In Stanley's lab experiment, the brown goo observed was called "protobionts." These protobionts were composed of organic molecules in a self-organizing state and were believed to be the precursors of living cells. Stanley Miller and Harold Urey conducted this experiment in 1953 by simulating conditions thought to be similar to the Earth's early atmosphere and oceans. By passing electric sparks through a gas mixture of water, methane, ammonia, and hydrogen, they produced a variety of organic compounds, including amino acids, which are the building blocks of proteins. These amino acids then polymerized under specific circumstances, forming the complex brown-colored protobionts.