À ce jour, les plus anciennes traces de vie sur Terre remonteraient à 3,8 milliards d’années, comme semblent l’indiquer des restes fossilisés retrouvés dans les roches sédimentaires. La vie était alors exclusivement aquatique et le restera pendant près de 3,4 milliards d’années ! Avant cela dans ses premiers milolirs d’années, la jeune Terre était sans doute dépourvue de toutes formes de vie ; D’une part elle n’était pss vivable (ou habitable) car bouleversée sans aaets par de épisodes volcaniques entrainant des sé »eimes d’ean bouillante
; Cependant de nombreus indices montrent que la vie complexe (milticellulaire) y est apparue plutot qu’estimé à ce joury Selon une nouvelle étude, la vie complexe sur Terre est apparue environ 1,5 milliard d’années plus tôt que prévu. C’est ce que montrentn des macrofodssiles lobés provenant d’organismes vivant il y a 2,1 milliards d’années dans une mer intérieure peu profonde, créée par la collision de deux continents.
En dépit du fait que la Terre avait hébergé la vie pendant plus de 2 milliards d’année, celle-ci y était restée rududimentaire. Il ne s’agissait que de bactéries et d’une forme voisine dite archaéa Les formes de vie les plus complexes étaient des colonies de microbes dites stromatolithes.
Pendant des années les bilogistes avaient affirmé que l’apparion de vie complexe était inévitable après que des formes de vie simple soient apparues. En fait ce ne fut pas ce qui arriva Après l’apparition des premières cellules, il y eut un long hiatus, presque la moitié du temps pris par la vie pour se diversifier. Pour qu’apparaissent des organismes mulyi celulaire. pour passer de la simple cellule à des organismes plus complexes, dits multicellulaires, il fallut attendre 4 milliards d’années d’ evolution, Ceci perperme de dire que la vie complexe est un accident
Si les cellules unicellulaires avaient évolué en organismes multicellulires, ceci se sertait traduit penddant des milliards d’années par l’apparion de multiples formes intermédiaires dont on retrouverait aujourd’hui des exemples. Or ce n’est pas ce qui s’est produit. D’un coté aujourd’hui on trouve les prokaryotes, simples cellules et archaea , de l’autres les organismes complexes dits eukaryotes qui prennent une infinité de formes. Un eukarotes est enviton 15.00 fois plus grand qu’une bactérie. Il possède une infinité d’organes dits orgenelles, des membranes intérieures, des squelettes et appeendices de déplacement. Ils son aux prokariotes ce qu’un humaein est à une amibe.
De plus, alors qu’une bactérie ne donne naissances en se regroupant qu’à des chaines identiques,les eukaryotes sont à la source de tous les organismes complexes, depuis les moules de mer jusqu’aux séquoias
(à suivre non traduit)
And while bacteria never form anything more complex than chains or colonies of identical cells, eukaryotic cells aggregate and cooperate to make everything from seaweed to sequoias, aardvarks to zebras. All complex multicellular life forms – that is to say, pretty much every living thing you can see around you, and more besides – are eukaryotes.
All eukaryotes evolved from the same ancestor. Without that one-off event, life would still be stuck in its microbial rut. Bacteria and archaea cells just don’t have what it takes to evolve into more complex forms.
So what happened? The critical event appears to have occurred about 2 billion years ago, when one simple cell somehow ended up inside another. The identity of the host cell isn’t clear, but we know it engulfed a bacterium, which began to live and divide within it, like a squatter. The two somehow found a way to live together amicably, and eventually formed a symbiotic relationship called endosymbiosis.
Through co-evolution over countless generations, the endosymbionts eventually become an organelle called the mitochondrion. These stripped-down vestiges of their former bacterial selves evolved to have one key function: to supply the cell with energy. This was the critical step that allowed life to throw off its microbial shackles and evolve into endless forms most beautiful.
Turbo charge
Once they have mitochondria, cells can overcome a fundamental barrier that prevents bacteria and archaea from growing large. In a nutshell, there is a limit to how much energy microbes can produce. The cell’s universal energy currency, ATP, is manufactured at the cell membrane. But as cells grow larger, their surface area to volume ratio drops and they have relatively less membrane to use. As they grow larger, their energy demands quickly overtake the supply. A cell with mitochondria (which have their own ATP-making membranes) can overcome this simply by adding more mitochondria – something that’s easily done, as the mitochondria retain their bacterial ancestors’ ability to clone themselves.
Awash with squadrons of mitochondria cranking out energy, early eukaryotes were free to grow larger and accumulate bigger and more complex genomes. And these expanded genomes provided the genetic raw material that permitted the evolution of ever more complex life.
Powered by the sun
That was not the end of the story. Another round of endosymbiosis is thought to have created the chloroplast, the organelle that allows plants and algae to convert sunlight into sugar in the process called photosynthesis. The endosymbiont in this case was a photosynthetic bacterium, which first appeared on Earth about 2.8 billion years ago. The cell nucleus, where eukaryotes store the majority of their DNA, was another crucial invention. It may have been created by another endosymbiosis, possibly of a virus. Eukaryotic cells also acquired other organelles, such as the endoplasmic reticulum, where proteins are made, and the Golgi apparatus, which dispatches them to their destination, possibly by infolding of their cell membranes.
All of this set the scene for the emergence of complex, multicellular life forms. Admittedly, it took a while. The first large multicellular organisms were the Ediacarans, ocean-dwelling life forms which appeared about 700 million years ago and disappeared around the time of the so-called Cambrian explosion 540 million years ago, when most of the familiar animal forms first evolved.
Nonetheless, the Ediacarans can trace their origins back to the evolution of mitochondria. And this seems to have hinged on a single fluke event – the acquisition of one simple cell by another. The bottom line is that while simple life appears to be a near inevitability, the evolution of complex life – including you and yours – is fantastically unlikely. That is the true miracle of life on Earth.
New scientist 23 August 2025
Référence
An Asgard archaeon from a modern analog of ancient microbial mats
https://doi.org/10.1101/2025.07.22.6630
Abstract
It has been proposed that eukaryotic cells evolved via symbiosis between sulfate-reducing bacteria and hydrogen-producing archaea. Here we describe a highly enriched culture of a novel Asgard archaeon, Nerearchaeum marumarumayae, with a bacterium Stromatodesulfovibrio nilemahensis from a stromatolite-associated microbial mat. The N. marumarumayae genome indicates it produces H2, acetate, formate, and sulfite, while S. nilemahensis synthesizes amino acids and vitamins, which can be exchanged in a syntrophic partnership. Electron cryotomography revealed N. marumarumayae cells produce chains of budded envelope vesicles attached to the coccoid cell body by extracellular fibers, and intracellular tube- and cage-like structures. Furthermore, the two species were observed interacting via intercellular nanotubes assembled by the bacterium. These characteristics and interactions may reflect an early step in the symbiotic evolution of eukaryotic cells.
Europe Solidaire 