On nomme procaryotes les organisme dont le noyau cellulaire n’est jamais entouré d’une membrane nucléaire, mais est mêlé au cytoplasme. Les procaryotes regroupent les bactéries et les cyanobactéries. Leur matériel génétique est le plus souvent réduit à un chromosome unique annulaire.
On nomme eucaryotes les organismes, unicellulaires ou multicellulaires, qui se caractérisent par la présence d’un noyau et généralement d’organites spécialisés dans la respiration, en particulier mitochondries chez les aérobies mais aussi hydrogénosomes chez certains anaérobies.
Les plus anciens eucaryotes attestés seraient âgés de 1,6 Ga, Ainsi certains acritarches dateraient approximativement de cette époque. Leur origine, toutefois, pourrait être encore plus éloignée. Grypania, vieille de 2,1 Ga, a été rapprochée des algues, et les Gabonionta, dans les formations de schistes noirs du Gabon, aussi anciens, suggèrent qu’une vie organisée faisant penser aux eucaryotes existait déjà. L’apparition des eucaryotes est encore plus ancienne. La présence de stérane, marqueur biochimique des eucaryotes dans des formations schisteuses australiennes suggèrent qu’à l’époque deux lignées s’étaient déjà différenciées il y a 2,7 Ga.
Les eucaryotes sont considérés comme représentatifs de la vie complexe Or aujourd’hui il est évident que leurs premiers spécimens sont apparus relativement tard dans l’histoire de la vie, au regard de la date présumée d’apparition des premiers procaryotes.
On comparera sur ce point le contenu de deux articles publiés dans Nature, respectivement en 2023 et 2010
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Article 1
Lost world of complex life and the late rise of the eukaryotic crown
- Nature
- volume618,
- pages 767–773 (2023)
Abstract
Eukaryotic life appears to have flourished surprisingly late in the history of our planet. This view is based on the low diversity of diagnostic eukaryotic fossils in marine sediments of mid-Proterozoic age (around 1,600 to 800 million years ago) and an absence of steranes, the molecular fossils of eukaryotic membrane sterols. This scarcity of eukaryotic remains is difficult to reconcile with molecular clocks that suggest that the last eukaryotic common ancestor (LECA) had already emerged between around 1,200 and more than 1,800 million years ago. LECA, in turn, must have been preceded by stem-group eukaryotic forms by several hundred million years3. Here we report the discovery of abundant protosteroids in sedimentary rocks of mid-Proterozoic age. These primordial compounds had previously remained unnoticed because their structures represent early intermediates of the modern sterol biosynthetic pathway, as predicted by Konrad Bloch4. The protosteroids reveal an ecologically prominent ‘protosterol biota’ that was widespread and abundant in aquatic environments from at least 1,640 to around 800 million years ago and that probably comprised ancient protosterol-producing bacteria and deep-branching stem-group eukaryotes. Modern eukaryotes started to appear in the Tonian period (1,000 to 720 million years ago), fuelled by the proliferation of red algae (rhodophytes) by around 800 million years ago. This ‘Tonian transformation’ emerges as one of the most profound ecological turning points in the Earth’s history.
Article 2
Nature Volume 466 Issue 7302, 1 July 2010
Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago
A series of well preserved centimetre-scale fossils in an extended fossiliferous level within black shales near Franceville, in Gabon, West Africa, may represent the earliest evidence so far reported for cell-to-cell signalling and coordinated growth behaviour on the scale of macroorganisms. Dated at about 2.1 billion years old, a billion and a half years before the rapid expansion in multicellular life forms known as the Cambrian explosion, their shape and regular fabric indicate a multicellular degree of organization.
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