Miller-Urey Experiment

What was the origin of life on earth? For thousands of years, the prevailing scientific explanation was spontaneous generation—that life was formed from nonliving matter—a theory apparently disproven by Louis Pasteur in 1859. In the 1920s, the Soviet biochemist Alexander Oparin and the British evolutionary biologist J. B. S. Haldane independently hypothesized that conditions on Earth about four billion years ago were such that organic molecules could have been formed from simpler inorganic molecules.

The Miller-Urey experiment was intended to simulate conditions that existed almost four billion years ago, resulting in the production of organic compounds, including amino acids. In the experiment, simple molecules were continuously bombarded with electric sparks that were likened to lightning storms, believed to be common during Earth’s early history.

During the 1950s, scientific curiosity was renewed in studying the origin of life, an interest long held by Harold Urey, a 1934 Nobel laureate. This was the subject of the Miller-Urey Experiment, performed in 1952 by Urey’s graduate student Stanley Miller and reported in 1953. The experimental conditions were intended to simulate those thought to have existed in the Earth’s atmosphere some four billion years ago, as theorized by Oparin in 1924. In the Miller-Urey experiment, a mixture of water and the gases ammonia, methane, and hydrogen, were continuously exposed to electric sparks —intended to simulate lightning storms, which were very common at that earlier time. After one week, organic molecules were produced and, more importantly, 2 percent of the products were amino acids, the building blocks of life. This experiment was initially interpreted as proof that life on Earth could have arisen from simple organic compounds. Moreover, evolutionary biologists generally believe that life today evolved from a common life form.

In subsequent years, this experiment and its results have been subjected to critical analysis, and a number of challenges have been made about its validity, results, and conclusions. Questions have been raised about the similarity between the compounds found in the early atmosphere and those in the experiment, and whether these chemicals were not exposed to far more electrical energy than would have occurred at that time. One of the most telling criticisms revolved about whether the amino acids on our early planet were not brought to Earth from an extraterrestrial source. In 1969, a meteorite struck earth in Murchison, Australia, and was found to contain more than ninety amino acids. The search for life outside our planet continues.