evolution: another eukaryote article (Introduction)

by David Turell , Monday, October 28, 2024, 19:01 (24 days ago) @ David Turell

Today:

https://www.quantamagazine.org/meet-the-eukaryote-the-first-cell-to-get-organized-20241...

"The eukaryotes invented organization, if we use the literal definition of “organize”: to be furnished with organs. Inside a eukaryotic cell are self-contained, membrane-bound bundles that perform special functions, called organelles. All eukaryotic cells — animal, plant, fungus or protist — have a nucleus that encloses and protects DNA. Nearly all of them have mitochondria, which produce energy to fuel biochemical reactions. (Any eukaryotic lineages that lack mitochondria used to have them and then lost them sometime in evolutionary history.) And across the evolutionary tree, different eukaryotes have evolved or procured additional organelles that assemble proteins, store water, turn sunlight into energy, digest biomolecules, get rid of waste, and more. If prokaryotes are a loose pile of papers on the floor, eukaryotes are a sophisticated filing system that binds pages into packets and labels them.

“'They’ve got the endoplasmic reticulum, Golgi apparatus, peroxisomes, lysosomes, vacuoles — all this machinery not present in bacteria or archaea cells,” said Thijs Ettema(opens a new tab), an evolutionary microbiologist at Wageningen University in the Netherlands.

"How this all happened isn’t entirely clear, but today, most experts agree that 2 billion or 3 billion years ago, an archaean cell engulfed a bacterial cell, which somehow escaped digestion and adapted to life inside its host. That bacterium evolved to become the organelle we now know as the mitochondrion.

"Since that original act, the eukaryote has transformed again and again. It first evolved into a smattering of unique unicellular creatures, such as the ancestors of modern diplomonads(opens a new tab), which swim with dual tail clusters, and the parasitic microsporidians(opens a new tab), which shoot out coiled tubes to infect victim cells.

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"At some point, in a sequel to mitochondrial capture, a eukaryote engulfed a cyanobacterium capable of photosynthesis — the process of using sunlight to harvest carbon from the air and spin it into energy. That branch of the eukaryotic family, freshly equipped with green organelles called chloroplasts, evolved into plants and other photosynthesizers.

"Then, within the last billion years, some individual eukaryotes began working together. Collectives became colonies, and they further organized when entire cells began to specialize, or perform unique functions within a complex multicellular body. Multicellularity unlocked even higher levels of sophistication, resulting in mushrooms, trees, hippos and humans.

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"Ettema and his team were surprised to find genes that looked suspiciously eukaryotic among the expected traces of archaea and bacteria. Upon further investigation, they found what might very well be the missing link in our understanding of eukaryotic evolution: a prokaryote(opens a new tab) with hallmark eukaryotic complexity in its genome. They named the microbe Lokiarchaea(opens a new tab) after its home vent.

"Ettema suspected that a cell from this family, later dubbed “Asgards,” might make a convincing candidate for that first hungry host that engulfed a bacterium that became a mitochondrion. If so, he proposed, that ancient Asgard archaean might have evolved into the eukaryotic branch while the rest of its family forged on as prokaryotes. (my bold)

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"Researchers have since dug up Asgard DNA all over the world, including in North Carolina and Yellowstone National Park. But only two labs have successfully cultured Asgard cells. Those cells reveal some eukaryote-like features(opens a new tab), such as a cytoskeleton built with the protein actin. The rest are still under investigation, and their cellular structures are unknown.

"Today, most researchers agree(opens a new tab) that Asgards are the closest known prokaryotic relatives of eukaryotes. Modern Asgards aren’t our ancestors, but we likely share an ancestor somewhere in deep time. “You could argue that eukaryotes are Asgards in the same way that birds are dinosaurs,” said Itay Budin(opens a new tab), a biochemist at the University of California, San Diego. “They’re kind of our cousins.”

"The process by which an archaean cell turned a free-living bacterium into its own cellular machinery — called endosymbiosis — remains largely obscured by evolutionary history. Most biologists believe that one prokaryote “swallowed” another in a process called phagocytosis. However, phagocytosis takes a lot of energy. In fact, it is so energy-expensive that some believe it would have been impossible without the first host cell already having energy-generating mitochondria — opening a tricky, microbial chicken-and-egg debate.

"Modern Asgards offer a clue that could get us around this mind bender: Their cytoskeleton suggests that they might use tiny, armlike projections called blebs to entrap prokaryotic food, without needing mitochondria. Maybe the first eukaryotic ancestor did, too."

Comment: to followers of this website, this is all old news. Somehow all of this got cobbled together or evolved. Perhaps all designed. Note the bold. Asgards are the best clue we have