1. The Basic Chemicals of Life
In the 1920s, scientists Aleksandr I. Oparin and John B. S. Haldane suggested that Earth’s early oceans contained large amounts of organic molecules.
They proposed these molecules formed spontaneously in chemical reactions that were activated by energy from the sun, volcanic eruptions, and lightning.

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Oparin and other scientists hypothesized that the early atmosphere was rich in hydrogen gas (H2) and hydrogen-containing gases, such as water vapor (H2O), ammonia (NH3), and methane (CH4).
They thought that if the atmosphere lacked oxygen, a variety of organic compounds made up of these elements could form.
The hypothesis was tested in the 1950s by American scientists Harold Urey and Stanley Miller.

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The Miller-Urey Experiment
In this experiment, Miller placed hydrogen-containing gases into a device.
He used electrical sparks to simulate lightning.
After a few days, Miller found organic molecules in his device, including some of life’s basic building blocks: amino acids, fatty acids, and other hydrocarbons (molecules made of carbon and hydrogen).

4. Miller-Urey Experiment

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The Miller-Urey experiment showed that, under certain conditions, organic molecules could form from inorganic molecules.
We now know that the molecules used in Miller’s experiment could not have existed in abundance on early Earth.
Four billion years ago, shortly after it formed, Earth did not have a protective ozone layer.

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Ultraviolet radiation from the sun would have destroyed any methane and ammonia in the atmosphere.
When ammonia and methane gases were absent from the Miller-Urey experiment, key biological molecules were not made.
However, the experiment shows that complex biological compounds can form from simple building blocks.

7. Life’s Building Blocks
One hypothesis that addresses the origin of life states that early biological molecules formed close to hydrothermal vents.
The heat from hydrothermal vents could have provided energy for chemical reactions.
Within the sea, biological molecules would have been protected from harmful solar radiation.

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Another hypothesis that addresses the origin of life states that organic molecules arrived on Earth in meteorites or comets.
Some meteorites contain amino acids.
Such molecules could have arrived on early Earth, when frequent meteorite impacts were common.

9. The First Cells
Research continues to discover how biological molecules grouped together to form cells.
Scientists must figure out how amino acids first linked together to form proteins.
Forming a Cell
Scientists have conducted research to determine how molecules became packaged together inside a cell membrane.

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When studying the behavior of organic molecules in water, scientists have observed that lipids tend to combine in water.
Certain lipids, when combined with other molecules, can form a tiny droplet that has a surface that resembles a cell membrane.
Further research has shown that short chains of amino acids can form tiny spherical structures called microspheres.

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Many scientists think that the formation of microspheres may have been the first step toward cellular organization.
Microspheres could not be considered cells unless they had the characteristic of heredity.
Origins of Heredity
Scientists have studied the origins of heredity by studying the formation of proteins.

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In the laboratory, scientists have not been able to make proteins or DNA form spontaneously in water.
They have been able to form short chains of RNA, the nucleic acid that helps to carry out the instructions of DNA.
RNA molecules may have been the first self-replicating molecule.

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In the 1980s, American scientists Thomas Cech and Sidney Altman found a certain type of RNA molecule, called a ribozyme, that can act like an enzyme.
They also showed that RNA can form spontaneously in water, without DNA.
They hypothesized that RNA was the first self-replicating molecule that stored information and catalyzed the formation of the first proteins.

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One idea of how RNA could have been involved in protein synthesis is shown here.

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Cech and Altman further hypothesized that RNA could have changed—evolved—from one generation to the next.
Scientists hypothesize that DNA and proteins eventually took over these roles in the cell.

16. Summary
The Miller-Urey experiment showed that, under certain circumstances, organic compounds could form from inorganic molecules.
Among the scientific theories that address the origin of life, one suggests that life began close to hydrothermal vents, and another proposes that organic molecules arrived on early Earth from a meteorite.
The formation of microspheres might have been the first step toward cellular organization.