A second key requirement for the emergence of life was the availability of carbon-based organic molecules. During the formation of the Earth from stellar dust particles, some organic compounds used by life would have been present. However, these would have been destroyed by the immense energy created by the planetary collision that led to the formation of the moon, as the collision turned the surface of our young planet into a sea of molten rock that would have ripped apart even the hardiest organic compounds. Molecules that were necessary for the first life were consequently either synthesized on Earth after the collision that formed the moon, or were formed in space before arriving on meteoroids, asteroids and dust particles entering our planet’s early atmosphere.
Evidence of complex organic compounds being generated in space comes from the examination of meteorites, with the most studied of these having crashed into Earth in late September 1969. The meteorite’s impact was observed close to Murchison, Victoria, Australia, and the meteorite was quickly collected before it could become contaminated by molecules from Earth. The Murchison meteorite contained seventy different amino acids (life uses twenty) among the 14,000 molecular compounds so far identified. The Murray and Tagish Lake meteorites have also been studied, but not in quite as much detail, and nucleobases have been discovered on all three rocks. In 2019, scientists even discovered sugars, including ribose, on meteorites. Space rocks can be rich in organic compounds, and some scientists have predicted that the Murchison meteorite contains millions of distinct organic compounds. The Murchison meteorite is also old, and much older than the Earth. Dating of silicon carbide particles found within it revealed it could have formed about 7 billion years ago. What this means is that complex organic compounds, including those life uses to build DNA, RNA and proteins, appear to be easily formed in space and can fall to Earth on particles of space dust or meteorites.
Phosphorus is another element that is key to life, being an ingredient of both DNA and RNA. Although phosphorus is a common element, much of it would have been rather inaccessible to early life, being locked up in rocks and minerals. Life would have needed more easily accessed phosphorus-containing molecules. Some may have arrived on the meteorites that brought water and organic compounds to Earth, but another possibility is that lightning strikes may have also played a role. The atmosphere on Earth around the time life emerged would have featured frequent lightning strikes, and laboratory experiments reveal these could have helped release inaccessible phosphorus, making it available for life in the form of molecules called phosphides, phosphites and hydrophosphites. I suspect that both lightning and meteorites played a role in providing accessible phosphorus for the first organisms, but this key molecule would have been present on the early Earth in a form that life could use.
Your cells have high concentrations of complex organic molecules that contain elements like phosphorus, carbon, oxygen and nitrogen. The chemistry of organic molecules that happens within a living modern-day organism such as you, me, an earthworm, or even a bacterium, is very different from the chemistry that happens in the external environment of today’s Earth. Within each of your cells you are continuously making large, complicated molecules such as proteins. Outside of your body, on the floor, or the table, the chemistry that is occurring is different. The chemical activity in the external environment is much, much simpler than that which is happening in your cells, and it does not involve large organic molecules to anywhere near the same extent.
Scientists describe the external inorganic environment as being at an equilibrium, with your cells being in a state that remains quite stable while you are alive, but which is very different from the external equilibrium of the environment outside of you. The difference between the steady state of cells in living organisms and the equilibrium state of the external environment can be thought of as a distance. The greater the difference between the chemistry in your cells and the chemistry in the external environment, the greater the distance. The distances observed between modern-day cells and the external environment in which they find themselves are much larger than the distances would have been when the first wisps of life emerged. It is physically and chemically impossible to instantaneously create a cell that is as different from the external environment as the cells in you and me are from the outside, non-living bits of our planet. Given this, early life must have evolved within a non-living environment that was rich in the compounds it needed, and as life became more efficient over time, it became better at commandeering these compounds, leading to a greater difference between the chemistry going on in the inside and on the outside of life’s early cells. As life got older, the distance between the chemistry going on within cells and within the external environment would have increased. The next challenge in understanding how life emerged is to understand how the organic molecules, water and phosphorus molecules from which the first life emerged become sufficiently concentrated that a simple cell might arise.
There are several competing hypotheses as to where organic molecules could be sufficiently concentrated for early life to emerge, with volcanoes playing a role in the two that are most strongly favoured by scientists. Life is thought to have evolved in either volcanic ocean vents or in freshwater hydrothermal fields like those found in Yellowstone, Iceland and Japan today. In volcanic oceanic vents, organic compounds are hypothesized to have become concentrated in tiny pores in volcanic rocks. In contrast, in hydrothermal fields on land, repeated wetting and drying events caused by rain rich in organic compounds followed by evaporation or large tides caused by the moon are argued to have created small pools suitable for life to emerge. Currently it appears that life could potentially have evolved in either freshwater or saltwater volcanic environments, with chemists yet to successfully rule either out. As we learn more about complex organic chemistry, emergence-of-life researchers may eventually conclude that life could only arise in one of these environments. Regardless of which environment it arose in, the next step is the start of a complex chemical reaction that is called autocatalytic.
An autocatalytic reaction involves a form of replication. A very simple example of an autocatalytic reaction is adding two compounds – for simplicity, let’s call them A and B – into a beaker in equal amounts, letting the reaction happen and observing only B at the end of it. B is called an autocatalyst, and in this case one molecule of B can convert one molecule of A into another molecule of B. A catalyst is a compound that enables a reaction, and adding ‘auto’ in front of the word means B makes copies of itself if it has the right material to do so – in this case molecules of compound A. We start with one A and one B molecule and end up with two Bs. Perhaps there is some waste product too. The reaction may occur giving out heat, or it may require energy to proceed. Which will depend on the chemical properties of A and B.