"Another approach is to date phylogenies by looking at a 'clock' of molecular changes that accumulate in the genetic code. Although that approach has been successfully used to decipher relations between organisms, calibrating it to measure the time elapsed since the divergence of phylogenetic branches is problematic; concerning the early evolution of life, there is no generally accepted approach. Most attempts to date early molecular phylogenetic trees used the emergence of eukaryotes (around 2.0 billion years B.P.) as a calibration point. Russell F. Doolittle and his co-workers at the University of California at San Diego recently attempted to extend the calibration further into the past, but this work is contested. The controversy centers on possible cases of horizontal gene transfer across phylogenetic branches that were ignored by these authors and on an insufficient correction for multiple substitutions. In addition, these backward extrapolations assume that the rate of molecular change at the time the eukaryotes originated is the same as it was during the metazoan evolution, when in fact it was probably much faster.

"The typical eukaryotic cell resulted from a symbiosis between different prokaryotic ancestors. Three prokaryotic components can be traced by comparing molecules in extant prokaryotes and eukaryotes. These components are the mitochondria (derived from purple bacteria), the plastids (from cyanobacteria), and the nucleocytoplasmic component (from archaebacteria). Other features in eukaryotic cells--for instance, the cytoskeleton--may also be of bacterial descent, but so far the molecular record has not yielded unambiguous clues as to their origin.

"The nucleocytoplasmic component of the eukaryotic cell branches off very early in the evolutionary radiation of the archaebacteria. There is an active dispute as to whether some of the archaebacteria are more closely related to the eukaryotic nucleocytoplasm than are others (proponents of the differing views are James Lake of the University of California at Los Angeles and Carl Woese of the University of Illinois at Urbana-Champaign). Regardless of how the debate is resolved, the ancestor of the eukaryotic nucleocytoplasm must have separated from the archaebacteria early in, or even before, the era when the major archaebacterial groups arose. In contrast, bacterial ancestors of mitochondria and plastids separated from the eubacteria lineage only after the evolutionary radiation that gave rise to the major eubacterial kingdoms. Therefore, it is likely that primitive eukaryotes lacking mitochondria and plastids were around a long time before they made it into the fossil record. I would be surprised if such eukaryotes did not date back to at least 3.0 billion years before the present.

1 Comments

1. 1. Dov Henis 05:27 AM 2/10/09

   Multicells-Life Date Pushed Back,
   But With Confused Terminology
   
   
   A. From "Animal ancestors may have survived snowball Earth"
   http://www.sciencenews.org/view/generic/id/40580/title/Animal_ancestors_may_have_survived_%E2%80%98snowball_Earth%E2%80%99
   Chemical fossils in Precambrian sedimentary rock push back the first date for animal life.
   
   1) Finding a molecule that was made by an organism, means that the biosynthetic ability to make that molecule must have evolved earlier than the Cambrian period, which extended 544 to 500 million years ago with marine invertebrates. Biosynthetic ability had to be around by at least 635 million years ago and, the researchers say, maybe as early as 751 million years ago.
   
   2) Theres evidence of eukaryotic life, organisms with DNA sequestered in a protective nucleus, from roughly 1.9 billion years ago, but proper multicellular animals dont appear on the scene until much later.
   
   
   B. Confused terminology
   
   The report mostly and clearly attributes ONLY to multicells organisms the terms animal ancestors, animal life, organism, biosynthetic ability. Yet it terms also an 'organism' a maybe, probable, 1.9 billion yrs old 'eukaryotic life', even if implying that it might have not been 'proper multicellular animals'.
   
   
   C. Suggested clarification of scenario and terminology
   
   From "Life's Manifest"
   http://www.the-scientist.com/community/posts/list/112.page#578
   
   - First were independent individual genes, Earth's primal organisms.
   - Genes aggregated cooperatively into genomes, multigenes organisms, with genomes' organs.
   - Simultaneously or consequently genomes evolved protective and functional membranes, organs.
   - Then followed cellular organisms, with a variety of outer-cell membrane shapes and
   functionalities.
   - Cellular organisms (3rd stratum organism): mono- or multi-celled earth organisms.
   
   Since evolution is definitely driven by culture, the evolution of multicelled organisms was preceded by evolution of cooperative community life culture of the monocelled organisms. Most presently observed biological processes and internal organs in multicelled organisms have originated and evolved by and during the evolution of the cooperative community life culture of the monocelled organisms...
   
   
   Suggesting,
   
   Dov Henis
   (Comments From The 22nd Century)
   http://blog.360.yahoo.com/blog-P81pQcU1dLBbHgtjQjxG_Q--?cq=1

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