= Evidence for bacterial ancestry of organelles
Resemblance is a clue, not sufficient evidence of <common descent>. A small size, two surrounding membranes, division, retained <DNA> or bacterial-looking <ribosomes> does not uniquely identify an ancestor. Internal membranes exist outside these <organelles>, and a similar function can favor similar structures. The stronger question is whether independently examined molecular characters support the same historical relationship.
A concrete genomic observation is the mitochondrial sequence reported in 1997 for <Reclinomonas americana>: $69{,}034$ <base pairs> containing 97 identified <genes>, including four for a bacterial-type multisubunit <RNA polymerase>. This is a reported <genome> inventory, not a calculation from the pictured shape of a <mitochondrion>. Retention of such <bacterial> features strengthens the ancestry argument; a living <Reclinomonas americana> is not itself an ancestral <eukaryote>. The inspected abstract supplies these observations but not the detailed sequencing protocol or contamination controls, so those details are not attributed to this report here. <An ancestral mitochondrial DNA resembling a eubacterial genome in miniature by B. Franz Lang and others>[Abstract, Nature 387 (1997), pp. 493–497: genome length, gene inventory and bacterial-type RNA polymerase]{ref}
A sequence-based investigation obtains <nucleotide sequences> by <DNA sequencing>, identifies comparable <genes>, makes a <sequence alignment> and uses <phylogenetics> to infer relationships. The readout is a proposed relationship with support and uncertainty, not a photograph of an ancient encounter. Comparisons of <plastid> sequences with <bacterial> sequences support cyanobacterial ancestry, while the exact closest sampled cyanobacterial group has depended on the chosen data and inference. A <bacterial> affinity in one <gene> is weaker than coherent evidence across several <genes> and cellular systems. <Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden>[“The endosymbiont: when and who?”: alternative phylogenetic placements and lateral gene transfer]{ref}
For such an analysis, useful checks include distinguishing <organelle> <DNA> from <bacterial> contamination, verifying sequence quality, comparing corresponding rather than unrelated <genes>, sampling several <bacterial> groups and testing sensitivity to the <sequence alignment> and evolutionary <mathematical model>. These are methodological requirements, not a claim that this article has rerun a particular published analysis. <Horizontal gene transfer>, <gene> loss and unequal evolutionary change can make an individual <gene>'s history differ from a whole <organism>'s history. A supported <bacterial> origin therefore need not identify an extant donor <species>, an exact date or the original mode of entry. <Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden>[“The endosymbiont: when and who?”: ancient origin and confounding lateral transfer]{ref}
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