The origin of life on Earth is one of the fundamental questions of science. While we are far from a definite answer, several interesting possibilities have emerged over the past decades. One, worked out in more detail in the 1980s, is the role of an "RNA world". The genetic information of higher organisms is stored in the double helix of DNA molecules. But there are closely related molecules, RNA (ribonucleic acid), which play a prominent role in modern cells; notably, they catalyze certain chemical reactions, and are essential for ferrying genetic information around inside the cell, and for the cell to synthesize specific proteins (the "executive orders" of cell government) based on genetic information. For some viruses, storing genetic information does not involve DNA at all; instead, all the information is encoded in virus RNA.
DNA and RNA
The key building blocks of both RNA and DNA are the nucleotides – in both cases, the pattern within the long, long chain of nucleotides determines the information carried by DNA and RNA. For DNA, this is sometimes expressed as a string where each character stands for one of the four possible nucleobases that form the key part of each DNA nucleotide, A for adenine, C for cytosine, T for thymine, and G for guanine (CGATTCACGATTACA...). In RNA molecules, thymine is replaced by Uracil, U. Another difference: While DNA is commonly found as the well-known double-stranded helix, RNA is more versatile in appearance – most common are single strands of RNA, folded in on themselves in what sometimes can become rather complicated shapes.
While RNA is essential for life as we know it, it also has several key properties that make it a good candidate for earlier, more primitive forms of life – before the advent of cells, to say nothing about multicellular organisms. The most important of these is the property to self-replicate – a given piece of RNA can gather the right nucleotides and arrange them into a copy of itself.
An early RNA world
The most promising current scenario for the emergence of life involves the formation of chains of nucleotides in the shape of RNA, self-replicating the emergence of simple cell precursors as fatty acids spontaneously self-assembled into membranes (a reaction that has been observed in the lab), forming primitive bag-like enclosures which allowed more complex chemical reactions to take place in their protected interior. From these simple beginnings, more complex mechanisms evolved, in particular those of DNA replication.
All the transitions of this scenario are, at this time, speculative, and for each step, alternative explanations and models exist – even for the notion of an RNA world preceding the DNA world, there are alternatives. But we live in exciting times, and there is a realistic hope that the next few decades will see standard model for the origin of life established. Progress will require not just imaginative scenarios, but concrete calculations and experiments, showing which evolutionary pathways are feasible and which aren't. Advances involve different areas of research: On the one hand, more and more hypotheses about the transition from pre-life to life have become amenable to experimental tests, as our knowledge of molecular biology increases. On the other hand, there are exciting new developments at the interface of molecular biology and astronomy.
Over the past few decades, astronomers have made considerable progress in understanding how planetary systems form around young stars, and in particular about the evolution history of the Earth and our own solar system. The new results profit from the wave of exoplanet discoveries, and from direct observations of young planetary systems that have only become feasible with the advent of the most recent generation of telescopes. Planet formation models, including models of the evolving chemistry of newborn planetary systems, set the scene for the conditions under which life could have emerged four billion years ago in our own solar system, and how it could have formed in other planetary systems.
Combining astronomy, chemistry and biology
Now, a study by the astronomers and planet formation specialists Ben Pearce, Ralph Pudritz, Dmitry Semenov and Thomas Henning is drawing together astronomy and prebiotic chemistry to shed light on the earliest era of the RNA world: the processes by which short RNA molecules band together to form longer molecules ("polymerization"), which then start to self-replicate in earnest in a later phase of chemical evolution.
Making longer RNA molecules is not easy, and requires certain well-defined conditions. One possible scenario locates the first steps towards life in the vicinity of hydrothermal vents in the deep ocean – fissures in the Earth's crust emitting water heated by the Earth's deeper, hotter layers. But there are questions about how longer polymers could form under these conditions; polymerization would seem to require a cycle of wet and dry conditions, unlikely to occur deep in the ocean. There is also the problem of obtaining a suitable supply of nitrogen in the shapes of molecules like hydrogen cyanide (HCN) or ammonia, necessary for forming the first stages of life as we know it.
The appeal of warm little ponds
An attractive alternative as a likely birthplace of like are "warm little ponds": small, stagnant bodies of water, in which chemicals can concentrate, and react, under much more favorable conditions than in the ocean. Ponds with walls made of clay or other minerals create particularly favorable conditions, which would foster certain kinds of chemical reaction. An important feature of such ponds is the existence of wet-dry cycles. Every so often, such a pond would dry out, concentrating its chemical content even more and allowing bonding to occur between nucleotides. At some later date, the pond would re-fill with water. Such cycles very probably played a role in shaping the chemical reactions in such ponds. The phrase "warm little pond" itself goes back to one of the earliest speculations on the origin of life: an 1871 letter from Charles Darwin to the botanist Joseph Hooker.
Warm little ponds would have been comparatively rare four billion years ago, when oceans dominated Earth's surface even more than they do today, and when continents were just starting to rise, consisting mostly of igneous rocks created from the mantle, such as basalts. Violent volcanic eruptions were commonplace, and the atmosphere was dominated almost completely by volcanic gases. So were could the organic molecules have come from that set off the evolution of the RNA world?
Building blocks from outer space
A plausible answer, perhaps surprisingly, is that the building blocks for emergent life on Earth or similar planets could have come directly from outer space. The disks, made of gas and dust, that surround young stars contain considerable amounts of ammonia (NH3) and hydrogen cyanide (HCN), both of which providing the nitrogen necessary for forming nucleobases. Iced-over dust particles in all but the innermost regions of such disks turn out to be surprisingly effective little chemical laboratories – in fact, experiments in a laboratory setting here on Earth show how molecules collecting on the icy surfaces of such dust grains can be processed into nucleobases when the grains are illuminated by UV light, such as would be the case for young stars.
In these experiments, scientists were able to show how three of the five nucleobases (namely uracil, cytosine, and thymine) formed spontaneously under such conditions. Meteorites are observed to contain considerable amounts of three of the five nucleobases (namely guanine, adenine, and uracil). It has been shown that these nucleobases are synthesized in the interiors of these meteorite’s “parent bodies,” namely large asteroids, during the formation of the solar system.
Meteorites and dust particles as a cosmic delivery service
Back to the warm little ponds – ideal environments for RNA molecules to become more complex, but where do the basic building blocks, the nucleobases in the pond, come from in the first place? The chemistry of the surrounding atmosphere, dominated by carbon dioxide (CO2), nitrogen gas (N2), sulfur dioxide (SO2), and water (H2O), is of no great help. Under the conditions on early Earth ("weakly reducing atmosphere"), even the occasional bout of lightning, as in the famous Miller-Urey experiment on the origins of organic molecules, will not produce a significant amount of nucleobases.
Meteorites falling on Earth, on the other hand, are a much more plausible source. At that time, roughly 4 billion years ago, meteoritic bombardment of Earth was between 100 million and 100 billion times more intense than nowadays, with between a trillion and a quadrillion kilograms of meteoritic material raining down on Earth's surface every year, carrying an estimated 2000 kilograms per year of intact carbon compounds that have survived the cosmic journey intact. In addition, there is a steady flow of interplanetary dust particles making their way to the surface of the Earth directly, carrying along whatever surface chemicals have formed. This much less spectacular arrival is nonetheless quite effective, amounting to an estimated 60 million kilograms of intact carbon compounds per year.
Meteorites seeding warm little ponds: a quantitative study
It's all very well to talk about a scenario such as meteorites or dust particles carrying nucleobases into small ponds, but unless the model is backed by quantitative data, it does not have much more explanatory power than a Just-So Story.
Pearce and his colleagues calculated a detailed model for this scenario. From a reconstructed history of the Moon's impact craters, they derived three possible scenarios for meteoritic bombardments of the Earth – a late bombardment model, with a late onset of intense meteoritic bombardment at about 3.9 billion years before the present, plus two additional models, both setting in around 4.5 billion years before the present, and representing the minimum and maximum amount of meteoric material compatible with the data, respectively.
They then calculated the probability of sizeable carbon-containing (carbonaceous) meteorites “seeding” these ponds. Specifically, these meteorites, which are originally between about 20 and 40 meter in diameter, break up into small pieces as they traverse Earth’s atmosphere. The astronomers calculated the probability of such small pieces landing near a suitably sized warm little pond on Earth (between 1 and 10 meter in diameter), close enough for some of its organic material to enter the pond. (For this calculation, they had to estimate the number of warm little ponds; they did so by assuming a similar prevalence of ponds on landmasses as today, and accounting for the overall smaller landmass area back then, based on models of geological evolution. To be on the safe side, they repeated their calculation both for ten times as many and for one tenth as many ponds.) The result is that thousands of wet little ponds would have been seeded in this way, providing them with building blocks for emergent life.
Simulating what happens in warm little ponds
What happens to the meteorite- or dust-born nucleobases once they have entered the pond? A number of them will be lost: While the pond is filled with water, during one of the wet phases, nucleobases will be dissolved in water (hydrolysis). Some of the water will seep through pores in the basalt base of the pond, taking nucleobases with it, and removing those nucleobases from any further chemical reactions within the pond. During the dry phases, when the pond is dried up and its chemicals deposited as sediments, UV radiation from the Sun will split nucleobases into simpler compounds (photo-disssociation) – unless these nucleobases are protected by sediments on top.
With new nucleobases introduced into ponds at a certain rates, and with various mechanisms for nucleobase loss, it is quite clear that only quantitative modeling can tell whether or not there remained sufficiently many nucleobases in a number of ponds for longer chains of RNA to form. Again, the researchers considered several possibilities for dryer and wetter, hotter and colder conditions on the early Earth. These conditions will also determine how fast or slow nucleobases group together to form RNA chains.
Building block delivery: meteors, not dust grains
The first interesting result to come out of the study is that meteors, not interplanetary dust grains are the main source for nucleobases that survive the various adverse conditions. The reason is simply that the steady deposition by dust grains competes directly with the mechanisms for nucleobase loss, such as seepage and photo-dissociation. Meteorites, on the other hand, will deposit a considerable amount of nucleobases in one go, leading to higher nucleobase concentrations at least for a shorter time.
But, as it turns out, that shorter time is enough for the nucleobases to form longer RNA molecules, and those, in turn, are not lost as readily as their shorter kin. In particular, these larger molecules do not seep away through basalt pores, due to their larger size. That is how longer RNA molecules, once formed, can survive to take part in more complex chemical reactions – and why this happens on the basis of the nucleobases deposited in bulk by meteorites, but not with the steady stream of nucleobases deposited by interplanetary dust particles.
The case for quickly forming life
The deposition model has interesting implications for the timing of the origin of life. Over time, the meteorite infall rate quickly decreases, so there is a comparatively short window of opportunity. Most of the nucleobase delivery by meteorites must have occurred rather early, until about 4.17 billion years before our time. This suggests that the RNA world should have formed rather early, as well, namely 200 to 300 million years after the surface of the Earth had cooled sufficiently to become habitable – that is, after the temperatures had dropped sufficiently for areas of liquid water, such as oceans and lakes, to form on Earth surface.
Again, we are probably a few years off finding a complete, consistent, generally accepted model of how life originated on Earth. The calculation published now by Pearce and his colleagues is one piece of the puzzle – demonstrating that meteorites are likely to play a major role in bringing the building blocks of life to Earth, and that under those circumstances, longer RNA pieces would have formed comparatively early in Earth's history. Overall, the calculations increase the feasibility of the warm little pond scenario, strengthening that scenario’s position in comparison with the competing hydrothermal vent scenario.
But on the path towards a standard model, we need quantitative analyses such as the one describe here – calculations that combine our knowledge about the geology of the early Earth, chemical conditions, properties of the molecules involved, and astronomical information about the properties of meteorites and interplanetary dust, to tell us which of the hypothetical steps from simple chemicals to self-reproducing living cells are feasible and which aren't. It is an exciting feature of current research on the origins of life that thanks to advances in many fields, from microbiology to the search for exoplanets and observations of planetary birthplaces around stars, we are steadily moving away from speculations and into the realm of quantitative analyses.