SE Luria, JE Darnell, D Baltimore and A Campbell (1978). General Virology, 3rd Edn. John Wiley & Sons, New York, p2 of 578.
SE Luria, JE Darnell, D Baltimore and A Campbell (1978). General Virology, 3rd Edn. John Wiley & Sons, New York, p4 of 578.
Steven Hawking, of black holes fame, apparently believes that computer viruses should count as life: they are obligate parasites which exploit the "metabolism" of the host computer they infect, they replicate in the form of their source code [=genome], and they newest and nastiest can mutate while they do so (Weekend Argus, 6-7 August, 1994).
Computer viruses infecting cells...Russell Kightley Media
Short stretches of DNA - no more than 500 nucleotides - have been amplified up in vitro by the technique of polymerase chain reaction or PCR, from mites entombed in amber up to 200 MYr BP, and from fossilised leaves up to 60 MYr old. This DNA can be sequenced and compared to that of morphologically related modern organisms.
FA Murphy and DW Kingsbury (1990). Virus Taxonomy. Chapter 2 in Fields Virology, 2nd. Edn. (BN Fields et al, Eds.) Raven Press, New York.
EG Strauss, JH Strauss and AJ Levine (1990). Virus Evolution. Chapter 9 in Fields Virology, 2nd. Edn. (BN Fields et al, Eds.) Raven Press, New York.
Eukarya probably arose from cells which probably most closely resembled Archaea-like organisms (Archaebacteria), about 1.4 billion years ago. Their key differences from Prokarya and Archaea - the possession of nuclear membranes and mitochondria - are difficult to explain. The former may be a simple adaptation to localise functions specific to DNA replication and transcription, and may have happened independently in at least one other of the Bacteria. The latter, however, is postulated to have had its origins in an endosymbiotic association of a free-living bacterium with another with aerobic respiration, possibly related to the present-day Agrobacterium, Rhizobium and Rickettsias: the latter would have been present in the cytoplasm of the former, and would gradually have lost its cell wall (though remaining enveloped), and much of its genome (much of whose function was taken over by the host), though retaining its own DNA replication, circular genome structure, and ribosomal RNA and protein genes. This would have been the origin of true Eukarya, all of whom would descend from this pioneering symbiosis.
Another endosymbiosis which became permanent happened at least once (and possibly several times) much later on in evolution, with the entry into symbiosis of a primitive member of the Eukarya and a photosynthetic bacterium: this was possibly an ancestral relative of Prochloron, which has similar chlorophyll, but may have been a cyano- (blue-green)bacterium. As happened with mitochondria, the chloropast percursor lost its wall, and much of its genome; however, as it occurred much more recently in evolutionary time, chloroplast genomes are usually much larger than mitochondrial genomes. Several eukaryotic marine algae have chloroplasts which appear to have different origins than those of all land plants: these may have been independent acquisitions. Still others have complicated multi-membrane layered chloroplasts which appear to have vestigial nuclei: these are probably derived from primitive photosynthetic eukaryotes which formed endosymbiotic associations with other eukaryotes.
Prescott et al., Microbiology: 2nd. Edn.
Strauss et al., Virus Evolution, in Fields et al., Virology, 2nd. Edn.
copyright Ed Rybicki, 1995, 2000