Endurance of life : the implications of genetics for human life / Macfarlane Burnet.

  • Burnet, F. M. (Frank Macfarlane), Sir, 1899-1985.
Date:
1978
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Endurance of life : the implications of genetics for human life / Macfarlane Burnet. Source: Wellcome Collection.

    29/248 (page 19)
    2 Complexity and Error 19 sedimentary rocks (about 3400 million years B.P.) show that water was already present as liquid seas and oceans, and that cycles of erosion and sedimentation were well established. The earliest microfossils with a reasonable-but not certain—claim to represent micro-organisms are dated around 3000 million years B.P. From about 2500 million to 2300 million years B.P. stromatolites appear in the geological record, which from comparison with similar modern structures are interpreted as mineral precipitation on sheets of primitive blue-green algae, the first chlorophyll-containing micro-organisms. The proterozoic era is therefore dated from 2500 miUion to 600 million years B.P. This includes the Precambrian, where in specially suitable rocks impressions of soft-bodied inver¬ tebrate forms have been found. The standard beginning of the fossil record of animals and plants is at the beginning of the Cambrian Period, 600 million years B.P. Happenings at the biochemical level in the period before 2500 million years B.P. that led up to the first appearance of self-repro¬ ducing units would be of immense interest to chemists, and there are many speculations. From our biological point of view, however, such details are irrelevant and we can start with a micro-organism essentially equivalent to a modem bacterium, at least as far as its mode of reproduction is concerned. This would necessarily imply many other similarities to the modern forms, and anything to be said will have to be based on an elementary understanding of the genetics of some modem prototype bacterium. By a combination of accident and design, the standard form used in virtually all laboratories that are interested in bacterial chemistry and genetics has come to be the common intestinal organism Escherichia coli, known to every biologist as '£■. coW. Three thousand million years ago the surface waters must have swarmed with such primitive micro-organisms living without oxygen, presumably on organic molecules in solution including fragments of dead and disintegrated micro-organisms. In the ab¬ sence of oxygen in the atmosphere, much lethal ultraviolet radiation from the sun must have penetrated to a moderate depth of water and killed a relatively large proportion of the organisms near the surface. In the very, very distant future, descendants of these micro-organ¬ isms would evolve into algae and other chlorophyll-containing plants that in due course would produce the molecular oxygen of the atmosphere and allow the development of an ozone shield against most of the destructive eifect of the ultraviolet. In the meantime, however, one of the main tasks of evolution must have been to develop ways by which DNA could be protected from ultraviolet