W Ford Doolittle

Date:
[1980]
Reference:
PP/CRI/H/6/13/5
Catalogue details

Licence: In copyright

Credit: W Ford Doolittle. Source: Wellcome Collection.

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    deserve to be considered together because together they represent a radical revision of the way in which we think about cellular evolution. The points are these, (i) Living cells arose very early in the history of the earth, (ii) Living cells diverged very early into three (organizationally prokaryotic) lineages; the archaebacterial lineage, the eubacterial lineage (as defined by Woese and Fox [3]), and that lineage which gave rise to the nucleus of eukaryotic cells (ten¿d the urkaryotic lineage by VJoese and Fox, and here called the nuclear-cytoplasmic lineage). ( i i i) Eukaryotic cells themselves resulted from the repeated fusion of representatives of two of the three 1 i neages. Living cells aro se v e ry early in t he history of the e arth The fossil record for multicellular eukaryotes is some 700 million years old and, until the late 1950's or early 1960's, was the only fossil record we had [4], Since then, microfossi 1 s which are cellular by several criteria have been discovered in deposits of increasing and almost unbeliev able antiquity. Organic microstructures from the Swaziland system of S. Africa which, at 3.5 billion years, is more than three-fourths as old as the earth itself, have now been shown by Knoll and Barghoorn [5] to be almost unquestionably biogenic, on five grounds: (i) chemical composition, (ii) unimodal size distribution, {i i i ) morphology, (iv) sedimentary context, and (v) the preservation of cells in the process of division. The oldest (3.8 billion years) sedimentary rocks are those of the Isua formation of Greenland. These too contain structures which appear cellular [6], The earth itself is only some 700 million years older than these rocks [7]. Even if we assume that conditions conducive to the accumulation of non- biologically synthesized precursors of biological systems existed from the
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