chapter  1
Denitions of Life
Pages 18

Before one can understand evolution, one must rst 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. Evidence of bacteria rst appears in 3.5 billion-year-old rocks. Therefore, simpler organisms must have existed prior to this time. These organisms probably self-assembled long before they evolved into the bacteria that were present 3.5 billion years ago. But, what were these rst organisms? The initial answer is that no one knows, but we can make some educated guesses on the characteristics of the rst organisms as well as the processes that led to them. The rst life on 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 Earth. Stanley Miller (1953) published the rst 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 charges (to simulate lightening), 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 also were produced. Since then, other experiments have been performed, including those that used different sets of gasses to more accurately re£ect 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 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 conditions in a meteorite when it passes through the atmosphere or near deep-sea volcanic vents.