Until recently, it seemed clear that prokaryotes had dominated the Earth for a billion years or more, before the first eukaryotes appeared. However the evidence is far from clear now. First, as we have seen, molecular reconstructions of the universal tree of life do not confirm that Eucarya arose later than Bacteria or Archaea, as had been expected. In fact, all three domains might have arisen at about the same time for all we know. Geochemical data from biomarkers has also given surprising evidence.
Biomarkers are organic chemical indicators of life. Most biomarkers are lipids, fatty and waxy compounds found in living cells. Some biomarkers are indicative of life in general, but others can be associated with particular domains or kingdoms. In 1999, Jochen Brocks, a research fellow at Harvard, and colleagues, announced new biomarker evidence from organic-rich shales in h Australia dated at 2.7 billion years ago. As expected, some of the ° biomarkers were indicators of cyanobacteria, but the investigators 1' also unexpectedly identified C28-C30 steranes, which are ¡5
sedimentary molecules derived from sterols. Such large-ring sterols are synthesized only by eukaryotes, and not by prokaryotes. So, this biomarker evidence confirms the existence of cyanobacteria at least 2.7 billion years ago, but it is also the oldest hint of the occurrence of eukaryotes, long before any fossils.
But how could eukaryotes, with their complex internal structure of nucleus and other organelles, have arisen from simpler prokaryotes? The most popular idea has been the endosymbiotic theory, proposed by Lynn Margulis, then a young faculty member at Boston University, in 1967. According to her theory (Fig. 7), a prokaryote consumed, or was invaded by, some smaller energy-producing prokaryotes, and the two species evolved to live together in a mutually beneficial way. The small invader was protected by its large host, and the larger organism received eukaryotic cell in an animal, a fungus, or certain protists eukaryotic cell in a plant or in certain protists
Primitive prokaryotic host cell
Él invagination (folding in)
...turned into chloroplasts o
...turned into chloroplasts
° 7. The endosymbiotic theory for the origin of eukaryotes supplies of sugars. These invaders became the mitochondria of modern eukaryote cells. Other invaders may have included worm-like swimming prokaryotes (spirochaetes) that became motile flagella (the whip-like appendages used by some micro-organisms to get around), and photosynthesizing prokaryotes that became the chloroplasts of plants.
The endosymbiotic model is immensely attractive, and some aspects have been confirmed spectacularly. Most notable is that the mitochondria and chloroplasts in modern eukaryotes are confirmed as prokaryotes, the mitochondria being closely related to alpha-proteobacteria and the chloroplasts to cyanobacteria. So the amazing thing is that a modern eukaryote cell has proven prokaryotic invaders that possess their own DNA and that coordinate their cell divisions with the divisions of the larger host cell.
Many experts reject the endosymbiotic theory, or at least most of it. They point out that the only real evidence for engulfment is for the mitochondria. There is no evidence to support the idea that the nucleus was engulfed, nor is it clear what kind of prokaryote did the engulfing, and in fact engulfment is seen today only among eukaryotes, and not among prokaryotes. So the alternative view, termed the protoeukaryotic host theory, is that an ancestral eukaryote, the so-called protoeukaryote, already equipped with a nucleus, indeed did engulf an energy-transferring prokaryote that became the mitochondrion. But this does not tell us where the protoeukaryote itself came from. Further doubt is cast on the classic endosymbiotic theory by the suggestion that neither Archaea nor Bacteria appear to be ancestral to Eucarya, and that biomarker evidence indicates an unexpectedly ancient origin for eukaryotes. Back to the drawing board!
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