Earth formed roughly 4.6 billion years ago, and for several hundred million years the planet’s surface was almost certainly too hot and heavily bombarded by comets and asteroids to be hospitable to any kind of lifeform. About a billion years later, life not only existed, but was also leaving evidence of its presence in the form of fossilized microbial mats.
So, what happened in the interim? How did life spring from non-life over the course of a half billion years or so?
1. Sparked by lightning
Atmospheric conditions at the time life appeared were very different from those that exist now, notes Jim Cleaves, chair of the chemistry department at Howard University and co-author of A Brief History of Creation: Science and the Search for the Origin of Life.
In the 1950s, he explains, Nobel Prize-winning chemist Harold Urey noted that most atmospheres in the solar system are dominated by nitrogen and methane; Urey reasoned that early Earth also had this type of atmosphere, and that the presence of life transformed it into one richer in oxygen. Urey also proposed that this earlier atmosphere “could be very efficient at making organic compounds, which could be the precursor to life,” Cleaves explains.
He tasked his research student Stanley Miller with developing an experiment to test this theory. What would become known as the Miller-Urey experiment created a closed system, in which water was heated and combined with molecules of hydrogen, methane, and ammonia. These were then zapped with electricity (to represent lightning) and cooled to allow the mixture to condense and fall back into the water, like rain.
The results were astonishing.
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Within a week, the experimental “ocean” had turned reddish brown because of the molecules combining to create amino acids, the building blocks of life.
Subsequent research has shown that the planet’s early atmosphere was somewhat different from the experiment created by Miller, and that the main components were nitrogen and carbon dioxide, with hydrogen and methane present in smaller quantities.
The principles espoused by Miller remain broadly sound, however, with lightning combining with asteroid impacts and ultraviolet radiation from the Sun to create hydrogen cyanide, which then reacted with iron brought up by water from Earth’s crust to form chemicals such as sugars. These chemicals may have combined to create strands of ribonucleic acid, or RNA, a key component of life that stores information; at some point, RNA molecules began replicating themselves, and life was possible.
How did these RNA molecules develop into complex cellular structures surrounded by protective membranes?
The key may be coacervates—droplets that contain proteins and nucleic acids and which are able to bind their components together much as cells do, but without the use of membranes; several researchers have hypothesized that such droplets acted as protocells that concentrated early RNA and other organic compounds.
2. Brought to Earth by outer space
Amino acids, as well as some of the other key building blocks of life such as carbon and water, may have been brought to early Earth from outer space, according to another theory. Comets and meteorites have been found to contain some of the same organic building blocks of life, and their early bombardment of Earth may have increased the availability of amino acids.
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According to Nobel Prize-winning chemist Jack Szostak of the University of Chicago, who heads the university’s interdisciplinary Origins of Life initiative, asteroid and comet impacts were almost certainly integral.
He notes that an early atmosphere of nitrogen and carbon dioxide would have been less conducive to some of the proposed chemical reactions that took place in Miller’s concoction of hydrogen, methane and ammonia; but, he explains, a moderate sized impact can create atmospheric hydrogen and methane on a transient basis, allowing for a temporary jolt of compound-creating conditions.
“It’s like having your cake and eating it,” he explains.
3. Hiding in Earth’s oceans
Another theory postulates that life may have begun deep in the ocean, around hydrothermal vents on the sea floor, but Szostak dismisses this hypothesis.
“If you look at the chemistry that takes you from simple starting materials up to nucleotides and RNA, there are multiple steps in there that require UV radiation from the sun to drive the reactions,” he explains. “Energy from the Sun is the is the is by far the biggest source of energy, even on the early planet. So, if there are multiple chemical steps that require UV, you can't be in the deep ocean.”
It is all but certain that life did begin in water, however.
“You need a solvent for the chemical reactions to take place,” Cleaves points out. “You need a liquid. And when you start talking about liquids, only a few are stable under planetary surface conditions. And, even in the early solar system, water happens to be the most abundant.”
Szostak argues that, far more likely than life starting in the deep ocean is that it became established “on the surface, probably in shallow ponds or in a hot springs type of environment: the kind of environment that's very common around impact sites or volcanic regions.” (Indeed, extensive volcanic activity also may very well have contributed to life becoming established, not least by generating enormous amounts of localized lightning.)
It is possible that, although all life on Earth today shares a universal common ancestor, an unknown microbial lifeform that has presumably long since disappeared, life itself may have started on multiple different occasions via several different pathways, only to be snuffed out by cometary impacts or simply fail to gain traction, until the RNA-based molecule that is the ancestor to us all became established.
“It may really have been a roll of a dice,” says Cleaves.
Early life remains mysterious
If that did happen—if life started and fizzled out more than once before it took root—we will almost certainly never know what might have been, as such putative lifeforms have left no trace of their existence.
Life could have followed a very different path, one that would not have led to flowers or trees or dinosaurs or humans. The key to comprehending all of it, says Szostak, is to replace the big picture question with a series of much smaller ones.
“Life is such a complicated system that the simplest bacteria or virus has thousands of moving parts. It's hard to understand how something like that could just appear out of nowhere,” he says. “And the answer is that it didn't. It happened step by step by step.”
