Thursday, February 26, 2009

"Did Anyone Observe the Origin of Eukaryotes?"

AUTHOR: Allen MacNeill

SOURCE: Original essay

COMMENTARY: That's up to you...

A creationist asked the following pretty typical question on another blog:
"Have prokaryotes ever been observed become eukaryotes via endosymbiosis?" [sic]

If by "observed", one means directly observed, then of course the answer is "no". As far as we can tell, this probably happened more than a billion years ago. But if only things that have been directly observed are valid, then virtually all of science, if not almost all human intellectual endeavors, are invalid and pointless.

Did anyone alive today "observe" the decline and fall of the Roman empire? Of course not. So, how do we know it happened? We read about it, or were told about it. We might also have directly observed some ruins in Rome or elsewhere in Europe or Asia Minor, and made some inferences about where they came from and how old they are.

But if direct observation is necessary to validate an assertion, then each of us is trapped in a tiny world whose borders are the limits of our own unaided perceptual apparatus. Not even most forms of logic would survive such an absurd and self-destructive limitation.

However, if one allows for indirect observation and logical inference, then the answer is "yes". There are multiple sources of empirical evidence for the assertion that eukaryotic cells arose as the result of the serial endosymbiosis of several prokaryotic ancestors. You can read a summary of this evidence here (scroll down; it's toward the end of the article).

Furthermore, this inference is made using the most reliable (i.e. "strongest") form of logical inference known to us: consilience. There are multiple, independently discovered and derived lines of empirical evidence pointing to the serially endosymbiotic origin of eukaryotic cells. That is, the evidence for the serial endosymbiosis theory is based on consilience, which is much more reliable that induction, deduction, or even abduction alone.

As in any case having to do with a very complex universe, there are "gaps" in our current model of the serial endosymbiotic origin of eukaryotes. There is also empirical evidence that is not entirely consistent with the model as it now stands. However, as more and more empirical evidence has been discovered, the vast majority of it has supported Lynn Margulis' original theory.

So, which method of validation shall we choose? Shall we voluntarily blind ourselves to the only kind of evidence that can validate things that have happened outside of our immediate perceptual environment, or accept what virtually all thinking people accept – that we must, almost everywhere and at almost all times, accept the validity of empirical evidence that we have not ourselves immediately obtained?

As always, comments, criticisms, and suggestions are warmly welcomed!


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At 2/26/2009 07:42:00 PM, Blogger NickM said...

Hi Alan -- nice post & link, but careful! The evidence is overwhelming for the symbiotic origin of mitochondria & chloroplasts, but pretty much no experts in the area, except Margulis & her group, accept the spirochete hypothesis for the origin of cilia/undulipodia/eukaryotic flagella (equivalent terms).

In fact, the evidence for this one keeps falling apart. The claim of DNA in the centrosome hasn't been replicated, the claim of eukaryotic tubulin (the major protein in cilia) in spirochetes hasn't been replicated, centrosomes/centrioles don't self-replicate anyway (they are now known to self-assemble in all kinds of situations).

That, plus spirochete motility is nothing like cilial/flagellar motility, either at the gross level or biochemically, and there are some severe, probably fatal problems with fusing the membranes of a spirochete with a eukaryote (as Cavalier-Smith pointed out in the 1980s).

Pubmed will bring up a number of recent papers exploring the much better supported autogenous idea, i.e. the cilium evolved out the cytoskeleton/mitotic spindle, which are themselves tubulin & dynein-based etc. and so much better relatives.

Cheers, Nick

At 3/05/2009 02:43:00 PM, Blogger -DG said...

I hadn't read the blog in a while and came in to post exactly what Nick has apparently already beat me to. There support for Serial Endosymbiosis (what little there was) is rapidly falling away. The evidence for the endosymbiotic origin of Chloroplasts and Mitochondria (as well as their descendent mitochondria-like organelles such as Hydrogenosomes, Mitosomes, and Apicoplast) as well as the secondary and tertiary endosymbiosis of photosynthetic eukaryotic algae by other eukaryotes is pretty overwhelming. But like Nick said, it's pretty much only Margulis and her groups sticking to their guns for the endosymbiotic origin of Flagella and cilia.

At 4/24/2012 05:27:00 PM, Blogger Fardad said...

Hi Alan.
This diagram is full of mistakes. Why are you using this?

At 4/24/2012 05:28:00 PM, Blogger Fardad said...

Why are you using this diagram? The mistakes in it are problematic.


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