ABSTRACT
Before one can understand evolution, one must first decide on the boundaries of life, and what is at and beyond those boundaries. Complex cells, such as bacteria, probably did not spontaneously assemble from a set of chemical compounds. Fossil evidence of organisms resembling bacteria first appears in 3.5-billion-year-old rocks. Therefore, simpler organisms must have existed prior to this time. These organisms probably assembled and evolved long before one of their members evolved into the bacteria that were present 3.5 billion years ago. But, what were these first organisms? The initial answer is that no one knows, but we can make some educated guesses about the characteristics of the first organisms, as well as the processes that led to them. The first life on the Earth may have had its beginning from sets of chemicals and reactions that may have been derived in different ways, which eventually mixed in chemical pools or near undersea volcanic vents on the Earth. Stanley Miller published the first studies that produced amino acids and some other compounds from the simple molecules thought to be present on early Earth (Figure 1.1). He mixed water, ammonia, methane, and hydrogen into a sealed container, added heat and electrical discharges (to simulate lightning), and withdrew the products from time to time. He found that at least four of the amino acids found in modern cells were formed, and several precursors of nucleic acids were also produced. Since then, other experiments have been performed, including those that used different sets of gasses to more accurately reflect the early Earth, some performed under high pressure, and some done under cold conditions. In total, at least 16 of the biologically relevant amino acids, as well as fatty acids, and rudimentary nucleic acids were formed. More recently, in vitro experiments have been performed that have produced nucleic acids under conditions thought to have existed on the Earth early in its history. Also, amino acids and other biological compounds have been found in meteorites and comets, and peptides (linked chains of amino acids) can be formed under warm to hot conditions under high pressure, similar to the conditions in a meteorite when it passes through the atmosphere, or near deep-sea volcanic vents.
