The antiquity of the green algae (Viridiplantae) has been hotly debated and is still controversial today. In his book, Life on a young planet (2003), Andrew H. Knoll published a diagram (p. 152, fig. 9.5) assigning to the first documented fossils of these algae an age of 750 million years (Ma) and to the separation of their ancestors from those of the red algae a date somewhat more than 1200 Ma. On the contrary, in my book, La vie invisible (Teyssèdre, 2002), I argue that the oldest known fossils of green algae date back at least to 1200 Ma, and that the ancestors of Viridiplantae and Rhodophyta separated from each other perhaps as early as 2000 Ma. The matter is of consequence and the disagreement obvious.
I have the greatest admiration for A.H. Knoll, whom I consider one of the most distinguished specialists of the Precambrian. I do not contest the facts he reported (with one significant exception: I maintain that some green algae, the Pyramimonadales, were attested long before 750 Ma). Several of the data I use come from Knoll's papers. The disagreement mainly concerns two points, one theoretical, the other methodological.
1) I disagree with the whole theory underlying the "short chronology" that Knoll assumed in preparing diagram 9.5 of his book, Life on a young planet (p. 152). This diagram looks to me like an attempt to reconcile some of the phylogenetic trees of the Eukaryotes (ibid., p. 127, fig. 8.2) with the hypothesis of an "evolutionary big bang" that Knoll presented in a famous 1992 paper. According to this hypothesis the radiation of the "crown-group" of Eukaryotes was explosive. It was induced by a rapid increase in atmospheric oxygen between 1200 and 1000 Ma. Since 1992 Knoll has nuanced his theory with many shades and derogations. However, to my knowledge, he has never formally stated that he may have been wrong. Concerning this topic I defend in my book, La vie invisible (Teyssèdre, 2002), three ideas. The first two are in agreement with Knoll's concepts, whereas the third is radically contrary:
a) the expansion of Eukaryotes correlates closely with the increase in atmospheric oxygen;
b) the effects of this increase did not occur as a continuous progression, but as successive stages separated by thresholds;
c) the phase that stimulated the expansion of the Eukaryotes did not occur between 1200 and 1000 Ma, as Knoll (1992) asserted, but considerably earlier, during a period separating two well-defined Huronian glaciations around 2400 Ma.
2) The second disagreement is a methodological one. It seems to me that it is now essential to confront paleontology with a science that is developing exponentially today: molecular phylogeny. In his most recent publications Knoll accepts this concept in a general way, but in my opinion he does not draw all the inferences from the fact that a paleontologist must master those aspects of this science that concern phylogenetic trees both as a method for control and as a heuristic tool. On the one hand, although molecular phylogeny does not always allow us to establish an exact taxonomy, it helps us to avoid gross errors due to homoplasy or morphological convergence. If a paleontologist avoids this interdisciplinary collation, he lays himself open to a misinterpretation of the phylogenetic relationships of the fossils. Furthermore, molecular phylogeny allows us to determine the sequence of nodal points, the order of the successive evolutionary stages in a phylum. This sequence is an important criterion for evaluating the probability that a fossil discovered at a given geological level does or does not belong to a given taxonomic clade. The reader may find in the annexes of this paper two examples of the usefulness of molecular phylogeny as a means of detecting false assertions (Teyssèdre, 2006, annex 15) or as a heuristic tool (Teyssèdre, 2006, annex 16).
In the last thirty years paleontological analysis of the Prasinophyceae has been reinvigorated, for morphological description of fossils is now supplemented by observation of cell ultrastructures using the TEM microscope and by biochemical analysis of the cellular wall. Eminent paleontologists distinguish themselves in these fields, following the pioneer work of Schopf, Hofmann, Bengston, Vidal, Knoll, Colbath, Jux, Le Herissé, Guy-Ohlson, Butterfield. Since 1995 several researchers have insisted explicitly as a principle on the necessity of combining ultrastructural and biochemical analyses with traditional descriptions. See for instance Arouri et alii (1999, 2000), Talyzina & Moczydlowska (2000), Versteegh & Blokker (2004), Javaux, Knoll & Walter (2001, 2004), Marshall et alii (2005). It seems to me that the time has come to enter a new phase, that would be the third one, in which a paleontology that aims at becoming explanatory and not merely descriptive would integrate its findings with those of molecular phylogeny.
I am currently working on research which could be entitled "Precambrian paleontology in the light of molecular phylogeny". The object of the study presented here is to demonstrate the antiquity of the Viridiplantae, by focusing on fossils that probably pertain to one of the divisions of this phylum, the Chlorophyta, and more particularly to the clade of the Pyramimonadales. I shall defend two proposals:
- The oldest known fossils of green algae do not date back to circa 750 Ma, as indicated in Knoll's diagram (2003, p. 152), but at least to 1200 Ma and probably much earlier.
- The divergence of green and red algae did not occur slightly before 1200 Ma, but about 2000 Ma.
The arguments I develop have been set out in nine headings, so that A.H. Knoll may, if he likes, clearly identify them in his counter-arguments. A series of documentary papers complementing my discussion can be found in the annexes (Teyssèdre, 2006, annexes 1-16).
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