Classifying Eukaryotes Easier than Evolving Them

first_imgIf you like stories with surprise endings, check out an otherwise boring paper by two Canadian evolutionary biologists, Alistair G. B. Simpson and Andrew A. Roger, in the Sept. 7 issue of Current Biology.1  Their subject is the real “kingdoms” of eukaryotes (that’s all creatures with nuclei, including plants, animals, and a host of single-celled organisms).  Unhappy with previous artificial classifications, including the “grab-bag” grouping called Protista (common in most textbooks), they propose more biologically realistic groupings based on phylogeny and the findings of molecular genetics.  The assumption is that a natural group should include an ancestor and all its descendants.    The paper lumbers along through their new proposed groupings: Opisthokonta (animals, true fungi, and all You Karyotes out there), Amoebozoa, Plantae (you guessed it), Chromalveolata (try that on Jeopardy), Rhizaria, and Excavata (not backhoe drivers, but a group of heterotrophic flagellates).  Each of these are “well-known” groupings that could be called kingdoms; there might be a few others that cannot be fitted into these six.  So far so good.  This is a paper about ancestors and descendants, right?  So where is the ancestor of all these eukaryotes?  Surprise:Identifying six natural groups of eukaryotes raises the question: what are the relationships amongst them?  Molecular phylogenetics could provide the answer in principle, but there are tremendous practical difficulties.  As we look further back in time, most historical signal is lost from present day molecular sequences, so that non-historical (artefactual) signals in the same data can easily obscure the true relationships. (Emphasis added in all quotes.)Keep reading.  Although they suggest that Amoebozoa and us might be related (sorry to hurt your self-esteem), the deep level structure and timing of the divergence of these groups “remains contentious.”  There is “huge disparity” in timing of the split, from 1.2 to 2.8 billion years.  Without new and better fossils, “the precise age of eukaryotes and the tempo of their divergence are unlikely to be resolved in the near future.”  In fact, eukaryotes are so different, the authors leave it unsolved whether there is any reason to think there was a common ancestor at all:Eukaryotic cells are drastically different from their presumably prokaryotic ancestors.  With the limited fossil record, researchers have tried to understand the evolution of the eukaryotic cell by identifying living eukaryotes that are ‘primitive’ in some aspects.  The primitive status of a group is untenable, however, if phylogenetic studies indicate that it is closely related to ‘complete’ eukaryotic cells.  In fact, all the groups of eukaryotes seriously suggested to be primitive eukaryotes now seem to be related to ‘complete’ forms (most fall within Excavata).  The last common ancestor of living eukaryotes now appears to have been a ‘complete’ eukaryotic cell.  It had a nucleus, endoplasmic reticulum and Golgi apparatus, and underwent mitosis and meiosis.  It had mitochondria capable of oxidative phosphorylation, amongst other functions, and was presumably aerotolerant.  It had a complex eukaryotic cytoskeleton including eukaryotic flagella (most likely a pair of them), and was heterotrophic, consuming food particles by phagocytosis.  The only major eukaryotic features that seem to be of later origin are plastids.  We are now left with an intriguing and difficult question: did living eukaryotes diverge [shortly after the rapid and drastic evolution of the eukaryotic cell, or was this cell assembled gradually, but with modern eukaryotes then replacing all intermediate forms?To be or not to be an ancestor, that is the question, and that is where this “evolutionary” paper ends.1Simpson and Roger, “The real ‘kingdoms’ of eukaryotes,” Current Biology, Volume 14, Issue 17, 7 September 2004, Pages R693-R696, doi:10.1016/j.cub.2004.08.038.If you thought the missing link was somewhere between Java Man and us, behold The Missing Chain.  Either way, the Darwin Party is stuck in Story Book Land.  Either evolution was so rapid it happened without a trace, or it was slow and gradual, but the later organisms replaced the earlier ones, who disappeared without a trace.  This is reminiscent of another paper last month that left us readers staring at nothing with the advice, “watch this space” (see 08/19/2004 commentary).  Sometimes no comment is the best comment.  Let the Darwinists admit to themselves they have no evidence for their belief system.  We suggest that the phrase “watch this space” would be the perfect theme for the Darwin Party Curriculum.  It could be the title of their next textbook on evidence for evolution.  Every page could contain the caption, “This page unintentionally left blank.”(Visited 10 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

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