Evolution: origin of eukaryotes (Evolution)

by David Turell , Wednesday, April 17, 2019, 19:55 (2045 days ago) @ David Turell

Alternatives to mitochondrial origin theory:

https://www.quantamagazine.org/rethinking-the-ancestry-of-the-eukaryotes-20190409/

"A particularly vexing mystery is the rise of the eukaryotes, cells with well-defined internal compartments, or organelles, which are present only in animals, plants, fungi and some microbes like protists — our evolutionary kin. The earliest eukaryotes left no clear fossils as clues, so researchers are forced to deduce what they were like by comparing the structural and molecular details of later ones and inferring their evolutionary relationships.

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"And those first eukaryotes may depart significantly from what most scientists expected, if some recent findings are any indication. Earlier this month, one team presented evidence that a signature event in eukaryote evolution — the development of the organelles called mitochondria — might have unfolded quite differently than was theorized. Meanwhile, other researchers have suggested that the earliest “ancestor” of all eukaryotes might not have been a single cell at all, but rather a mixed population of cells that avidly swapped DNA. The difference is subtle, but it might be important for understanding the evolution and diversity of the eukaryotes we see today.

"The very first cells — the first life forms on this planet — were prokaryotes, but they were not all alike. Even early on, two very distinct lineages emerged, the archaea and the bacteria. The archaea might have been the first to thrive because even now they can survive in extreme environments like hot vents and super-saline pools. But it’s also possible that archaea and bacteria split from the first cells at the same time and began to diversify independently from the start. Figuring out definitively when and how the split occurred is probably impossible given how much time has passed; fossil evidence is nonexistent, and organisms from both branches have swapped genes extensively through horizontal gene transfer (as opposed to the “vertical” transfer of genes down through generations), which complicates analyses of their genomic history.

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"It would be a struggle to distinguish the cells of this first eukaryotic common ancestor, or FECA, as such. It didn’t yet have a nucleus, for example. It didn’t have mitochondria to convert sugars and other molecules into more metabolically usable forms of energy. It didn’t even have microtubules, the structural proteins in eukaryotic cells that allow for compartmentalization by enabling the cell to shuttle things where they need to go.

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"The scientists concluded that mitochondria most likely arose out of a partnership between archaeal cells that fermented certain small organic molecules and alphaproteobacteria that survived by oxidizing certain other ones: The bacteria could use the electrons and hydrogen that the archaeal cells shed as wastes. (The researchers call this the “reverse flow model” because according to a previously popular theory, the bacteria would have donated hydrogen to the archaea’s metabolism.)

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"For example, some modern archaea that live under oxygen-free conditions and metabolize hydrocarbons depend on bacteria to accept their electrons. “A similar type of interaction may have characterized the presumed archaeal ancestor of eukaryotes.”

"Over time, horizontal transfers of genes from other bacteria would have provided more of the machinery for the metabolic processes performed by mitochondria as we know them. Meanwhile, gene transfers between archaeal hosts and their bacterial symbiotes, along with the loss of some superfluous genes on each side, would have cemented what had been separate symbiotic cells into a permanently unified eukaryotic state.

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"Overall, the genesis of eukaryotes remains mysterious because all eukaryotes alive today arose from an organism that was already complex. Somehow, over an unknown number of millennia, FECA turned into the last eukaryotic common ancestor, or LECA — an organism ancestral to every other subsequent eukaryote living or extinct, including ones currently unknown to science. LECA is a lot easier to imagine because it probably looked similar to some of today’s microbial eukaryotes. “It turns out that everything that has a nucleus also has mitochondria, a Golgi apparatus and everything else,” said W. Ford Doolittle, “LECA appears to have already been a fairly sophisticated eukaryotic cell.”

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"many of O’Malley and Leger’s colleagues do agree that it makes sense to think of LECA as a population of cells. But there has been some pushback to that idea, too. According to O’Malley, some scientists insist that LECA had to be a single cell, one that split and then split again and again and again, eventually giving rise to all other eukaryotic cells.

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"The trouble is, we may never know what LECA looked like because no fossils or remnants of DNA will ever reveal its nature directly. Even the best genomic methods can’t literally turn back time and allow us to watch how a sequence changed. It’s basically impossible to concretely determine what LECA’s genome or pangenome looked like."

Comment: The prokaryote cell and the eukaryote cell are as different as as tribe of cave dwellers are to a modern city's population. The highly organized cell functioned just as the simple cell did, but it led the way to complex evolution. Not by chance.