The origin of the eukaryotes--the kingdom of life that includes all of the higher plants and animals, including ourselves--took place in the heavily obscured early history of the earth. Consequently, there is still much speculation involved in answering this question. Carl Woese, a professor of microbiology at the University of Illinois at Urbana-Champaign and the discoverer of archaebacteria, offers one reply:
"Evidence from microfossils strongly suggests that life arose on the earth long ago, probably within a few hundred million years of the planet's formation. Sedimentary rocks 3.5 billion years old (and perhaps those 3.8 billion years old) contain what appear to be fossil stromatolites, which are natural colonies formed by photosynthetic bacteria; within the stromalites one can see microscopic forms reminiscent of bacteria. If these presumed bacteria are direct ancestors of extant photosynthetic bacteria, life was already well developed by then, having passed through the stages that led to the most recent universal ancestor and the splitting of the ancestral lineage into the primary lines of descent.
"By comparing molecular sequences to infer genealogies, molecular phylogeneticists tell us that the two primary lines of descent lead to the eubacteria (or common bacteria, which include the photosynthetic bacteria) and to a second common lineage that subsequently divided to form the archaea (which like the eubacteria are prokaryotes) and the eukaryotes (which include all higher plants and animals). All these events appear to have preceded the oldest fossil stromatolites. So the eukaryotic lineage appears to be very ancient, about as ancient as the two prokaryotic lineages.
"The key unanswered question here concerns when on the eukaryotic line the eukaryotic type of cell formed. Eukaryotic cells seem structurally far more complex than their prokaryotic counterparts (from which they arose), so biologists generally believe that many evolutionary steps must have separated the two. Nevertheless, the eukaryotic stem on the phylogenetic tree of life spawns many branches before one gets to the split that separates the ancestors of plants from the ancestors of animals, which seems to have happened more than a billion years ago. There seem to have been many earlier branchings from the eukaryotic stem, all represented by unicellular eukaryotes (such as the slime molds, the flagellates, the trichomonads, the diplomonads, the microsporidia, among others).
"Clearly, eukaryotic history goes back far into the era when the earth's atmosphere contained little or no oxygen, well over two billion years ago. Our current concept of the origin of the eukaryotic cell is in flux, however, and an evolutionary sequence that appears simple when conceptualized on a phylogenetic tree diagram may be far more complex and interesting in reality. We know that the eukaryotic cell is of ancient origin, but we do not yet know the evolutionary dynamic that underlies its formation."
J. Peter Gogarten in the department of molecular and cell biology at the University of Connecticut at Storrs, gives a broader overview:
"The question is the subject of an ongoing and lively controversy. The best guesses for the time when eukaryotes evolved range from just below 2.0 billion years to around 3.5 billion years before the present.
"One of the less ambiguous sources of information is the fossil record. Work by Gonzalo Vidal of the University of Uppsala in Sweden indicates that single-celled planktonic eukaryotes certainly date back to 1.7 billion years B.P. and very likely to at least 2.2 billion years B.P. The early fossil record is very sparse, however, and small eukaryotic cells present in the fossil record would not necessarily have been positively identified. My colleagues generally agree that the fossil record provides only a most recent estimate for the time when eukaryotes were already abundant; they might have been around a long time before they made it into the fossil record in a recognizable form.
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Add CommentMulticells-Life Date Pushed Back,
Reply | Report Abuse | Link to thisBut 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