Evolution: how the complex cell formed (Introduction)

by David Turell , Sunday, October 29, 2023, 17:02 (181 days ago) @ David Turell

A series of guesses and some accepted:

https://knowablemagazine.org/article/living-world/2023/how-endomembrane-system-of-eukar...

"More than 1.5 billion years ago, a momentous thing happened: Two small, primitive cells became one. Perhaps more than any event — barring the origin of life itself — this merger radically changed the course of evolution on our planet.

:One cell ended up inside the other and evolved into a structure that schoolkids learn to refer to as the “powerhouse of the cell”: the mitochondrion. This new structure provided a tremendous energetic advantage to its host — a precondition for the later evolution of complex, multicellular life.

"But that’s only part of the story. The mitochondrion is not the only important structure within complex, eukaryotic cells. There’s the membrane-bound nucleus, safekeeper of the genome. There’s a whole system of internal membranes: the endoplasmic reticulum, the Golgi apparatus, lysosomes, peroxisomes and vacuoles — essential for making, transporting and recycling proteins and other cargo in and around the cell.

"Where did all these structures come from? With events lost in the deep past and few traces to serve as evolutionary clues, it’s a very tough question to tackle. Researchers have proposed various hypotheses, but it is only recently, with some new tools and techniques, that cell biologists have been able to investigate the beginnings of this intricate architecture and shed some light on its possible origins.

***

"Scientists proposed that it already was fairly complicated, with a variety of membrane structures inside it. Such a cell would have been capable of engulfing and ingesting things — a complicated and energetically expensive eukaryotic feature called phagocytosis. That might be how the mitochondrion first got into the host.

"But this idea, called the “mitochondria late” hypothesis, doesn’t explain how or why the host cell had become complex to begin with.

***

"In short, Gould, Garg and Martin’s hypothesis explains why endomembrane compartments evolved: to solve problems created by the new guest. But it doesn’t fully explain how the alphaproteobacterium got inside the host to begin with, says cell biologist Gautam Dey at EMBL in Heidelberg, Germany; it assumes the endosymbiont is already inside. “This is a massive problem,” Dey says.

"In short, Gould, Garg and Martin’s hypothesis explains why endomembrane compartments evolved: to solve problems created by the new guest. But it doesn’t fully explain how the alphaproteobacterium got inside the host to begin with, says cell biologist Gautam Dey at EMBL in Heidelberg, Germany; it assumes the endosymbiont is already inside. “This is a massive problem,” Dey says.

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"Martin’s main objection is that the inside-out model does not provide an evolutionary pressure that would have caused the nucleus or other membrane-bound compartments to arise in the first place. The inside-out model “is upside-down and backwards,” Martin says.

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"In 2017, cell biologist Heidi McBride of McGill University in Montreal reported that cells lacking peroxisomes could generate them from scratch. Working with mutant human fibroblast cells without peroxisomes, her team found that these cells put proteins that are essential for peroxisome function into mitochondria instead. Then the mitochondrial membrane released them as little bubbles, or vesicles.

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"...a 2021 report from the lab of biochemist Adam Hughes at the University of Utah found that when yeast cells are fed toxic amounts of amino acids, their mitochondria will shed vesicles that are loaded with transporter molecules. The transporters move amino acids into the vesicles, where they won’t poison the mitochondria.

"Hughes also discovered that the vesicles shed by the mitochondria can form long, tubule-like extensions with multiple layers, reminiscent of the layered stacks of the endoplasmic reticulum and the Golgi body. The structures persist in the cell for a long time. “They’re definitely their own unique structure,” Hughes says.

***

"It may never be possible to know for sure what happened such a very long time ago. But by exploring what can happen in today’s living bacterial, archaeal and eukaryotic cells, scientists can get more clarity on what was possible — and even probable. A cell moves into another cell, bringing benefits but also problems, setting off a complex cascade. And then, McBride says, “all this stuff blooms and blossoms.'”

Comment: all of this complexity is best explained by a designer at work. Please see the beautiful diagram of a cell with all of its many parts identified.