Origin of Life Theories

Strands of DNA over the earth.

Getty / Oliver Burston

Scientists from all over the world have studied the origins of life as far back as recorded history spans. While religions relied on creation stories to explain how life on Earth began, science has tried to hypothesize possible ways that inorganic molecules (the building blocks of life) got together to become cells. There are several hypotheses about how life started on Earth that are still being studied today. So far, there is no definitive proof for any of the concepts. However, there is evidence that can point to a likely scenario. Here is a list of common hypotheses about how life on Earth began.

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Hydrothermal Vents

Black Smoker hydrothermal vent in the Pacific Ocean.
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The early atmosphere of the Earth was what we would now consider a quite hostile environment. With little to no oxygen, there was not a protective ozone layer around the Earth like we have now. This means the scorching ultraviolet rays from the Sun could easily reach the surface of the Earth. Most ultraviolet light is now blocked by our ozone layer, which makes it possible for life to inhabit the land. Without the ozone layer, life on land was not possible.

This leads many scientists to conclude that life must have begun in the oceans. Considering most of the Earth is covered in water, this assumption makes sense. It also is not a leap to realize ultraviolet rays can penetrate the shallowest areas of water, so life may have begun somewhere deep in the ocean depths to stay protected from that ultraviolet light.

On the ocean floor, there are areas known as hydrothermal vents. These incredibly hot underwater areas are teeming with very primitive life still to this day. Scientists who believe in the hydrothermal vent theory say these very simple organisms could have been the first forms of life on Earth during the Precambrian Time Span.

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Panspermia Theory

Meteor Shower Heading Toward Earth

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Another consequence of having little to no atmosphere around the Earth is that meteors often entered the Earth's gravitational pull and got sucked into the planet. This still happens in modern times, but our very thick atmosphere and ozone layer help burn the meteors up before they reach the ground and cause damage. However, since those layers of protection did not exist when life was first forming, the meteors that struck the Earth were extremely large and caused a lot of damage.

With the commonality of these large meteor strikes, scientists have hypothesized that some of the meteors that struck the Earth may have been carrying very primitive cells, or at least the building blocks of life. Panspermia Theory does not try to explain how life formed in outer space, but that is beyond the scope of the hypothesis anyway. With the frequency of meteor strikes all over the entire planet, not only could this hypothesis explain where life came from, but also how it was spread out over various geographic areas.

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Primordial Soup

Diagram of primordial soup

 Carny / Wikimedia Commons / CC BY 2.5

In 1953, the Miller-Urey experiment was all the buzz. Commonly referred to as the "primordial soup" concept, scientists showed how the building blocks of life, such as amino acids, could be created with only a few inorganic "ingredients" in a lab setting that was set up to mimic the conditions of the early Earth. Previous scientists, such as Oparin and Haldane, had hypothesized that organic molecules could be created from inorganic molecules that could be found in the early Earth's oxygen lacking atmosphere and oceans. However, they were never able to duplicate the conditions themselves.

Later, as Miller and Urey took on the challenge, they were able to show in a lab setting that using just a few ancient ingredients like water, methane, ammonia, and electricity to simulate lightning strikes. This "primordial soup" was a success and yielded several types of building blocks that make up life. While, at the time, this was a huge discovery and lauded as the answer to how life began on Earth, it was later determined that some of the "ingredients" in the "primordial soup" were in fact not present in the atmosphere as previously thought. However, it was still important to note that organic molecules were made relatively easily out of inorganic pieces and may be how life on Earth began.