For a long time it was assumed that the most important molecular building blocks of life were formed on terrestrial realms – for example in underwater vents, hydrothermal pools or liquid-filled pores in rocks. There, it was believed, in the early days of our planet, conditions suitable for the formation of organic molecules such as DNA, amino acids or even components of the cell membrane could have reigned. When these cells came together in a “primordial soup”, the first cells appeared – and thus the first life appeared on our planet.
Comets and meteorites as bringing to life?
But now the focus is on a different scenario. According to this, the origin of life on Earth can have two cosmic and terrestrial components. The organic molecules that formed in space and reached Earth could provide the organic building blocks, which then evolved on Earth and led to the formation of the first cells.
At least there were plenty of space-to-Earth transportation options in our planet’s early days: At that time, young Earth was plagued by countless remnants of planetary formation, including comets from the icy outer regions of the solar system. In addition, about 15,000 tons of interplanetary dust still fall into the Earth’s atmosphere every year – after all, about half of it penetrates the Earth’s surface, as suggested by analyzes of Antarctic snow samples in 2021.
The basic building blocks of all three “ingredients” of life
Comets and meteorites can aid life and bring organic molecules to Earth. But where did these particles come from? Studies of meteorites, as well as chemical measurement data from comets, provide the first answers: scientists have repeatedly discovered DNA bases in meteorites – the chemical “letters” of the DNA of the genetic molecule, the sequence of which forms the genetic code.
In the ice of comets, under conditions of space, the sugar molecules that make up the “backbone” of DNA – ribose and deoxygenases – can also form. The Rosetta space probe, which visited comet Churyumov-Gerasimenko a few years ago, has detected more organic compounds. She identified the amino acid glycine in the comet’s envelope of gas and dust – thus proteins are one of the building blocks of a second important group of life molecules.
The scientists also tracked a third important component of the first cells into space: in the interstellar molecular cloud, they used spectroscopic measurements to reveal the signature of ethanolamine, an important component of the cell membrane. The molecule forms the simplest head and the second most common phospholipid found in cell membranes. Molecules from the three important classes of building blocks for life have already been discovered in space: bits of genetic information, proteins as the “working horses” of the cell, and building blocks of the cell membrane as a protective covering for the first life forms.
Interstellar dust as a cosmic chemistry laboratory
But where and how might these biomolecules originate in space? In recent years, there has been increasing evidence that dust from interstellar clouds can be a particularly productive “factory” of more complex organic molecules. The dust and gas of these cold molecular clouds are thought to form nurseries of new stars and planets. Most of the material in these clouds comes from the condensed gas found between stars throughout the universe. However, some dust has also been released during supernovae – explosions of massive, ancient stars.
The crucial point, however, is that grains of cosmic dust are usually surrounded by a layer of ice and stick together into small clumps of rocky material and ice. The ice layers of these particular dust clumps are key to the chemical reactions that can create organic molecules in space. Because water and carbon monoxide, as well as other molecules, can “attach” to their surfaces and come into close contact with each other – close enough to interact with each other. Thus, the ice layers become a laboratory for cosmic chemistry.
In fact, many complex organic molecules have already been detected in interstellar dust clouds. In addition, laboratory experiments under simulated space conditions recently showed that peptides can form even on ice-covered dust grains – short chains of many amino acids that form the precursor to longer chains of proteins.
Also helpers of extraterrestrial life?
This does not clearly prove that the building blocks of early terrestrial life were in fact “celestial” and perhaps even of interstellar origin. The research so far makes it at least possible. This, in turn, reinforces the hypothesis that other planets could have received the molecular building blocks for life to arise in this way—and thus the existence of extraterrestrial life is most likely.