How did complex life evolve? The answer could be inside out

A new idea about the origin of complex life turns current theories inside out. In the open access journal BMC Biology, cousins Buzz and David Baum explain their 'inside-out' theory of how eukaryotic cells, which all multicellular life - including us - are formed of, might have evolved.

Scientists have long pondered the question of how simple "prokaryotic" , like bacteria, which are little more than a membrane-bound sack, evolved into more complex eukaryotic cells, which contain numerous internal membrane compartments. These compartments include the nucleus, which holds genetic information in the form of DNA; the , which shunts proteins and lipids around the cell; and mitochondria which act as the cell's powerhouse. The mitochondria also contain their own distinct DNA, which is one good indicator of their once having been separate organisms. The trouble is that no one has identified eukaryotic cells that are intermediate in complexity, making it much harder to know how they evolved.

At present, the most widely accepted theory is that mitochondria derive from a bacterium that was engulfed by an archaeon (plural = archaea), a kind of prokaryote that looks similar to a bacterium but has many molecular differences. The eukaryotic membrane systems, including the nuclear envelope, then formed within the boundaries of this archaeal cell through the invagination of the . This fits with much current data, but a few problems remain. Most significantly, no archaeal cells are known that invaginate membranes.

Furthermore, it seems unlikely that mitochondria were engulfed since engulfing food requires a lot of energy, which in eukaryotes is provided by mitochondria, and engulfment likely also requires mitochondrial-derived lipids.

David Baum, University of Wisconsin, says: "All agree that eukaryotes arose from a symbiotic relationship between two cell types: bacteria that became mitochondria and a host cell, archaea, or a close relative of archaea, that became the cytoplasm and nucleus. This symbiosis explains the origin of mitochondria, but what about other eukaryotic structures, most notably the nucleus?"

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 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. The Baums use some examples of modern archaea that produce bleb-like protrusions to support the credibility of their ideas, and draw on many common features of eukaryotes that are easily explained by the inside-out model.

Like any good scientific theory, the inside-out model leads to predictions that can be tested in modern cells. The Baums hope, therefore, that their theory will stimulate empirical research, since there is still a lot that it not known about the biology of prokaryotic and eukaryotic cells.

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 doesn't have to be right to be useful, if it provokes people to think. And to test it."

Buzz Baum, University College London, says: "Even if the hypothesis or parts of it are refuted, we are optimistic that the effort to evaluate it will spawn new cell biological discoveries and, in so doing, will improve our understanding of biology of eukaryotic cells as they grow and divide. Although students studying cell biology may come to think that it's too late for them to contribute to a field where almost everything is known, this simply isn't the case. As the model helps to make clear, there is still much to be discovered about the basic logic of eukaryotic cell organisation."


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New study upends current theories of how mitochondria began

More information: BMC Biology, www.biomedcentral.com/1741-7007/12/76
Journal information: BMC Biology

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Citation: How did complex life evolve? The answer could be inside out (2014, October 27) retrieved 18 June 2019 from https://phys.org/news/2014-10-complex-life-evolve.html
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Oct 27, 2014
Do rickettsiales have pre-cristae junctions?
I am more interested in the origin of the inner membrane structure, do any bacteria have anything like them.

Oct 27, 2014
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Oct 28, 2014
i love reading about the origins of life research, from the very beginnings of organic molecular self assembly, the arrival of the proto cells and the beginning of metabolism cascade chains.

this research has a history going back many decades and is very fascinating.

however, this particular question----how multi-cellular life came to be will NEVER really be known to any satisfactory degree. at best, what will happen is that one day someone proves that they are able to replicate some of the mechanisms in some model of archea and bacteria and then shows something novel in the way of exhibiting features of what might become a multicellular species.

it strains the imagination to believe a research lab will simply perfectly recreate a transition from single cells to multicellular ones in one big gap jump. and even if they do, it still wont be full proof that that is how the gap jump was originally made.

i'm inclined to believe there were many of intermediary species lost.

Oct 28, 2014
To seek evidence for the impossible is a lost cause, but this exercise paid a salary and support families. Some scientific disciplines have become as the legal profession. The goal is not to reach the true picture but to work on a problem as long as possible. Sensations are made from time to time to maintain the interest of sponsors and a sense of progress in the audience. Impressive animations in popular programs that promote it, captured the imagination of young people and create a feeling in them that science can do anything, but this is ilusion. In fact we improve only known technology, but there are no real fundamental discoveries for several decades due to mistaken principles and this is real crisis.

Oct 28, 2014
Too little, too late.

The nuclear membrane has the wrong topology due to its openings (akin to how a cup with an ear has a different topology from a deep plate) so presumably it and ER evolved separate from the outer membranes.

And the mitochondrion likely derives from a parasitic clade of alpha-proteobacteria, so it wasn't engulfed but inserted itself. [ http://www.ploson....0110685 ]

Such insertion methods likely evolved already in the RNA/protein UCA lineage, since dsRNA viruses has such insertion mechanisms (secretes muralytic cell wall degrading enzymes), in which case they have been used long before the mitochondrion event. [ http://en.wikiped..._%CE%A66 ]

But yes, testable hypotheses are good. I just don't expect this will turn out positive.

Oct 28, 2014
@teslaberry: This isn't research on the emergence of life (about 4 billion years ago), it isn't research about complex multicellular life (about 0.5 billion years ago), this is research about the emergence of eukaryotes (about 1 billion years ago).

Multicellular lineages has evolved 20+ times independently, complex multicellulars a handful of times times (7 IIRC - too many "plant" lineages to bother with, once in animals and once in fungus). We have evolved it twice in the lab in less than a year. [ http://www.newsci...a9PmG8fY ]

Oct 28, 2014
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Oct 28, 2014
And no one has identified intermediate species in the evolutionary roadmap, either. They just aren't there.
@verkle
blatant lie based upon faith based delusional belief supported only in your fear, hypocrisy and illiteracy

guess you don't read much science, which makes me wonder, why do you bother posting to a science site when you obviously cannot intelligently supply proof of your claims?

feel free to refute the following: http://www.tim-th...ils.html
when you attempt your faith based delusional reply, make sure you address all of the referenced material in the link, which is extensive and includes the links at the bottom which directly prove you are a liar and going to h*ll based upon the tenets of your own religion

thanks


Oct 28, 2014
verkle claimed
As long as you cling to the defunct theory of evolution, things like complex life will never make sense. No matter how many scientists and $$$ you throw at it to try to figure it out.
Already told you, its massive amount of permutations over massive amount of time with massive number of environments. Didn't you notice all life is made from common things and all life has Amines - ie From ammonia - early earth's atmosphere...

verkle with blinkers onq]And no one has identified intermediate species in the evolutionary roadmap, either. They just aren't there. You Lie.
They are there, have been found & examined.

Why are you UNABLE to respond to those notices on other threads where you have made the same lies, are you so blinkered ?

Tell us how your deity is such a good & caring communicator ?

Told us about first aid, ensures timely & constructive feedback & crystal communications with ALL ?

Or is it an ego/status based claim in a book - Moses ?

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