Doolittle event

There are a number of other possibilities for the explanation of the Doolittle event (Mooers and Redfield, 1996). One of these could be due to the analytical method used by Doolittle, as he assumed a relatively constant rate of amino acid substitution. This assumption may not be justified and should be checked. 

It is not only the vast difference of about 1.8 billion years which is so surprising the timescale for the separation point of the main branches of the tree of life is clearly shifted. The catastrophe hypothesis put forward to explain this difference appears unlikely, since there are no signs of such a phenomenal obliteration of all life on Earth. Another explanation could be that the data from the amino acid sequences provide only information on the way in which life forms diverged, but not on the timescale (Schopf, 1998). This interpretation of the Doolittle event by Schopf was provided at a time when doubts had not yet been cast on the dating of the first fossils at 3.45 billion years, published by him in 1993. 

If, in the near future, results were to appear which show that the first traces of life are not in fact 3.45 billion years old, the Doolittle event would return to the discussion and become more probable than it is at present. Thus, the tree of life is still decorated with many questions, which will hopefully decrease, and finally disappear, in the next few years. 

These observations suggest a lateral transfer of cellulose synthase from cyanobacteria to D. discoideum. However, while the primary and secondary endo-symbiotic events that led to the evolution of plastids in plants and algae provide a clear mechanism for the transfer of a cyanobacterial cellulose synthase to photosynthetic organisms, such a mechanism is lacking for D. discoideum. Cyanobacterial genes are known to exist in eukaryotes which have secondarily lost plastids. However, there is no evidence for the existence of an endosymbiotic relationship between ancestors of D. discoideum and a cyanobacterium. Therefore, if a lateral transfer occurred, it was likely xenologous, possibly via a food ratchet mechanism (Doolittle 1998). 

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