Let’s face it: The young planet Earth was not a pleasant place to be. After coalescing from the protoplanetary disk of gases and dust left over from the formation of the Sun 4.5 billion years ago, the Earth’s surface had finally cooled to solid rock 1 billion years later (in between, it got hit by a planet-sized object to form the Moon and went through something called the “iron catastrophe,” which somehow isn’t being used as a band name). At this time, the Eoarchean Era, Earth’s atmosphere consisted solely of gases spewed by the planet’s many active volcanoes, meaning you’d find toxic ammonia and methane, but little free oxygen and no ozone layer to stop the Sun’s radiation. But crucially, an atmospheric pressure 10 times that of today’s allowed liquid water to exist on the scorching hot surface, and that was the key to life.
A paper published last month in Nature Communications presents a new take on how that first stirring of life could have formed. In particular, it looks at how “liquid crystals” could have driven the formation of primitive DNA.
The Earliest Days
Abiogenesis is the term for the spontaneous generation of life out of lifeless materials. Yet though this process has a name, it’s mechanics have been the subject of fierce debate since the time of Aristotle. For many centuries, it was thought that abiogenesis occurred all the time – maggots seemed to appear without cause in rotten meat, for example. Eventually, however, the work of Louis Pasteur and other scientists proved that biogenesis (“life from other life”) is the observable cause of all life on Earth, but that didn’t solve the question of where the first living organism came from.
Two overarching explanations have emerged for the origins of life on Earth – that it started here spontaneously, or that it was “seeded” from somewhere else. The “panspermia” theory, as the latter is called, has some interesting research associated with it, but it isn’t the topic of this blog post. The other is that microscopic life spontaneously formed from Earth’s primordial soup of inorganic, volcanic compounds.
Since the 1980s, the dominant theory on the origins of life has been “the RNA World,” which hypothesizes that all life arose from a primitive form of ribonucleic acid, a single-stranded molecule found in all life and even some non-living viruses. RNA is relatively simple, and most importantly is able to self-replicate from non-living molecules, making it a strong candidate for the first biological reproduction. However, the molecules that comprise RNA, known as nucleotides, are complex, leading many scientists to be skeptical of how enough of them could have formed randomly at the same time to form primitive RNA.
In a paper titled “Abiotic ligation of DNA oligomers templated by their liquid crystal ordering,” Italian and American scientists provide by an answer, by showing how a unique physical process could have driven the assembly of nucleic acids to form RNA and DNA.
Liquid crystals are atoms in a state of matter between solid crystals and liquids. They are truly in the middle – they retain the homogenous alignment of solid crystals, but have the ability to flow like liquids. Most people have encountered them in the form “liquid crystal displays,” or LCDs – screens that use liquid crystal molecules to respond to electrical signals to form a picture.
Organic molecules can also be liquid crystals, and this paper found that very short DNA oligomers, or fragments, start to order themselves randomly in solutions in crystalline structures that could have been the basis for nucleotides, and later DNA and RNA.
“We envision our findings as a paradigm of what could have happened in the prebiotic Earth based on the fundamental and simplifying assumption that the origin of nucleic acids is written in their structure,” the authors wrote in the discussion.