A humbling reversal?

I have long noted that the case for non-teleological evolution was stronger in the past than it is in the present.  Consider this tiny example.

Below is a figure from Eukaryotic Evolution: Getting to the Root of the Problem (Simpson and Roger, Current Biology, Vol. 12, R691–R693, October 15, 2002).

The figure on the left is the eukaryotic phylogenetic tree from 1993 and before.  Simpson and Roger explain it as follows:

A decade ago, phylogenies based on small subunit ribosomal (r)RNA sequences provided an intuitively appealing evolutionary tree of eukaryotes. Complex eukaryotes, including animals, fungi, plants and most algae, emerged as a broad radiation usually called the ‘eukaryotic crown’ [1]. Below this ‘crown’, more bizarre, and generally simpler, organisms diverged in a ladder-like succession. The small subunit rRNA tree was ‘rooted’ with mitochondrion-lacking unicellular eukaryotes such as diplomonads, parabasalids and microsporidia forming the basal branches (Figure 1a).

Yes, this was intuitively appealing from a non-teleological, neo-darwinian viewpoint.

Those early branching lineages reflected a simple, bizarre state without mitochondria and provided a nice image of a primitive beginning.  Then, after hundreds of millions of years of selection pressure for multicellular existence, a more complex and sophistitcated “crown” appeared, including animals and plants.

In a sense, this phylogeny nicely reflected the words of Richard Dawkins, in his book, “The Greatest Show on Earth”:

One thing we can say, on a basis of pure logic rather than evidence, is that Darwin was sensible to say, ‘from so simple a beginning.’…..The beginning had to be simple, and evolution by natural selection is still the only process we know whereby simple beginnings can give rise to complex results.”

But alas, with the analysis of more genes, the tree  changed dramatically, no longer reflecting a bizarre simple beginnings evolving into the complex results of the crown groups (see the tree on the right).  As Simpson and Roger explain:

Stechmann and Cavalier-Smith [3] tentatively favour a rooting between amoebozoa plus the fusion cluster on one side, and the opisthokonts alone on the other. This would place animals, fungi and their relatives in the ‘basal’ position in the eukaroytic tree: a humbling reversal for humans when compared to our previous lofty ‘crown’ position under the small subunit rRNAbased model.

Now that’s a strange sentence.  Why in the world would it be humbling to belong to a lineage that occupied the ‘basal’ position in the eukaroytic tree?  I sure don’t think it is humbling to realize that human cells may more closely reflect the ancestral eukaryotic state than algae, diatoms, or paramecia.  That animals, fungi and their relatives may be at the ‘basal’ position in the eukaroytic tree fits more comfortably with front-loading than the old ‘crown’ view.  It’s almost as if these complex, ancestral eukaryotic cells were poised to become animals very early on, while the algae, diatoms, or paramecia missed the cue and shot off on tangential pathways.

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9 responses to “A humbling reversal?

  1. It’s almost as if these complex, ancestral eukaryotic cells were poised to become animals very early on, while the algae, diatoms, or paramecia missed the cue and shot off on tangential pathways.

    K-E-W-L!
    thanks for that!

  2. I hate to sound like a broken record, but this seems to intensify the problem of getting from prokaryotes to eukaryotes. Are you still sticking with the one, initial design event, or have you thought of going with two design events?

  3. No, I would not invoke any second design event for eukaryotes. So far, the evidence suggests that bacteria and archebacteria fused to form a lineage that led to the eukaryotes. In the past, I have explored the needless complexity of archebacteria and how this makes sense in the light of front-loading. I have also reviewed some of the preadaptations involved in the evolution of mitochondria. A second design event would need to account for the preadaptations and needless complexity.

    However, I would agree that the prokaryotic to eukaryotic transition was the most radical leap in all of evolutionary history. And I think it highly unlikely that this transition occurred gradually over a long period of time.

  4. So far, the evidence suggests that bacteria and archebacteria fused to form a lineage that led to the eukaryotes.

    That may explain mitochondria and chloroplasts, but not the nucleus.

    However, I would agree that the prokaryotic to eukaryotic transition was the most radical leap in all of evolutionary history.

    Not of you look at it as eukaryote to prokaryote transitions.

    It was you, was it not, that posted something on Telic Thoughts referencing an article about that very thing?

    Is there any reason why front-loading had to start with prokaryotes?

  5. That may explain mitochondria and chloroplasts, but not the nucleus.

    I don’t agree. The archaeal chromosome is a single, circular piece of DNA, like bacteria. And like bacteria, it is not surrounded by a nucleus. Yet unlike bacteria, the DNA is associated with proteins much like eukaryotic histones. And unlike bacteria, they archae use transcription factors and RNA polymerases much like the eukaryotic versions. In other words, even though archaea are so bacteria-like that they were once classified as exotic bacteria, a closer look shows a cell loaded with machinery that foreshadows the eukaryotic cytoplasm and nucleus.

    Is there any reason why front-loading had to start with prokaryotes?

    It wouldn’t have to. But it would seem more likely in that you can only front-load metazoan-like life to come into existence if you first terraform the planet to prepare the stage. And prokaryotes appear to be the superior terraformers (as I have discussed on this blog).

  6. Looks like the last common ancestor of eukaryotes was about as complex as an individual animal cell.

  7. The archaeal chromosome is a single, circular piece of DNA, like bacteria.

    And a eukaryote’s nuclear chromosomes are linear.

    But anyway my point is one could start with populations of single-celled eukaryotes front-loaded to not only produce those terra-forming prokaryotes but to also sit in waiting for the right epigenetic trigger(s) [once the terra-forming reached a certain point] to bust out the metazoans in them.

  8. Mike: I would agree that the prokaryotic to eukaryotic transition was the most radical leap in all of evolutionary history.And I think it highly unlikely that this transition occurred gradually over a long period of time.

    I wonder if you could elaborate on that last point.

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