Life is a very complicated form of an autocatalytic reaction. For example, we could replace molecule A with prey and molecule B with a predator. A lion eats zebras and uses the energy from the zebra’s flesh and bones to produce lion cubs. Lions will continue to do this until zebras (or other prey) become scarce. I am not the first person to notice this: the equations that chemists use to consider one type of autocatalytic reaction are identical to the equations that ecologists use to describe how predators and prey interact.
We don’t know what the autocatalytic reaction was that was the key to life getting started. Perhaps it involved a number of chemical steps which resulted in molecules of nucleobases and amino acids making copies of themselves, a bit like DNA and RNA do today, but in a simpler way. Some scientists have argued that RNA evolved before DNA, and that RNA was involved in an early form of autocatalytic reaction. The RNA-first hypothesis, as it is called, is attractive in that RNA encodes information much as DNA does, but it is more reactive and can act as a catalyst in a way that DNA cannot. DNA is stable and non-reactive. Arguments against the RNA-first world are that each RNA molecule is not particularly stable, and replication errors may have been too high for autocatalytic reactions involving it to persist for long. Regardless of the details of the first autocatalytic reaction, it would have had three properties. First, some of the chemicals involved would have become locally abundant. Second, at least one of the chemicals involved could have taken a variety of forms, mutating from one form to another – replication would not have always been perfect. Third, the various forms would have competed against one another, with the superior form winning.
Any compound that can make copies of itself can spread fast. Autocatalytic reactions repeatedly double the number of self-replicating molecules, as long as conditions allow. One molecule makes two, two molecules make four, and so on. The astonishing power of such exponential growth is hard to comprehend, but the numerical consequences of continuing to double numbers is nicely illustrated by an old Indian proverb.
A rich king of old was a keen and competent chess player who liked to play strangers he encountered during his travels. One day he encountered a sage who had played chess all his life. The king challenged the sage to a game, and to motivate him he offered him a prize. The sage said that if he won, he would like the king to put one grain of rice in one of the corner squares on the chessboard, two grains on the next square, four grains on the next, and on each new square to double the number of grains of rice that were on the previous one until all sixty-four squares were full. The king agreed, and subsequently lost the game.
When trying to pay the sage his due, the king quickly realized his folly. By square 21, the king had already handed the sage over a million grains of rice. By square 64, the king would have needed over 200 billion tons of rice, covering all of India in a 1-metre-thick ricey blanket. Quite why the sage wanted so much rice is not explained, but the proverb shows how powerful a force exponential growth can be. In just ten cycles of an autocatalytic reaction, one molecule can produce 512 copies of itself, with over a million copies produced by cycle 20. Such staggering growth cannot continue indefinitely. It ceases when any required resource becomes unavailable. In an autocatalytic chemical reaction, a key chemical will become used up.
The first autocatalytic chemical reaction on Earth that was the precursor to life was not itself alive. It had to become more complex, and involve membranes and other compounds before it could be counted as living. In order to take these steps, imperfect replication was required. The reaction did not always result in identical copies of key molecules being made all the time. On occasions the new molecules being formed would be imperfect copies of the parent molecule. Some of these new versions would have been better autocatalysts than their predecessors, being more effective at commandeering the chemicals required to run the reaction, being able to use a new chemical or energy source, or being more energy-efficient. Each new autocatalyst that was better than its less effective ancestor would spread faster, outcompeting it and eventually driving it extinct. The generation of new variants through imperfect replication, followed by competition for resources, is the basis of evolution. Evolution works for autocatalytic chemicals as well as living organisms. It also works for viruses. Most scientists do not consider viruses as being alive, but they are able to invade living cells to make copies of themselves.
We all became familiar with imperfect replication, exponential growth and the power of competition during the COVID pandemic. Despite politically motivated statements by parts of the US government and conspiracy theorists, Sars-Cov-2, the virus that causes COVID, originated in wild animals, and most likely in bats. The first variant that emerged in China was called the L-strain, and this mutated to form the S, V and G strains. A mutation occurs when the genetic code is not properly replicated such that the genome of the initial virus particle, or virion, differs from that of the copy. Most of us paid little attention to these mutations, but in September 2020 we all listened when scientists announced the alpha variant that arose in the UK and quickly started spreading around the world. The Beta and Gamma variants subsequently emerged in South Africa and Brazil, but they made few ripples. In 2021 the Delta variant arose in India and spread across the globe, replacing other variants. In turn Delta was replaced by Omicron variants.
Viruses consist of genetic code wrapped up in a protein coat. Sars-Cov-2’s genetic code is written in RNA, and the protein coat includes a spike that is key to the virus infecting our cells. When a virion breaks through a host’s cell membranes, the RNA uses the cell’s machinery to make new copies of the virus. As viruses make copies of themselves, they often make the host ill and sometimes even kill it. Humans became a host – a resource – for Sars-Cov-2, and each new variant arose from an existing virus through imperfect replication. Mutations to the virus’s genetic code occasionally occurred when the host cell failed to make a perfect copy. Some of these mutations resulted in new spike proteins, the part of the virus that helped it enter host cells, and some of these new spike proteins made the virus better at infecting new host cells. Other mutations would have made the virus useless and unable to infect new host cells, but these strains were destined to become rapidly extinct and we knew nothing of them.
Each successful strain was replaced by a new one that could spread around the globe because it was better at infecting people than the last strain. Humans were the resource these variants were fighting over, and the Omicron variants were best at being transmitted between people and at making copies of themselves. Omicron, Delta, Beta and Alpha all grew exponentially, but Omicron was able to do it fastest because it was better at infecting people. It commandeered its resource – us – at a faster rate than its less competitive ancestors. Early autocatalytic compounds on the early Earth would have done the same thing with the compounds they needed to make copies of themselves. The species of molecules that could most rapidly use available chemicals to make copies of themselves would displace the less competitive types. Competition coupled with accurate, but occasionally imperfect replication means the most efficient type wins out.
Most research on autocatalytic reactions focuses on relatively simple cases involving only a few chemicals involved in a cyclic chain. The reactions that began in the environment where life arose were probably more complicated than this. I imagine a soup of large numbers of different, complex organic molecules concentrated in either a marine or freshwater environment. In this soup, networks of reactions took place. More specifically, there would have been an autocatalytic reaction involving a handful of chemicals. We can think of these reactions as being analogous to the replicating germline of modern-day life. However, other chemical products produced by this core reaction could have facilitated the autocatalytic reaction, playing a role analogous to that of the disposable soma. If this speculation is correct, separation between the germline and the disposable soma may have been a feature of precursors of early life. However, for such a complex network of reactions built around an autocatalytic core to become life, it would need to be enclosed in a membrane. Where did membranes come from?