Protein precursors became an area of vigorous research when Stanley Miller, working with Harold Urey at the University of Chicago, published his experimental results in 1953. Miller placed CO 2, H 2 O, NH 3, and CH 4 (methane) in a system of glass tubes such that by boding the water the mixture of gases was forced past an electrical spark and returned to the water again. When this system was allowed to operate for some days there appeared a dark precipitate in the water. Upon analysis this precipitate turned out to contain various amino acids.
Miller's basic experiment, including various modifications, has been repeated many times to reveal that not only can many essential amino acids be formed by this technique, but nonessential amino acids, significant amounts of proteins, and various other organic molecules. The answers to our first two questions regarding proteins are clearly in hand. Amino acids are the precursors of proteins, and they are readily formed spontaneously. And from them certain uninformed proteins are made. Now what can we conclude from this? Regarding the formation of amino acids there are two important points. First, it has become clear that an atmosphere in which amino acids will appear must be a reducing one (one in which H2 is present). That was true of Miller's experimental set-up. In an oxidizing atmosphere (one in which 02 is present) no amino acids were obtained.
Second, not all the essential amino acids are formed in this type of experiment and some amino acids were formed that are not found in organisms today. All of this is not so much a lesson as a puzzle. Why were only the 20 essential amino acids incorporated into living systems when some of them are not readily formed spontaneously, and why were others readily formed that are not incorporated? We cannot answer that question today.
And, then, the uninformed proteins require study. These proteins can be formed in large amounts from certain amino acids. That is, certain proteins are favored as are the aminoacids that constitute them. None of them has been shown to have enzymatic activity. These proteins tend to aggregate and form small spheres and, most interesting, these spheres differentiate in a way that suggests cellular structure. In the electron microscope they show a dense outer layer; internally, there is a mass of material of differing densities. Biochemist Sidney Fox and his collaborators have been especially active in studying these proteinaceous spherules. They suggest that these entities represent a structure that could anticipate cells at a later stage in the origin of life. Such microspheres are optimistically called protocells. A better term, coacervate, was first used by the Russian biologist Oparin, in 1924, in his pioneering examination of the origin of life. We will return to Oparin's coacervate after we have considered, in more detail, the transition from precellular living systems to cellular ones.