Science, Tech, Math › Animals & Nature Origin of Life Theories Share Flipboard Email Print Oliver Burston/Getty Images Animals & Nature Evolution History Of Life On Earth Human Evolution Natural Selection Evolution Scientists The Evidence For Evolution Resources Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Dinosaurs View More By Heather Scoville Science Expert M.A., Technological Teaching and Learning, Ashford University B.A., Biochemistry and Molecular Biology, Cornell University Heather Scoville is a former medical researcher and current high school science teacher who writes science curriculum for online science courses. our editorial process Heather Scoville Updated July 10, 2019 While religions have relied on creation stories to explain how life on Earth began, scientists have tried to hypothesize possible ways that inorganic molecules (the building blocks of life) joined together to form living 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 theories. However, there is strong evidence for several scenarios. 01 of 03 Hydrothermal Vents Ralph White / Getty Images 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 where it would have been 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 to this day. Scientists who believe in the hydrothermal vent theory argue that these very simple organisms could have been the first forms of life on Earth. 02 of 03 Panspermia Theory Adastra / Getty Images Another consequence of having little to no atmosphere around the Earth is that meteors often entered the Earth's gravitational pull and crashed 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 great damage. Because of these large meteor strikes, scientists have hypothesized that some of the meteors that struck the Earth may have carried very primitive cells, or at least the building blocks of life. Panspermia theory does not try to explain how life began in outer space; that is beyond the scope of the hypothesis. With the frequency of meteor strikes all over the planet, not only could this hypothesis explain where life came from, but it could also explain how life spread out over various geographic areas. 03 of 03 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 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 atmosphere of the young Earth. 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 such as water, methane, ammonia, and electricity to simulate lightning strikes—a combination of materials they called the "primordial soup"—they could generate several of the 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 of early Earth. However, it was still important to note that organic molecules were made relatively easily out of inorganic pieces, and this process may have played a role in the development of life on Earth.