Microbiology 148 (2002), 21-27
The enigma of the origin of life and its timing
Martin A. Line1
School of Agricultural Science, University of Tasmania, Hobart, Tasmania 7001, Australia1
Tel: +61 3 6226 2636. Fax: +61 3 6226 2642. e-mail: Martin.Line@utas.edu.au
Keywords: life origin, panspermia, last universal ancestor, last common community
Overview
The First 3±5 billion years of evolution on Earth was dominated by unicellular life, significant multicellularity being independently evolved on numerous occasions from about 1200 million years ago in Eukarya (Carroll, 2001) or earlier in Bacteria. Rather than being novel, much of recent evolution in terms of biochemistry is derived, being attributable largely to ` tinkering with the available equipment, adapting existing organs to new purposes' (Nisbet & Sleep, 2001). An example of the manner of working of `modern' evolution is seen in the origin of some forms of fermentation in Bacteria, which was derived from respiration, which was derived from photosynthesis, which were derived from nitrogen fixation (Burke et al., 1993). Nitrogen fiation was probably present prior to the divergence of the three domains of life, and because of its complexity, was almost certainly derived from an earlier function, perhaps as a detoxyase responsible for detoxifying cyanides or other chemicals present in the early atmosphere (Fani et al., 2000). It has been suggested that nitrogen fixation arose late in the Archaean, C2±2±2±3
gigayears ago (Gya), due to decreasing abiotic production of fied nitrogen in the atmosphere (Navarro-Gonza!lez et al., 2001), but phylogenetic evidence strongly indicates an origin earlier than 3±5 Gya.
There is an enormous leap from pre-biotic chemistry to the complexity of DNA replication, protein manufacture and biochemical pathways existing at the time of the primary divergence of life. Although progress is being made on the evolution of some structural components and biochemical pathways, there remain numerous unsolved `chicken and egg' problems. Margulis
(1996a) said, `To go from a bacterium to people is less of a step than to go from a mixture of amino acids to that bacterium', yet accumulated evidence from the physical and biological sciences indicates that advanced life existed at a very early stage of Earth's development. While derived evolution is clearly more efficient than de novo evolution, why then did it (coupled with symbiosis) take a further 3 gigayears to arrive at plants and animals, when most of the basic, often unique, developments appear to have been established in the first 500 million years or less? Hence the enigma: an origin of life on Earth appears highly improbable, an origin elsewhere is highly conjectural. While this conundrum has been identified in various forms for several decades, its magnitude has dramatically increased over the last five years as new constraints are placed on the timing of the primary divergence of the domains of life (Shen et al., 2001). The problem can be likened to one faced by cosmologists a few years ago, and since resolved, that the universe appeared to be younger than the oldest stars in our galaxy. The cumulative evidence suggesting an extraterrestrial origin for life on Earth is the subject of this review.
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