By Robert Hazen, George Mason University
The first experiments to understand studying the origin of life were devised and conducted in the early 1950s, at the University of Chicago. They were done by chemist Harold Urey and his graduate student, Stanley Miller. These are often called the Miller–Urey experiments. What were they? What did they prove?
Mimicking Darwin’s ‘Warm Little Pond’
Alexander Oparin’s ocean, referred to as the ‘primordial soup’ and Charles Darwin’s ‘warm little pond’, all posited the requirements of water, energy and chemicals. It also pointed to an experimental strategy, a very simple one, for studying the origin of life. It was on the basis of these ideas, put forth by Oparin and Darwin, that Miller and Urey began their experiments.
They started with glassware. There was a sealed flask partially filled with water, representing a primitive ocean. The rest of the flask was filled with a simple mixture of gases to represent an early atmosphere. Miller and Urey used methane, ammonia, and carbon monoxide; with some hydrogen thrown in. The water was heated—resulting in Darwin’s ‘warm little pond’.
Organic Molecules
They then set off electric sparks, exposing their chemicals to ultraviolet radiation, in order to represent that early atmosphere, and the dynamic, energy-rich environment of the early atmosphere.
At first they started off with clear water, but within a couple of days that water became cloudy, and then it started turning shades of brown and pink, indicating that new molecules, new chemicals, were being produced in that water. When Miller and Urey analyzed that water, they found a complex mixture of organic molecules. They found carbon-based molecules that are the building blocks of life. These were made simply from water and air, with lightning or ultraviolet radiation to trigger the chemical reactions.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Miller’s Findings
Stanley Miller’s first publication was a bombshell. He published A production of amino acids under possible primitive Earth conditions, an article which appeared in the May 15, 1953, issue of the prestigious journal, Science. It was the same year when Watson and Crick discovered the double helix DNA structure and that amino acids are only one of the building blocks, one of the key molecules of life.
Thus, putting the two together, when Miller’s results first came out, the press had a field day. Thinking that scientists were just one step away from making life, they wrote about synthetic life crawling out of test tubes in the laboratory, and so forth. In reality of course, they were very far from the truth. It took many, many years to understand the full significance of the Miller-Urey experiments.
Limitation of the Miller–Urey Model
Thousands of subsequent experiments, during the last four decades, have defined the promise—and also limitations—of this idea, that life arose at the surface of the Earth. Time after time, Miller-Urey experiments have demonstrated the synthesis of life’s most basic building blocks; it’s become a cottage industry in chemistry departments. But as exciting and important as these results are, they are still daunting problems.
First and foremost, the building blocks that Miller and Urey make are only the first step. They’re only the single building blocks; one has to assemble them into polymers, into large arrays.
Living things are made up of much more complex groups of these smaller molecules, and the trouble is, these larger molecules just aren’t stable in an environment of lightning and ultraviolet radiation. The polymers tend to break apart into the individual amino acids. For example, one just can’t form proteins easily. This leads us to a major question: Where did life arise? If life can’t possibly have arisen right at the surface of the ocean, because the environment was too harsh, where else did it?
Hydrothermal Origin-of-life Theory
An alternative hypothesis has arisen in recent years to challenge the Miller and Urey model. It posits that the ‘warm little pond’ on the surface of the ocean is not the only wet, energy-rich environment on Earth. We also have deep hydrothermal zones, especially places on the ocean floor near volcanic vents. Those are places where hot, mineral-rich waters flow through cracks in the rock, an active site which could also be an alternate site for life’s origins.
This theory gained credence in 1977, when an astonishing new series of communities: new ecosystems of life living right on those ridges, right where the volcanic waters were coming out, was discovered by Jack Corliss, an oceanographer, at the University of Oregon. These deep ecosystems were dominated by microbes, a single-celled organisms at the very base of the food chain.
It was these discoveries that prompted Corliss and his co-workers to publish a remarkable and an original idea: that submarine hot springs might actually be the site of life’s origins.
Common Questions about the Miller–Urey Model
Alexander Oparin‘s ocean, referred to as the ‘primordial soup’ and Charles Darwin’s ‘warm little pond’, all posited the requirements of water, energy and chemicals. It also pointed to an experimental strategy, a very simple one, for studying the origin of life.
When Miller and Urey analyzed the water, they found a complex mixture of organic molecules. They found carbon-based molecules that are the building blocks of life. These were made simply from water and air, with lightning or ultraviolet radiation to trigger the chemical reactions.
Jack Corliss and his co-workers published a remarkable and an original idea: that submarine hot springs might actually be the site of life’s origins.
