Giardia retired from missing link status, according to articles in Nature vol. 426, p127 and 172, 13 Nov 2003. A single celled organism named Giardia intestinalis has been regarded as a living fossil from the time when simple cells called prokaryotes were evolving into more complex cells called eukaryotes. The difference between these two types of cells is that eukaryotes store their DNA in a membrane bound nucleus and make chemical energy in complex structures called mitochondria. Evolutionary theory says that prokaryotes turned into eukaryotes by evolving a nucleus and then engulfing other prokaryotes, which then became mitochondria inside the cell that engulfed them. The cell that originally did this "still has biologists dumbfounded" but evolutionary biologists have used organisms like Giardia as evidence for the theory because Giardia has a nucleus but not mitochondria.

Scientists have now found that Giardia has structures similar to mitochondria called mitosomes. These are much smaller than mitochondria and do not make chemical energy but they do make iron-sulphur clusters - usually a function of mitochondria. Therefore, the scientists came to the conclusion that Giardia once had mitochondria but they have been reduced in function. This means Giardia can no longer be used as an example of an evolutionary intermediate between prokaryotes and eukaryotes.

Editorial Comment: It is about time biologists gave up on the engulfed bacterium theory and admit that mitochondria and mitosomes are exactly what they appear to be - complex structures well designed for carrying out chemical reactions. Giardia have never shown any evidence of being anything other than fully functioning single celled orgnisms that have not evolved from any other single celled organisms. (Ref. Giardia, mitochondria, mitosomes)


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