The origin of life on Earth is one of the most difficult scientific questions to investigate. The emergence of living systems from non-living chemistry happened only once, as far as we can tell, 3.5-4.0 billion years ago. It’s a process that likely requires a lot of raw material (on a planetary scale) and a lot of time (up to 500 million years), and so is not something we can replicate in a lab. But life-origin researchers have made lots of progress none-the-less. One important question is where the prebiotic molecules that ultimately lead to life came from. For about a century one hypothesis has been that comets delivered much of this feedstock, in a steady rain over the course of the prebiotic era on Earth. However, a new study now casts doubt on that hypothesis.

Why was this ever a hypothesis in the first place? Why couldn’t all the chemicals necessary to develop into life have existed or been made right here on the Earth? The short answer is – they could, but there are unanswered questions and the cometary hypothesis would have been a convenient way to answer a lot of those questions. Prebiotic molecules must have included molecules with lots of carbon, hydrogen, nitrogen, and oxygen – all elements that would have been abundant on the early Earth. Researchers have demonstrated the plausible formation through chemical reactions of all the necessary components of life, either by creating abiotic synthesis in the lab or demonstrating viable chemical pathways. This includes amino acids, nucleobases, nucleotide precursors, lipids, sugars, and peptides. All the raw material was floating around in the “primordial soup” and could have spontaneously formed into all the building blocks of life. So what’s the problem?

Well, scientists are persnickety and want to dot every “i” and cross every “t”. In this case they want to know as much as possible about the conditions on early Earth, during the era when life would have formed, and exactly how it happened. Specifically, it is not enough that all the chemistry was there. It had to be all there in the same place at the same time and in sufficient concentrations. If the prebiotic chemicals were too dilute, in the world’s oceans, that would not allow for sufficiently active chemical reactions. Further, you need some energy in the system. Once you have life, no problem – critters can make high energy molecules powered by sunlight, and those high energy molecules can power the entire living ecosystem. But what were the high energy molecules of the prebiotic soup and how were they created? Further, what environments allowed for all of the prebiotic building blocks to exist in high concentrations all at the same time?

There are plausible hypotheses for all of these questions, scientists are just trying to work out what the most likely scenarios were and even which ones actually existed on earth. Lightening in lakes and ponds of water could have cause lots of high energy molecules in high concentrations, for example. Geothermal vents could provide minerals, catalysts, heat, and chemical gradients. Shallow pools over clay beds could also provide another environment that could drive many prebiotic reactions. All plausible, but still with lots of details to be worked out.

But this is precisely why the comet hypothesis was interesting – comets could theoretically deliver all the prebiotic molecules necessary for life in a highly concentrated form. We have actually found many prebiotic molecules on comets, so we know they are there. If such comets were raining down on the prebiotic Earth, that may be all that was necessary. This is where the new study comes in. It is mainly a computer simulation of comet impact on the early Earth, using updated knowledge about the dynamics of cometary impacts and the condition of the early Earth’s atmosphere. The simulations try to estimate the angle and speed at which the comets would have hit the atmosphere, and the effects of their impact. They focused on one molecule in particular, HCN – hydrogen cyanide. HCN is a high energy molecule thought to be important to prebiotic chemistry. It has a high energy triple bond between carbon and cyanide that can be used to provide energy for lots of organic reactions. Also, its ability to survive impact is a good marker for other prebiotic feedstock.

What they found is that the vast majority of comet impacts would have been too violent for any significant HCN to survive. Therefore, comet impacts successfully delivering HCN or other prebiotic feedstock would have been rare events, too rare to contribute significantly to prebiotic chemistry on Earth. They cannot rule out any contribution, but their simulations make it very unlikely that comets were the major source of prebiotic feedstock for life on Earth. Incidentally, Mars would have been a more friendly environment for cometary delivery of such molecules. That does not mean it happened, but it is more plausible on Mars than Earth.

Where does that leave us? It’s a single study, a computer simulation, so is likely not the last word. But – if this study’s conclusion is essentially correct, then the cometary hypothesis for prebiotic molecules is likely not true. This means the life-origin researchers need to focus on terrestrial origins for prebiotic molecules. Other extraterrestrial sources, like asteroids, are even less likely than comets.

The study is clarifying, and will help focus future research into life origins.  The overall impact may be small, however, because for decades life origin researchers have already been focusing on terrestrial origins, which has been the major focus of research.