The Baums’ inside-out theory provides a gradual path by which eukaryotic cells could have evolved. The first stage began with a bacterial cell whose outer membrane forms protrusions, which the Baums call ‘blebs’, that reached out from the cell. These protrusions trapped free-living mitochondria-like bacteria between them. Using the energy gained from being in close contact with bacteria (and using bacterial-derived lipids), cells were able to get bigger and expand the size of their blebs.
The sides of the blebs formed the endoplasmic reticulum and their inner surfaces formed the outer membrane of the nucleus, with the original outer membrane of the archaeon becoming what we now call the inner nuclear membrane. Finally, the fusion of blebs with one another led to the formation of the plasma membrane. The result was the eukaryotic cell as we now know it. This inside-out theory is explained in more detail using a diagram in the research article (see notes to editors).
David Baum explains the differences between the outside-in and inside-out theories using a metaphor: “A prokaryotic cell can be thought of as a factory composed of one large, open building in which managers, machinists, mail clerks, janitors, etc. all work side by side. In contrast, a eukaryotic cell is like a factory complex, composed of a several connected work spaces: a single control room and specialize rooms for receiving, manufacturing, shipping, waste disposal, etc. The traditional theories propose that the factory complex arose when partitions were built within a single hangar-like building. The inside-out theory, in contrast, imagines that a series of extensions were added around an original core building — now the control room — while others functions moved out into new, specialized quarters.”
The inside-out theory is radically different from all existing theories because the action in building the eukaryotic cell is outside the boundaries of the ancestral cell. As David Baum, who came up with an outline of the model 30 years ago, when still an undergraduate, noted: “The inside-out model ought to be an obvious alternative to the outside-in models, but maybe you have to be a naive undergraduate to consider such an inverted perspective.”
We can’t know how these very early evolutionary steps occurred, but we can look at current processes for inspiration.
Wait. Hold that last thought, will you?
Some of us thought the whole discussion was supposed to be about “how these very early evolutionary steps occurred,” but it turns out we can’t know?
The trouble with modern archaea that produce bleb-like structures, introduced as evidence, is that they don’t go on to do what is proposed here.
Commenting on the inside-out theory Miranda Robertson, Editor of BMC Biology, says: “Not everyone is going to be convinced by this theory — any reconstruction of events in a past as far distant as the origin of eukaryotes is going to have areas of uncertainty which it would be futile to try and fill in. But a theory doesn’t have to be right to be useful, if it provokes people to think. And to test it.”
Agreed, for sure, but how can it be tested? Well, “blebs” is a great new word, anyway.
See also: Did a low oxygen level delay complex life on Earth? (For a more conventional approach to such questions.)
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