The origin of life on Earth remains one of mankind’s greatest mysteries. While religions tried to explain it through creation myths, scientists hypothesized that life could have originated from inorganic molecules, or from space.
Although there is still no definitive answer, there is evidence that points to a likely scenario. Here are some of the most popular hypotheses for the origin of life on Earth.
Chemical evolution, or abiogenesis
In evolutionary biology, the term “chemical evolution” is used to refer to the hypothesis that says the building blocks of life, that is, amino acids, were formed through the combination of inorganic molecules.
Also called abiogenesis, this is a well-known hypothesis for the origin of life on Earth.
Earth’s primitive atmosphere was quite hostile compared to today’s atmosphere. It was mostly composed of methane, hydrogen, water vapor, and ammonia.
In addition to containing almost no oxygen, the ozone layer that today protects us from deadly radiation from the Sun also did not exist. Consequently, ultraviolet rays were constantly hitting the Earth.
Taking into account that new atoms are only created in the core of stars or during supernovae explosions, all the atoms that exist on Earth today have been recycled for billions of years.
This leads us to two conclusions: either the elements that later gave rise to life were already on Earth when it was formed, or they came from outside, through meteors.
Within the hypothesis that inorganic elements were already on Earth, there are several other hypotheses about where on the planet the chemical evolution could have started.
The primordial soup
The idea that the mixture of gases present in the primitive atmosphere could create amino acids was proposed by scientists Oparin and Haldane in 1924.
They hypothesized that organic molecules could be created from inorganic molecules found on the ocean floor. However, they were unable to prove it.
In 1953, scientists Miller and Urey carried out an experiment that became known as “primordial soup”.
The experiment showed how amino acids could be created using only a few inorganic ingredients, in a controlled environment that mimicked conditions found on the primitive Earth.
Initially, the experiment was a success, yielding several other hypotheses about the composition of life. However, years later it was discovered that some of the elements of the primordial soup were not present in the primitive atmosphere.
Still, the theory was important in showing that organic molecules could be formed from inorganic elements with relative ease.
Considering that the necessary inorganic elements were already on Earth, most theories agree that the transformation of inorganic molecules into organic ones started in the oceans.
The surface of the primitive Earth was also mostly covered by oceans, and the bottom of these oceans was protected from ultraviolet radiation. In addition, on the ocean floor, there are structures known as hydrothermal vents.
According to this hypothesis, these vents could have expelled hydrogen-rich molecules, which ended up accumulating in rocky corners, providing mineral catalysts for the reactions.
Even today, these extremely hot underwater areas are full of primitive life forms.
Life may have started because of lightning
In the Miller-Urey experiment, electrical sparks were used to generate amino acids from inorganic molecules, suggesting that lightning might have helped start life on Earth.
Volcanic clouds in the primitive atmosphere could contain methane, hydrogen, and ammonia. And being stimulated by lightning, these elements could have given rise to the first organic molecules.
A beginning under the ice
Another hypothesis suggests that life may have started under the ice. 3 billion years ago, the Sun was a third less bright than today, so the oceans were covered by ice.
This thick layer of ice could have protected the first organic compounds from ultraviolet radiation and meteor impacts.
The low temperature could also have helped the molecules to survive longer, giving enough time for important reactions to take place.
Panspermia: life from space
The panspermia hypothesis holds that primitive cells and amino acids arrived on Earth through meteors.
Unlike other hypotheses that try to explain how the building blocks of life originated from inorganic molecules, proponents of the panspermia hypothesis argue that life may have formed in space, and only then reached Earth.
This hypothesis would explain not only how life came about, but also how it spread across the globe.
One of the great advocates of this hypothesis was Stephen Hawking, one of the reasons behind his interest in space exploration.
Meteors that fall to Earth are always analyzed. And amino acids are commonly found in them, which reinforces this hypothesis, since these same compounds could have fallen into the primitive oceans, producing simple proteins and essential enzymes for the first prokaryotic cells on Earth.
Which of these hypotheses do you think is correct? Leave your thoughts in the comments below.