Some 4.5 billion years ago, the newly formed planet Earth was devoid of animals, plants and bacteria. Yet just a few hundred million years later, the first primitive life forms emerged. How exactly this happened is one of science’s greatest mysteries, but we may be closer than ever to finding out what sparked it. And now researchers are making strides at recreating the process in the lab.
When the planet first formed, conditions on Earth were too inhospitable for life; violent volcanic eruptions spewed hydrogen sulphide into the atmosphere, there was little oxygen, and the planet faced frequent bombardment from asteroids.
Yet we know that as little as 200 million years later, Earth was a much more welcoming place. Fossil records show that the world was brimming with simple single-celled organisms from around 3.7 billion years ago. So how did these first lifeforms get going?
There’s a consensus that for life to exist you need organic, carbon-containing compounds like methane, coupled with water and a source of energy. This spark would kickstart the chemical reactions needed to create more complex molecules, such as amino acids – the building blocks of proteins, and RNA – a nucleic acid present in all living cells with structural similarities to DNA. But what provided the spark, and could we recreate it?
Lightning storms
One idea is that intense ultraviolet radiation and lightning present on the early Earth could have provided the energy for amino acids, and later molecules such as DNA and RNA to form in the oceans.
Support for this theory came in 1952, when University of Chicago graduate student Stanley Miller teamed up with Harold Hey, a Nobel laureate in chemistry, to try to recreate the atmospheric conditions of early Earth. They injected ammonia, methane and water vapour into an enclosed glass container, then passed an electrical spark through the beaker to simulate a lightning strike. Amazingly, amino acids spontaneously formed. However, later research showed that the atmospheric conditions modelled by Miller and Hey were unlikely to have existed on Earth at the time. Another problem is that for four billion years the planet was mostly covered with ice, and lightning rarely strikes in such conditions.
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However Jeffrey Bada, a former student of Miller, and a professor of marine chemistry at the Scripps Institution of Oceanography in San Diego, believes that that lightning could have formed within volcanic ash clouds. There is good reason to think that such conditions produce intense lightning storms. In 2022, the underwater volcano Hunga Tonga–Hunga Haʻapai in the southern Pacific Ocean erupted and spewed a mix of gases, ash, and seawater vapour 33 miles (52.8km) into the atmosphere. The result was a stunning 25,508 lightning strikes in just five minutes.
“There were a lot of small volcanic islands on the early Earth,” argues Bada. “I suspect these volcanoes were erupting quite violently, and there were a lot of them.”
Lightning hitting certain gases in pools may have created the right conditions to create amino acids, the building blocks of protein (Credit: Getty Images)
The volcanoes would have spewed gases like carbon monoxide and hydrogen into the atmosphere. According to Bada, the intense lightning that accompanied eruptions could have provided the spark to convert these gases into amino acids.
In recent study, Bada worked with colleagues at Munich University in Germany to simulate volcanic lightning in apparatus containing carbon monoxide and hydrogen gas.
“I processed the results in my lab and sure enough we found amino acids,” says Bada.
The amino acids would have initially formed in the atmosphere, before falling onto the flanks of volcanoes. Once there they may have washed into little ponds and lakes, where life would have found it easier to propagate.
This echoes earlier arguments life couldn’t have started in the open ocean, as any carbon-based chemicals produced would immediately drift away, and not come close enough to react with other molecules. In shallow pools, however, heat from the Sun would evaporate water, which would concentrate chemicals like hydrogen cyanide together, allowing them to encounter each other more frequently. Researchers have recreated such a process in the lab, successfully creating the three main molecular building blocks of life – DNA, proteins, and lipids – from hydrogen cyanide.
But some scientists remain sceptical about this theory.
“To my mind, the problem with [life] starting in pools is there isn’t an obvious driving force there,” says Nick Lane, professor of evolutionary biochemistry at University College London (UCL).
“The theory is that UV radiation could have energised cyanide molecules, making them react, but it’s questionable how much cyanide there was on early Earth. The earliest forms of life are thought to have grown from hydrogen and carbon dioxide, not cyanide, and the chemical pathways are completely different, so you’re not starting from the same point that life did,” says Lane.
Hydrothermal vents
Lane believes that hydrothermal vents at the bottom of the ocean are a more likely candidate for where life began. These labyrinth-like structures are like a haven away from the open ocean; here hot, mineral-rich fluids bubble up out of little gaps in the Earth’s crust.
Some scientists believe volcanic islands which formed on the early Earth may have incubated life (Credit: Getty Images)
“Hydrothermal vents provide hydrogen in large amounts, and we think the early oceans were rich in CO2, so vents could have been the ideal mixing zone for these chemicals to come together,” says Lane.
When hydrogen reacts with CO2 it forms carboxylic acids. From these you can make chains of fatty acids – a major component of cell membranes – and amino acids.
The pores at the centre of hydrothermal vents could have played a vital role in catalysing the reaction between hydrogen and CO2. According to Lane, they have a structure almost like a cell, with a membrane containing iron sulphur minerals. The outside of the pore is also positively charged relative to the inside, which is known as a proton gradient. This is the same process which occurs in cells.
“That cell like structure effectively breaks down the barrier between the reaction of hydrogen and CO2,” says Lane.
“They are both quite stable gases – they don’t normally react very easily – but the combination of iron sulphur minerals and proton gradients makes them react.”
In the last decade, researchers have begun to demonstrate experimentally that prebiotic chemicals can be formed under hydrothermal conditions. In 2019, Lane and his team at UCL even succeeded at creating simple ‘protocells’ in an environment similar to that of hydrothermal vents.
Lane took a selection of fatty acids and fatty alcohols, which previous research had shown could have formed under hydrothermal conditions, and tried to get them to spontaneously form a rudimentary cell membrane. Amazingly, it worked.
“We got a bilayer membrane and there’s watery stuff inside,” says Lane.
“We found that it was actually easier to do it in conditions similar to hydrothermal vents. You need alkalinity, warm temperatures, and salt water.”
Meteor strikes
Another theory is that falling meteorites could have provided the spark that led to the formation of the first organic compounds. Meteorites contain high levels of metals such as iron, nickel, cobalt and uranium, commonly used as catalysts on Earth. When a meteorite enters the atmosphere it gets heated up, and these metals become oxidised.
“On early Earth we had an atmosphere composed of mostly CO2 and nitrogen, so I thought that CO2 could have become activated under these conditions,” says Oliver Trapp, professor of organic chemistry at the Ludwig Maximilian University of Munich.
Submarine volcanic vents, research suggests, may have been a more likely enviornment for life to develop (Credit: Getty Images)
In a study this year, Trapp’s PhD student Sophia Peters took iron particles taken from meteorites and volcanic ash, and mixed them with various minerals thought to have been present on the early Earth. These minerals serve as a kind support structure, onto which iron particles attach themselves.
Since the Earth’s atmosphere at this stage contained no oxygen, Peters removed almost all of the oxygen from the mixture. She then brought the mixture into a pressure chamber filled mainly with carbon dioxide (CO2) and hydrogen molecules to replicate the high atmospheric pressures on the Earth’s surface.
The experiment worked. Organic compounds including alcohols, acetaldehyde, and formaldehyde formed. Acetaldehyde and formaldehyde are the building blocks of many of life’s most important molecules, including fatty acids, nucleobases, sugars and amino acids.
What’s more, the researchers showed that when you mix together aldehydes with other chemicals thought to have been present in Earth’s early atmosphere, such as cyanide, ammonia and hydrogen sulphide gas, then something very interesting happens.
“We were able to produce organic molecules that are able to directly modify their own structure, and catalyse the production of other, similar molecules” says Trapp.
These molecules, known as organocatalysts, undergo a process of evolution by natural selection, whereby the most ‘successful’ molecules go on to reproduce. According to Trapp they could have played an important role in the emergence of life.
“What we have found is a hidden layer between the first small organic molecules, and later self-replicating compounds like RNA,” says Trapp.
And what of RNA? Today this molecule, found in every cell, plays a key role in turning the instructions held in the DNA of your genome into functional proteins in your cells. However, many scientists believe that on primordial Earth, RNA molecules capable of replicating themselves did many of the jobs that modern cells do, such as catalysing the formation of proteins.
These RNA molecules could have eventually formed the ribosome – a factory present in every cell of the body, which uses the information present in DNA to construct proteins. The ribosome is constructed largely of RNA.
Today oceans teem with life, but vast seas wouldn’t have had the right conditions to kickstart life, sceintists believe (Credit: Getty Images)
In a ground-breaking 2022 experiment, scientists led by Ada Yonath at the Weizmann Institute of Science in Israel managed to create an early primitive version of the ribosome in the lab, showing how it might have arisen on the early Earth.
Yonath won the 2009 Nobel Prize in Chemistry for her work determining the structure of the ribosome. This structure reveals a pocket at the heart of the giant molecule. This pocket is found in the ribosomes of all organisms, from bacteria to humans. And inside is where the amino acids link up to form a protein.
“The pocket allows the amino acids to be positioned in exactly the right place so that a peptide bond can be made,” says Yonath.
“This type of bond can be made spontaneously by two amino acids, but the frequency and efficiency would be ten thousand, a hundred thousand or even a million times less than if there is a pocket.”
In her study, Yonath copied the design of bacterial ribosomes, including the pockets in which protein synthesis occurs. She then prepared these protoribosomes in a laboratory dish. To check whether the primitive ribosomes were capable of producing proteins, the researchers then added a solution containing amino acids, salts and other ingredients. To the team’s delight, the synthetic ribosomes were able to join amino acids together.
“We think that what we did in the lab is analogous, or at least imitating what happened in nature,” says Yonath.
“Initially there would have been a few RNAs that coiled round on themselves and formed a little pocket. From these pieces, a rudimentary, functional ribosome may have arisen.”
The proto ribosomes that were more successful at catalysing bonds between amino acids would have stuck around for longer, and eventually, through a process of natural selection, the ribosome would have been born.
Does this bring us any closer to understanding the origin of life? Well, we now have several possible explanations for how life’s first organic compounds could have formed – the energy could have been provided by a lightning strike, meteorites, or hydrothermal vents.
However, successfully recreating life’s first compounds in the lab could go some way to showing how the process might have taken place somewhere else in the Universe aswell.
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