How life began

Editor’s Note:

I recently discovered the work of Nick Lane, who in my mind is something akin to the David Deutsch of biochemistry. I make that comparison because I haven’t been this excited reading a science book since The Beginning of Infinity; Nick’s ideas about the origins and development of life on earth are so fascinating that to get the swirl of excited thoughts out of my head the next three posts on Cosmographia are going to be devoted to relaying his theories about life, both its origins and subsequent development, and how they might relate to the Fermi Paradox — or why we haven’t detected any alien life yet.

This first post will attempt to relate Lane’s ideas about the origins of life, which are split across a few of his books, into a single (and hopefully compelling) narrative; the second will look at the evolutionary jump from simple prokaryotes to the more complex eukaryotes, and why it took so long; and then the third will attempt to tie these ideas into questions about what this all means for the prospect of alien life out there in the wider universe. I should point out that a lot of this story is not settled science yet, not every scientist agrees with Lane, and many of the details might get disproved in time. But then that’s the beauty of science — our understanding of the world continually evolves as we get closer and closer to the truth. Anyway, that’s enough preamble — I hope you enjoy!

The Voyage of Life, Childhood — Thomas Cole (1842)

Our planet is roughly 4.5 billion years old, or a little less than a third as old as the universe itself. In the early days of our solar system, when the young Sun was beginning to emit a pale, weak light, the accretion disc of cosmic dust and rock that surrounded our star began to coalesce into planets. In the case of the Earth, it seems that two of the primary ingredients in this planetary milieu were a mineral called olivine, and icy planetesimals, or water.

The first geological period in the Earth’s history is called the Hadean aeon, named after the Greek god of the underworld. This gives the impression of a hellish landscape of erupting volcanos and a toxic atmosphere — indeed, this is what I, and I’m sure you, were taught it was like in school. In fact, recent evidence suggests that we should have named the aeon after Poseidon instead; the early Earth was a water world.

Crystals of zirconium silicate, smaller than a grain of sand, survive from this early period in the Earth’s history, some 4.4 billion years ago. The crystals indicate that they were formed at low temperatures and in the presence of water. They show too that the early atmosphere was dominated by oxidised gases — carbon dioxide, water vapour, nitrogen gas, and sulphur dioxide — not the ‘primordial soup’ of methane, ammonia, and hydrogen, which was once postulated to have given rise to life after a few lightning strikes.

It seems likely that the early ‘Earth’ was in fact an ocean, with perhaps a single body of water enveloping the entire planet, punctuated by few, if any, volcanic islands. The picture was far from calm, however.

Back then the Earth spun madly on its axis, with a single day lasting only seventeen hours. The moon, created after a Mars-sized world wrecked into ours, loomed much larger in the sky than it does now, so the tides were colossal. Meanwhile, the atmosphere was a red, smoggy mess, full of dust and particulates, while meteorites, comets, and asteroids tore bright lines through the sky far more often than they do now. Underneath the surface, the ocean, made acidic by vast quantities of dissolved carbon dioxide, roiled and churned, heated as it was from cracks in the crust where magma boiled up from below, and from a forest of submarine volcanoes.

There was another common geological feature in this turbulent underwater world too, one that would prove crucial for life’s beginnings: alkaline hydrothermal vents. Unlike the more familiar ‘black smokers’ — violent, superheated vents spewing acidic fluids - alkaline vents are much more gentle. They form where seawater seeps deep into the ocean floor, reacting with a rock called peridotite. This rock, rich in the mineral olivine I mentioned earlier, undergoes a process called serpentinisation when exposed to water. The reaction produces hydrogen gas and alkaline fluids, which then bubble back up through the ocean floor. As these warm, alkaline fluids meet the cooler, more acidic ocean water, they react to form mineral deposits like iron sulphides, releasing energy in the process.

When left for thousands of years, the vents create towering structures of permeable rock. They look a bit like chimney stacks, but instead of a single hollow flue like an ordinary chimney, they are instead riddled with countless arrays of microscopic chambers and channels, making the rock porous, like a sponge. It just so happened that in the iron-rich ocean of the early Earth, these miniature, protected spaces inside the vents were perfect for forming and concentrating organic molecules — the building blocks of life.

The ‘chimney’ of an alkaline hydrothermal vent, part of the Lost City hydrothermal field in the Atlantic Ocean. These structures can grow to be hundreds of metres tall and can last over a hundred thousand years. CC BY-SA 4.0

The biochemical foundation of all life on our planet is the reaction of hydrogen with carbon dioxide. Most of the time this happens indirectly, and requires energy produced by another biochemical process, like photosynthesis or respiration. But in an alkaline vent in an oxygen-free ocean, like that of the early Earth, this reaction could occur naturally and spontaneously. The raw hydrogen bubbling up within the vents at 60-90°C was catalysed into a reaction with the carbon dioxide dissolved in the seawater by the very structure of the vent itself, which was made from natural catalysts like iron, nickel, and molybdenum sulphides. This reaction created organic molecules directly, while also releasing the substantial amount of energy required to power further organic reactions, creating a positive feedback loop.

Then, because the microscopic chambers of the vent’s interior kept the organic compounds from floating out into the wider ocean, they remained in the ‘organic hatchery’ where they could take advantage of the abundant supply of carbon, hydrogen, and free energy to react again and again, creating bigger and bigger compounds. To risk an oversimplification, the first few reactions created small lego bricks, like formate and methanol, which were then able to be combined into bigger and more complex lego structures — polymers like nucleotides, amino acids, and fatty acids.

Speaking of fatty acids, when concentrated above a certain threshold, as they would if they were collecting in the pores of an alkaline vent, they spontaneously form into cell-like vesicles — a sort of rudimentary cell formation. What’s more, if the vesicles are continuously fed by new fatty acids forming in the vent, upon reaching a certain size they would naturally split in two. This is due to surface-area-to-volume constraints, similar to how a large bubble splits in two if it gets too big. The geochemical reactions occurring spontaneously within the alkaline vents thus gave rise to organic compounds, primitive cell structures, and began a rudimentary form of cell division.

So, within the micropores of our alkaline vents we have the accumulation of ever more complex organics, which have given rise to protocell formations. These are formed by the natural outcome of the physical interactions of the organic compounds. Then, because of the same thermodynamic disequilibrium driving their formation in the first place, further organic synthesis begins to arise within the protocells themselves. This next stage gives rise to the most sophisticated molecules yet, those of the genetic code — RNA, and, eventually, DNA. It’s only now that geochemical selection gives way to natural selection, where genes and proteins begin to compete for survival within vent pores. This process eventually gave rise to sophisticated proteins like ribosomes and the ATP synthase, which are still universally conserved across life to this day.

Geochemistry has thus given rise to biochemistry. The alkaline vent systems provided the hydrogen, carbon, catalysis, protected space, state of thermodynamic disequilibrium, and the energy flux required to form the first simple organic molecules. They then acted as hatcheries for ever more complex molecules, until protocells spontaneously formed from fatty acid accumulations. These cell-like formations continued within themselves the synthesis of organics, becoming increasingly complex over time. Eventually, some of these protocells developed the ability to replicate genetic material and produce proteins — the hallmarks of true cellular life.¹

The most successful of these early lifeforms became what scientists call the Last Universal Common Ancestor, or LUCA for short. LUCA represents not the very first living thing, but rather the organism from which all current life on Earth descended. At this stage, these early cells were still fragile things, unable to leave the tranquil nurseries of the vent systems. It’s unclear how long after life first emerged that they lay clinging to their birthplace, tethered as if with an umbilical cord. But eventually they — we — left. To colonise the outside world, natural selection bifurcated life for the first time.

The two earliest, still extant branches of life seem to have evolved independently within the vents from LUCA: bacteria and archaea. These are both types of single-celled microorganisms, but with distinct cellular structures and metabolic processes. Each developed its own type of cell membrane, DNA replication, and other protein syntheses, and rapidly spread across the world-ocean of the early Earth.

In the four billion years since, the many species of bacteria and archaea have evolved a diverse variety of metabolic pathways, a few grew a couple of extra features, like flagella (tails), as well as a few other bits and pieces. But otherwise they have morphologically remained remarkably unchanged for all that time. In fact, it took two billion years for the next major step in evolution to occur — the emergence of eukaryotic cells. With this the last of the three domains of life had finally appeared, allowing the eventual rise of the ‘baroque complexity’² of plants, animals, protists, and fungi.

This all begs the question, why did this shift to complexity take so long? Find out next time.

1. There’s quite a lot of complexity to the biochemistry that I’ve left out of this explanation to make it as understandable as possible for the general reader. To summarise briefly: the energy needed to form organic molecules and chemical energy carriers came from natural proton gradients. These gradients flowed across the inorganic vent pores, and later across organic membranes. One key energy carrier formed in this environment was likely acetyl phosphate, an early precursor to ATP (the ‘currency’ of energy transfer used in all life today). These energy-rich molecules could then drive the formation of other important biological compounds. I also didn’t mention at all the importance of the reverse Krebs cycle, a metabolic pathway that likely played a key role in early organic synthesis. For a more detailed explanation into all of this I’d suggest reading Lane’s books directly. He’s such a good writer he can make incredibly complex biochemistry understandable (and interesting!), even for the uninitiated.

2. To steal Lane’s phrase.

Notes

Lane, Nick (2010). Life Ascending: The Ten Great Inventions of Evolution

Lane, Nick (2015). The Vital Question: Why is Life the Way it is?

Lane, Nick (2023). Transformer: The Deep Chemistry of Life and Death