Abiogenesis - Current Models - From Organic Molecules To Protocells - Coenzyme World

Coenzyme World

Recent studies, applying the equivalent of Moore's Law to biological evolution and extrapolating backwards, propose that life began "9.7±2.5 billion years ago", billions of years before the Earth was formed. In the case of evolution, empirical evidence suggested a doubling of complexity every 376 million years. As the age of trees can be measured by the number of rings, the hypothesis that the age of life could be measured by biological complexity (i.e., the length of functional non-redundant DNA in the genome) was studied. If log-transformed complexity is plotted against the time of origin of large evolutionary lineages, then the points fit to a straight line (see figure). The exponential increase in complexity can be explained by a positive self-activating feed back loop. The regression line hits zero (i.e., one nucleotide) at "9.7±2.5 billion years ago". If this model is correct, and since our Solar System is 4.6 billion years ago, then life somehow arrived to Earth from older stellar systems. This hypothesis was criticized by Eugene Koonin who suggested that the rates of early biological evolution might have been much faster due to the absence of competition on early Earth. Chris Adami argued that "it is inconceivable that life began with just a few nucleotides" (see discussion). To answer this criticism, Sharov proposed a hypothetical abiogenesis scenario that starts from coenzyme-like molecules that are functionally equivalent to single nucleotides.