One potential explanation is that Earth was seeded from space with the building blocks of life. The idea is that space is filled with clouds of gas and dust containing all the organic molecules necessary for life.
Indeed, astronomers have observed these blocks of buildings in clouds of interstellar gas. They can see amino acids, the precursors of proteins and the machinery of life. They can also see the precursors of ribonucleotides, molecules that can store information in the form of DNA.
But there is another crucial building block for life – molecules that can form membranes capable of encapsulating and protecting life molecules in compartments called protocells. On Earth, the membranes of all cells are made up of molecules called phospholipids. But these have never been observed in space. So far.
Forerunners of life
VÃctor Rivilla of the Spanish Center for Astrobiology in Madrid and his colleagues made the first detection in space of ethanolamine, a crucial component of the simplest phospholipid. The discovery suggests that the interstellar medium is teeming with all the precursors of life. âThis has important implications not only for theories about the origin of life on Earth, but also on other habitable planets and satellites anywhere in the Universe,â the team explains.
The group made their discovery by analyzing light from an interstellar cloud of gas and dust called Sagittarius B2, just 390 light years from the center of the Milky Way. Astronomers have long known this region as a rich reservoir of organic molecules, ice and dust particles.
Ethanolamine has the chemical formula NH2CH2CH2OH. The team simulated the spectrum that this molecule is expected to produce at the cold temperatures assumed to exist in the cloud. They then searched for and found clear evidence of this spectrum in the light that had passed through the cloud.
Although they have never been spotted in space, astronomers have found ethanolamine in meteorites. How it got there has been the subject of debate with some researchers claiming that it could only form through an unusual set of reactions on a parent asteroid.
The new finding suggests that ethanolamine is much more prevalent. On Earth, it forms the hydrophilic head of phospholipid molecules which self-assemble into cell membranes. Rivilla and her colleagues say her discovery in interstellar clouds suggests that “ethanolamine could have been transferred from the proto-solar nebula to planetesimals and minor bodies in the solar system, and then to our planet.” This could have led to the formation of cells in the prebiotic soup from which our earliest ancestors emerged.
A more radical idea is that ethanolamine could allow the formation of protocells in the interstellar medium itself. This is rich in other prebiotic components such as water and amino acids, which these protocells would have naturally encapsulated. The result would then be ready-to-use crucibles of prebiotic goop ready to seed the Earth, or any passing body.
Of course, none of this ultimately answers the question of how life began on Earth. But the work shows that there is no longer a mystery about the origin of the building blocks of life. “These results indicate that ethanolamine forms efficiently in space and, if delivered to early Earth, it could have contributed to the assembly and early evolution of early membranes,” say Rivilla et co. The question now is: what happened next?