Many discoveries made in the last ten years in the field of Precambrian paleontology suggest that Eukaryote radiation can be traced farther back in time than was thought previously. Only five milestones among the more significant ones are outlined here:
- A highly complex assembly of Eukaryotes already existed circa 1100-1200 Ma in the Thule Supergroup of Greenland (Samuelsson et alii, 1999). It includes at least three discrete types of Pyramimonadales, along with various spheromorph and acanthomorph acritarchs.
- The Eukaryotes had already reached a high level of diversity in the Roper Group, Australia, circa 1450 Ma (Javaux et alii, 2001, 2004).
- Multicellular organisms exhibiting a functional differentiation between several cell types, the Longfengshaniids, occur in the Tuanshanzi Formation, China, circa 1650-1700 Ma (Zhu & Chen, 1995).
- The date of the sudden increase in atmospheric oxygen at the beginning of the Proterozoic has been determined precisely. It occurred between the two last Huronian glaciations, i.e. between 2450 and 2320 Ma (Bekker et alii, 2004).
- The presence of Eukaryotes circa 2700 Ma at Wittenoom, Australia, is attested by sterans, biomarkers that only Eukaryotes can synthesize (Brocks et alii, 1999).
Note. A discovery requires confirmation. In the Dashiling and Qingshicun Formations of the Hutuo Group, China, ca. 2400 Ma, Sun & Zhu (1998) have collected microfossils that they have assignated to 19 genera and 31 species. Among them are large spheromorphs, coccoids connected by a filament (Polysphaeroides formosus) and an enigmatic triangular theca (Triangulomorpha crassa) that have been interpreted as Eukaryotes. If this attribution is correct, and if these fossils are actually 2400 millions years old, Precambrian chronology will have to be reconsidered.
All these discoveries are posterior to Knoll's famous paper on the "big bang" of the Eukaryotic crown-group (1992). They render less plausible the hypothesis that he was defending then, and that is still the basis for diagram 9.5 in his book, Life on a young planet (Knoll, 2003, p. 152, fig. 9.5). They are more in agreement, I think, with several ideas that I defended in my book, La vie invisible (Teyssèdre, 2002), and that I attempt to delineate in this paper:
- At about 750 Ma the evolution of multicellular green algae was already far advanced.
- Several specialized types of Pyramimonadales, of which two (Pachysphaera and Pterosperma) still exist today, were present more than 1200 Ma ago. Some of them may go back as far as 1500 Ma.
- The presence in the 1150-1250 Ma Ruyang Group of a very derived type of Streptophyta, the Zygnematale Spiromorpha, indisputably implies a long prior evolution of Viridiplantae.
- Phycomas of Pyramimonadales may be as old as 1730 Ma. In any case, acritarchs similar in their "mesospheromorphic" size and their mode of dehiscence to present-day phycomas existed at Chuanlinggou at this time.
- Small acritarchs, the envelope of which contained an acetolysis-resistant biopolymer, were numerous and diverse from 2000 Ma. Probably among them were primitive Viridiplantae.
The date of 750 Ma, from which we started, is far behind us.
The author thanks Emmanuelle Javaux, Alain Le Herissé and an anonymous reviewer for having led him, by their constructive suggestions, to improve a first version of the manuscript. Special thanks are due to Claudine Teyssèdre for having provided an English translation of this first version and to Nestor Sander for his assistance in amending the final English text.