Life on Earth is believed to have originated around 4 billion years ago, but to this day, scientists still lack precise evidence regarding the origins that spurred life to “sprout” on this planet.
Earth formed approximately 4.6 billion years ago, and for several hundred million years afterward, the planet’s surface was too hot and frequently bombarded by comets and asteroids, preventing any form of life from developing.
The origins of life on Earth remain an unanswered question. (Source: Ames Research Center, NASA).
However, about a billion years later, life not only existed but also left evidence of its presence in the form of fossilized microbial mats.
So, what happened during that time that allowed life to emerge? Scientists have proposed several hypotheses regarding how life appeared under such harsh conditions.
1. Life Originated from Lightning Strikes
Jim Cleaves, Chair of the Chemistry Department at Howard University and co-author of the book “A Brief History of Creation: Science and the Search for the Origin of Life,” notes that the atmospheric conditions at the time life emerged were vastly different from today’s conditions.
He explains that in the 1950s, Nobel Prize-winning chemist Harold Urey pointed out that the majority of the atmosphere in the Solar System was dominated by nitrogen and methane.
Urey suggested that the primordial Earth also had this type of atmosphere, which could have set the stage for the creation of organic compounds, potentially the precursor to life.
Cleaves tasked his student Stanley Miller with developing an experiment to test this theory, creating a closed system in which water was heated and combined with hydrogen, methane, and ammonia molecules.
Then, a chemical reaction was triggered by electricity (symbolizing lightning), and the mixture was cooled to condense and fall back, similar to rain.
The results were astonishing. Within a week, the “ocean” in the experiment turned reddish-brown due to molecules combining to form amino acids, the building blocks of life.
The origins of life on Earth may stem from multiple causes. (Source: National Geographic).
Subsequent research indicated that the initial atmosphere of Earth was slightly different from Miller’s experiment, with nitrogen and carbon dioxide being the primary components, while hydrogen and methane were present in smaller amounts.
Nevertheless, the principles employed by Miller in his experiment remain valid, with lightning combining with meteorite impacts and ultraviolet radiation from the Sun to produce hydrogen cyanide, which then reacted with iron brought up from Earth’s crust by water to form chemicals such as sugars.
These chemicals could combine to create strands of ribonucleic acid, or RNA, a crucial component of life that helps store information; at some point, RNA molecules began to self-replicate, and life could exist.
2. Life Initially Brought to Earth from Outer Space
According to another theory, amino acids, along with several other essential components of life such as carbon and water, may have been delivered to primordial Earth from outer space.
Comets and meteorites containing some organic molecules have been discovered, and their impacts with Earth could have increased the availability of amino acids.
Nobel Prize-winning chemist Jack Szostak from the University of Chicago asserts that asteroid and comet impacts were almost certainly indispensable.
He notes that the initial atmosphere containing nitrogen and carbon dioxide would have been less conducive to some chemical reactions during Miller’s process of combining hydrogen, methane, and ammonia; however, a moderate impact could temporarily create hydrogen and methane in the atmosphere, allowing for a temporary alteration in the conditions necessary for compound formation suitable for life.
3. Life Concealed in Earth’s Oceans
Another hypothesis suggests that life may have begun deep beneath the oceans, around hydrothermal vents on the ocean floor, but Szostak dismisses this idea.
According to him, many chemical reactions required to produce nucleotides and RNA—the factors that form life—need catalysis from ultraviolet radiation from the Sun.
Thus, life could not have originated from the deep ocean, where sunlight cannot penetrate. However, it is almost certain that life began in water.
Szostak argues that it is more likely that life formed “on the surface, possibly in shallow ponds or hot springs: types of environments commonly found around meteorite impact sites or volcanic regions.”
So far, Earth is one of the rare planets where life has emerged in the universe. (Source: Earth)
It is possible that all life on Earth today shares a common ancestor, a form of microbial life that may have long since vanished, but life itself may have begun at various times through different pathways, being wiped out by impacts from comets or simply unable to adapt, until the RNA-based molecules that are our ancestors formed.
The Origins of Early Life Remain a Mystery
If that truly happened—if life began and perished multiple times before taking root—we are almost certain to never know what transpired, as such hypothetical life forms left no trace of their existence.
Life may have followed a very different evolutionary path, one that would not lead to the formation of trees, dinosaurs, or humans.
This scientist remarks: “Life is such a complex system that even the simplest bacteria or viruses have thousands of parts. It is difficult to understand how such a complex process could arise unexpectedly. And the answer is, it does not. It happens step by step.”
