How Eukaryotes Got Their Mojo

Warning: Serious biological technogeekery ahead.

For much of earth’s history, the only form of life was single-cell, primitive bacteria. Those early primitive cells are called prokaryotes. Basically, they were small sacks of fluid with the contents mixed together, higgedly-piggedly. Sometime about 1.5 billion years ago, a few lucky prokaryotes ingested or otherwise acquired another cell or another cell’s functions, and instead of digesting it, incorporated that alien cell into the host’s cellular structure. That cell-in-a-cell evolved into mitochondria, the biologic engines that drive most cellular life today, including you, gentle reader.

For a long time – and certainly in the middle Pliocene when WC took his high school biology class – it was treated as an event that happened all at once, presented as an instance of a form of endosymbiosis that gave that pioneering cell an evolutionary advantage. With that advantage, the combined cells out-competed their more primitive prokaryotic cousins and came to dominate life on our planet. It’s a nice, simple story and, like most nice, simple stories, it’s likely wrong.

Prokaryotic cells like modern-day bacteria and archaea tend to be small and simple, with few if any internal structures. Eukaryotic cells such as those of modern-day plants and animals are much more complex and have many internal structures, called organelles, that carry out specific functions.

if you were unlucky enough to study cell biology in undergraduate school – WC’s class was at the criminal hour of 7:30 AM – you were made to memorize all of the structures and their functions in a modern, eukaryotic cell: nucleus, nucleolus, ribosomes, rough and smooth endoplasmic reticulum, Golgi apparatus, cytoskeleton, lysosome, and centriole. WC gets drowsy by associative reflex, just thinking about it.1 More than one researcher has noted that the sheer number of internal components in a modern cell suggest the appearance of mitochondria was a process, not an event. Gradualism, not catastrophism, to borrow a distinction from geology.

The biologic evidence for gradualism is mounting. Sophisticated gene sequencing is being used to trace back to the ancestors of both the parent cell and the mitochondrial endosymbiotic in a wide variety of modern eukaryotic cells. The idea was to trace back to the ancestors of each. To the researchers’ surprise, they found genes that seemed to come from a wide range of other bacteria. If the merger of the ancient mitochondria and the primitive prokaryotic cells was a dance, those ancestral cells danced with more than one partner. Indeed, they danced with everyone in the room.

it’s difficult to know what those other dance partners might have been, because much of the life from that era is long since extinct. But the genes for all those sophisticated internal components, and even from the mitochondria, pretty clearly didn’t come from a single ancestor.

So, the researchers say, the model for evolution of eukaryotic cells is changed:

It’s reasonably clear that eukaryotes evolved from more primitive, prokaryotic cells like bacteria and archaea, and that those cells in turn eventually acquired (or had already acquired) the complex traits like internal, membrane-bounded structures called organelles, but the exact sequence of events remains poorly understood, and many species presumed to be intermediate stages – the (X) in the drawing – are now extinct.

In particular, the debate is whether the ancient symbiosis that led to mitochondria occurred early in the process – suggesting that it was a key trigger – or was a later refinement.

Consider mealybugs. Mealybugs have two bacterial endosymbionts, one nested inside the other in the mealybugs’ cells.2 The endosymbionts make essential amino acids that the mealybug can’t get from its diet. For that amino acid synthesis to work, mealybug cells had to evolve an apparatus that transported proteins back and between what were once independent organisms, allowing the proteins from the mealybug cell to journey across two sets of endosymbiont membranes for use by the innermost endosymbiont, and then back across the endosymbiont membranes to the rest of the cell. Geonomic analysis shows that apparatus is a mosaic made up of genes that originated with the bugs themselves, others that came in with their endosymbionts and still others that were picked up by horizontal transfer from other microbes in the environment.

The mealybugs research has begun to reveal how endosymbionts become integrated with their hosts and suggest that horizontal gene transfers from various sources might have been quite frequent early in the evolution of eukaryotes, too. It doesn’t show that is what happened in the formation of the mitochondria, but it suggests that it’s possible.

Other cell biologists are unpersuaded. They point out that the evidence is all from modern eukaryotic cells, which may have long ago evolved the ability to do this kind of “feature shopping.” Or perhaps the foreign genes in mitochondria were incorporated into the proto-mitochondrial cell before it merged with that proto-bacterial cell all those millions of years ago.

Research continues. But WC always finds it interesting when longstanding assumptions in science are challenged and questioned by new evidence. That’s what science is all about.

1 Any errors in describing the biologic processes in this blog post can be blamed upon the well-known fact that mammals, and especially primates, don’t learn as well or retain information as thoroughly when it is inflicted on them too early in the morning.
2 Greater fleas have little fleas/Upon their back to bite ’em/And little fleas still lesser fleas/And so, ad infinitum.. —  Augustus De Morgan. De Morgan’s rhyme probably traces to (and improves upon) Jonathan Swift‘s long satirical poem “On Poetry: A Rapsody” of 1733.

One thought on “How Eukaryotes Got Their Mojo

  1. “ingested or otherwise acquired another cell or another cell’s functions,” ?
    I’m not sure if you intended that breezy summary to be amusing or not but it definitely tickled my funny bone.🤔😉😁

    Liked by 1 person

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