title: Genome increase as a clock for the origin and evolution of life.
abstract: BACKGROUND
  The size of non-redundant functional genome can be an indicator of biological complexity of living organisms. Several positive feedback mechanisms including gene cooperation and duplication with subsequent specialization may result in the exponential growth of biological complexity in macro-evolution.
  
  
  RESULTS
  I propose a hypothesis that biological complexity increased exponentially during evolution. Regression of the logarithm of functional non-redundant genome size versus time of origin in major groups of organisms showed a 7.8-fold increase per 1 billion years, and hence the increase of complexity can be viewed as a clock of macro-evolution. A strong version of the exponential hypothesis is that the rate of complexity increase in early (pre-prokaryotic) evolution of life was at most the same (or even slower) than observed in the evolution of prokaryotes and eukaryotes.
  
  
  CONCLUSIONS
  The increase of functional non-redundant genome size in macro-evolution was consistent with the exponential hypothesis. If the strong exponential hypothesis is true, then the origin of life should be dated 10 billion years ago. Thus, the possibility of panspermia as a source of life on earth should be discussed on equal basis with alternative hypotheses of de-novo life origin. Panspermia may be proven if bacteria similar to terrestrial ones are found on other planets or satellites in the solar system.
  
  
  METHODS
  This article was reviewed by Eugene V. Koonin, Chris Adami and Arcady Mushegian.
authors.affiliation: Laboratory of Genetics, National Institute on Aging (NIA/NIH), 333 Cassell Dr,, Baltimore, MD 21224, USA.
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Sharov