Until 1859, it had always been accepted that life on Earth had always been around, unchanged and unchanging. Until the publication of a single book that year turned everything on its head, and changed the theological and scientific worlds forever: Charles Darwin’s On the Origin of Species. (The full title is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.) This book presented evidence, drawn from Darwin’s own studies and observations, that all species descended from common ancestors, adapted to their particular environments via evolution through natural selection. This is the theory of evolution.

This was a major breakthrough in biological science, but didn’t address the obvious question of how the very first species — the most common ancestor of all — itself came to be.

There is no doubt Darwin had pondered this very question himself. In a private 1871 letter he wrote to his botanist friend Joseph Dalton Hooker:

It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present.— But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter w^d be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.

In other words, could the hydrosphere (’warm little pond’) and the atmosphere (’light, heat, electricity’) form the beginnings of a biosphere (’a protein compound’)?

Darwin’s thinking may have been inspired by the German chemists Friedrich Wöhler and Adolph Kolbe. In 1829, Wöhler had synthesised urea, a compound found in life, from ammonium cyanate, a compound never found in life. This one experiment was enough to disprove ‘vitalism’, or the theory that only living organisms could produce organic compounds via a ‘vital’ force. A later experiment by Kolbe in 1845 eliminated any remaining beliefs in vitalism, when he synthesised acetic acid (vinegar) from carbon disulfide.

It would be another half-century before the thoughts Darwin shared privately with his friend independently entered the public domain in the 1920s, not once, but twice!

In 1924, the Russian biochemist Aleksandr Ivanovich Oparin published The Origin of Life (Происхождение жизни, Proiskhozhdeniye zhizny), in which he advanced a theory similar to Darwin’s: that life arose gradually from inorganic molecules in early Earth’s primaeval ocean. Five years later, in 1929, the British geneticist John Burdon Sanderson Haldane independently proposed very similar ideas and also wrote a paper titled The Origin of Life.

(The same idea or invention originating from multiple sources at the same time and independently is not unprecedented in scientific, mathematical and engineering history — the invention of calculus and even the theory of evolution itself are two other examples. Cultural and language barriers of the time, and slow communication methods, hindered the sharing of scientific ideas and research amongst nations — Oparin’s 1924 work was not published in English until 1936, for example.)

Oparin’s and Haldane’s theories are jointly known as the Oparin-Haldane hypothesis, also known as abiogenesis, the origin of life from non-living matter.

Both men suggested that early Earth’s reducing (as opposed to today’s oxidising) atmosphere, together with an energy source such as lightning and/or ultraviolet light, could synthesise, in water, a wide range of organic (carbon-containing) compounds from inorganic molecules.

Early Earth’s atmosphere was devoid of oxygen gas, but was believed to be rich in methane (CH₄), ammonia (NH₃), hydrogen gas (H₂) and water vapour (H₂O). (It is now believed that carbon dioxide (CO₂) and nitrogen gas (N₂) were more likely to be present than methane and ammonia respectively.) Lightning was a very regular event back then, and a lack of ozone in the atmosphere would have allowed more ultraviolet light to reach the Earth’s surface.

With lightning and/or ultraviolet light inputting energy into this ‘primordial soup’ and initiating chemical reactions, methane (or carbon dioxide) could theoretically have provided the carbon atoms, ammonia (or nitrogen gas) the nitrogen atoms, water the oxygen atoms, and all four molecules the hydrogen atoms needed to form the basic organic molecules that could have bootstrapped very early life. To this very day all amino acids (the building blocks of proteins), carbohydrates and lipids contain carbon, hydrogen and oxygen, and all amino acids additionally contain nitrogen.

It was further suggested that these early organic molecules would have further reacted to produce still more and more complex molecules over time. Some of these were theorised to form coacervates (microscopically self-forming spherical clusters of lipids), which in turn absorbed other organic molecules. Such structures when observed under a microscope can be observed to grow, change shape, take in molecules from surrounding water, and even divide in two, just like modern cells today. The Oparin-Haldane hypothesis relies heavily on the formation of coacervates, as their structure and behaviour makes them strong contenders as the precursors of all modern cells, which are actual self-forming rounded lipid membranes enclosing complex organic molecules.

In 1952 Stanley Miller set up his now famous experiment of subjecting a circulating mixture of methane, ammonia, hydrogen gas and water vapour to electrical sparks to simulate lightning. The water vapour was allowed to condense in a trap, and became pink in colour after one day, and deep red after a week. This solution when analysed was found to contain amino acids — this was an amazing discovery.

This classic experiment has since been criticised, with a major criticism being that while amino acids formed, the means by which they could assemble into peptides and proteins did not. Another criticism concerns the composition of gases which were believed to have been present in the early atmosphere, but which later turned out not to be accurate. However, these criticisms miss the point  completely — that organic molecules found in life as we know it could be generated abiotically from inorganic molecules at all. This experiment was deemed worthy of publication by Science in 1953, the prestigious journal second only to Nature.

At the time of Miller’s experiment, proteins (made of amino acids) were believed to be the genes and thus carriers of genetic material. It wasn’t until Alfred Hershey and Martha Chase showed conclusively that same year in 1952 that deoxyribonucleic acid (DNA) was the component of genetic material that decades of debate ended and investigations into how genes functioned could began.

The double-helix structure of DNA was determined the following year in 1953 by Francis Crick and James Watson, who drew heavily on the x-ray crystallography work of Rosalind Franklin they had seen without her knowledge, and whom neither credited. (Franklin may well have been the one to determine its structure had this not happened.)

The ground-breaking discovery of DNA’s structure and function brought home the realisation that the origins of life were much more involved than previously thought — generating simple amino acids and surmising lipid cells was one thing, but how on Earth (literally) could such an incredibly complex, self-replicating molecule come to be?

Another theory, Pseudo-panspermia (that the ’seeds’ of life on Earth came from space), had supporting evidence arrive in 1969 with the arrival of a meteorite in Murchison, Victoria, Australia. The Murchison Meteorite contained 7 billion year old silicon carbide particles (older than our 4.5 billion year old Earth), and, incredibly, over 70 amino acids. Was it possible that Earth’s primordial soup was seeded from space as well as, or instead of, incubating organic molecules from simple compounds? Perhaps it was both — one random ancient rock from the vastness of space, randomly striking Earth of all places, and bearing (some) amino acids found here on Earth, does suggest that the existence of organic molecules isn’t all that unusual.

In 1977 Jack Corliss and his team discovered an abundance of bacteria, giant tube worms, large clams, and other life inhabiting undersea volcanoes and deep sea hydrothermal vents, where hot, chemically-rich water was pumped through holes in the rocks of the sea floor. In 1981 he suggested that such vents could have existed closer to the surface of early Earth 4 billion years ago, in contact with the atmosphere and where ultraviolet light could penetrate, and proposed that these vents could have been where life originated.

While not definitive, the biochemistry of the organisms which inhabit hydrothermal vents has revealed supporting evidence for origins of life in these environments.

Noteworthy is the concentration of metals such as iron, magnesium, manganese, zinc and copper around hydrothermal vents. These elements could well have acted as catalysts in reactions between early organic molecules, just as they act as catalysts in biochemical reactions today (which is the reason we need these ‘trace elements’ in our diets). For example, iron is at the core of haemoglobin, magnesium is at the core of chlorophyll, and manganese, zinc, copper and other metals drive many enzymatic reactions in cells. Organic molecules in the primordial ocean of early Earth may well have reacted with each other in the presence of metal catalysts to form more complex organic molecules, and so on.

Today, energy continually enters the ecosystems on the Earth’s surface via sunlight through photosynthesis, whereby light photons split the chemical bonds in water molecules to release that bond energy. But sunlight cannot reach the deep ocean floor, and here, energy enters those ecosystems instead via microbes breaking the bonds of chemical compounds continuously emitted by the vents. The fact that this happens at all gives insights into early life.

These chemical bond-breaking microbes are chemoautotrophs (from the Ancient Greek words χημεία, chemo, ‘chemical’; αὐτός, autós, ‘self’; and τροφή, trophḗ, ‘nourishment’: self-nourishment from chemicals), and the chemicals from which they extract energy include inorganic ones that were known to exist in early Earth’s atmosphere, such as ammonia and hydrogen gas. Chemoautotrophs also synthesise their own organic molecules from carbon dioxide, another gas believed to be in the atmosphere of early Earth. It is quite conceivable that these microbes — or their ancestors — were amongst the first lifeforms of early Earth, when surface conditions were more like those of the vents around which they are found today.

Other discoveries based on the chemical environment of underwater volcanoes and hydrothermal vents, assuming the presence of ultraviolet light and metal catalysts, include the synthesis of two RNA nucleotides and lipid and amino acid precursors.

If those lipid precursors became lipids that self-assembled into early coacervates, which absorbed other organic molecules and provided an enclosed environment for further reactions to occur, these well could be the beginnings of life. For if those enclosed reactions were able to produce energy that both fed (sustained) the coacervate and built molecules that enabled the coacervate to grow (more lipids for its membrane perhaps), that coacervate would be metabolising. And if that coacervate grew such that its volume was too much for its lipid membrane, and the lipid membrane had to take on more lipid molecules so as to divide into two new coacervates, then the original coacervate would be reproducing. Metabolism, growth and reproduction are three characteristics that separate life from non-life.

Such coacervates would be cell precursors rather than actual living cells as we’d know these today. All living cells today, from bacteria, protozoa and fungi, to plants and animals, contain DNA and RNA for reproduction, and the means by which these molecules could have arisen is still not known.

Until the conditions that give rise to an actual living cell with fully functional DNA and RNA are known, we cannot know with certainty that life can fully arise abiogenetically. Nonetheless, molecules essential to life have been synthesised from non-life, and coacervates have been observed to grow and divide of their own accord. These alone suggest that it is at least plausible that the biosphere arose from the atmosphere and hydrosphere of early Earth.

This is reinforced with knowledge that the biosphere is the youngest of the three, dating from around 3.5 billion years ago, with the hydrosphere about 4 billion years old and the atmosphere (though much different to today’s) as old as the Earth, about 4.5 billion years old.

First an atmosphere, with its soup of simple molecules and the four elements needed to build organic molecules: carbon, hydrogen, oxygen and nitrogen.

Then a hydrosphere forming as the Earth cools, containing not only its own soup of metal catalysts and inorganic compounds, but also providing the aqueous medium in which chemical reactions can occur.

With these two in place, all that was then required was time — here, half a billion years or so — for the right molecules to meet up under the right circumstances, react, and set in motion a long process by which the first truly living, self-sustaining, reproducing cell could arise. And from that first living cell, the beginnings of a biosphere.