By 1950 scientists had used a va riety of methods to determine that the earth was 4.6 billion years old. However, the oldest fossil records of even tiny bacterial cells were no older than 3.5 billion years. That meant that Earth had spun through space for over a billion years as a lifeless planet before life suddenly emerged and spread across the globe.
How, then, did life start? Most agreed that life had to have emerged from inorganic chemicals. While this theory made sense, no one was sure if it could really have happened. Through the late 1940s Harold Urey, a chemist at the University of Chicago, teamed with astronomers and cosmologists to try to determine what Earth's early environment looked like. They determined that Earth's early atmosphere would chemically resem ble the rest of the universe, 90 percent hydrogen, 9 percent helium, with the final 1 percent made up of oxygen, carbon, nitrogen, neon, sulfur, silicon, iron, and argon. Of these helium, argon, and neon don't re act with other elements to form compounds.Through experiments, Urey determined that the remaining elements, in their likely composition in Earth's early atmosphere, would have combined to form water, methane, ammonia, and hydrogen sulfide.
Enter Stanley Miller. In 1952 this 32-year-old chemist decided to test the prevailing theory and see if life could be produced from Urey's mix of chemical compounds. Miller carefully sterilized long sections of glass tubing, flasks, and beakers. He built what looked like a sprawling erector set of support poles in his lab and clamped flasks, beakers, and connect ing glass tubes to this structure. He filled one large beaker with sterilized water. He filled other flasks with the three gasses Urey had identified as part of Earth's early atmosphere-methane, ammonia, and hydrogen sulfide. Miller slowly boiled the beaker of water so that water vapor would rise into his enclosed “atmosphere” of a labyrinth of glass tubes and beakers. There it mixed with the three other gasses in swirling clouds in a beaker labeled “atmosphere.”
Miller realized he needed an energy source to start his life-creating chemical reaction. Since other scientists had determined that the early at mosphere contained almost continual rolling thunder and lightning storms, Miller decided to create artificial lightning in his atmosphere. He hooked a battery to two electrodes and zapped light ning bolts across the“atmosphere” chamber. A glass pipe led from this chamber and past a cooling coil. Here water vapor recondensed and dripped into a collection beaker that was connected to the original water beaker.
After one week of continual operation of his closed-cycle atmosphere, Miller analyzed the residue of compounds that had settled in the collection beaker of his system. He found that 15 percent of the carbon in his system had now formed into organic compounds. Two percent had formed actual amino acids (the building blocks of proteins and of DNA). In just one short week, Miller had created the building blocks of organic life! Virtually all scientists were amazed at how easy it was for Miller to create amino acids, the building blocks of life.
In 1953 the structure of the DNA molecule was finally discovered. Its structure fit well with how Miller's amino acid molecules would most likely combine to create longer chains of life. This was another bit of evidence to support the idea that Stanley Miller had dis covered of how life on Earth began.
