environment, water molecules are far more prevalent, forming ice on the surface of any growing clusters of particles that could inhibit the reactions that form peptides.
By replicating the reactions likely to occur in the interstellar medium in the laboratory, the study shows that, although the formation of peptides is slightly reduced, it is not halted. Instead, as rocks and dust amalgamate to form larger bodies such as asteroids and comets, these bodies heat up and allow for the formation of liquids. This enhances peptide formation in these liquids, and there’s a natural selection of further reactions resulting in even more complex organic molecules. These processes would have transpired during the formation of our own Solar System.
Many of the building blocks of life, such as amino acids, lipids, and sugars, can form in the space environment. Many have been detected in meteorites.
Because peptide formation is more efficient in space than on Earth, and because they can accumulate in comets, their impacts on the early Earth might have delivered quantities that accelerated the steps towards the origin of life on Earth.
So, what does all this imply for our chances of discovering extraterrestrial life? Well, the building blocks for life are dispersed throughout the universe. The specificity of the conditions required to enable them to self-assemble into living organisms remains an unresolved question. Once we ascertain that, we’ll have a clearer understanding of how prevalent, or not, life might be.
