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Eukaryotes - all descended from a single eukaryote?

  1. Jul 8, 2010 #1


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    The first of several posts on the evolution of life, mostly in the Archean and Proterozoic eons.

    This thread is about the origin of eukaryotes.

    I understand there are several different eukaryote phylogentic trees, but that (by definition?) all eukaryotes belong to a monophyletic domain - or is that just one hypothesis among many?

    Is the last common ancestor of eukaryotes - if there was one - a eukaryote? I expect there is no firm answer to this question, but I'd like to know what the main, current, hypotheses are.

    Related to this is the question of the origin of the eukaryote organelles, specifically the ones which have their own DNA.

    First, are the only eukaryote organelles with DNA mitochondria and plastids? If not, what others, even rare, eukaryote organelles have DNA?

    Second, as far as is known, do all eukaryote mitochondria have a common ancestor? Or do there appear to have been several separate instances of endosymbiosis?

    Third, same question for the plastids, perhaps separate questions for the various kinds (e.g. chloroplasts).
    Last edited: Jul 8, 2010
  2. jcsd
  3. Jul 9, 2010 #2
    There are of course many, many open questions in phylogenetics and systematics. Within margins of error, phylogenetic trees of eukaryotes generally converge. This is often termed consilience of independent phylogenies and is strong empirical evidence for common descent. What you might be saying is that there are many branches in the domain Eukarya itself, and this is of course true; not all Eukarya belong to the same phylum. Eukarya is, as I understand it, most likely monophyletic.

    It is important to separate the questions "was the last common ancestor of all currently living eukaryotes itself a eukaryote?" with "was the last common ancestor of all living and extinct eukaryotes (the "first ever eukaryote") itself a eukaryote?" The answer to the first question is probably yes, and the answer to the last may be currently unknown.

    Consider an analogy with humans. The most recent common ancestor of all humans alive today with respect to matrilineal descent is mitochondrial Eve. She lived around 200000 years ago, but this does not mean that she was the first human female, other females probably lived around her time, but their lineages eventually when extinct. We do not know very much about her personally, but it is safe to say that she did not die childless.

    As for your question of endosymbiosis, it is currently accepted that the ancestors of current mitochondria may have undergone endosymbiosis once, but chloroplasts have multiple origins. For instance, plastids are present in many different groups of unicellular eukaryotes which are closely related to organisms lacking plastids.
  4. Jul 22, 2010 #3


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    Thanks Mkorr.

    I kinda expected that the answer to the question "was the last common ancestor of all living and extinct eukaryotes (the "first ever eukaryote") itself a eukaryote?" would be "not known, and may never be known", but I thought it was worth asking.

    I'd read that chloroplasts seem to have had multiple origins, so thanks for the confirmation (I'd like to explore this in greater depth, but in another thread).

    Are mitochondria and plastids the only eukaryote organelles with their own DNA?

    And on terminology, are all chloroplasts plastids (I think yes)?
    Are all plastids chloroplasts (I think no, but don't know what ones aren't)?
    Do all plastids have their own DNA (I think yes)?
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