From Ralph Cooper
Colin Tudge’s article on the virtues of eating plants
(17 November, p 40)
gave us some good food for thought. It could be true that one or two vitamins
started life as toxins and that many other plant products not yet classified as
vitamins are beneficial. But when he generalises about toxins and vitamins, he
should know that biochemical ecology is much more complex than he implies.
Consider the small group of compounds fundamental to cell metabolism that
used to be called B vitamins. If they were originally plant toxins to which
animals became adapted and dependent, why do many bacteria have similar
requirements? Looking wider, it is indeed a mystery why so many of us non-plant
organisms cannot synthesise more of our chemical needs for ourselves.
It is not just vitamins. About half of the 18 amino acids needed by humans in
large quantities are “essential” and so, like a few bacteria, we need to eat
protein. Have we lost the knack of synthesising these necessary nutrients
because the food that gave our ancestors their energy always provided enough
vitamins and protein at the same time? Or is it that all of our ancestors, right
back to the original bacteria living in the primeval soup, simply failed to
evolve the necessary biosynthetic enzyme systems, despite the apparent selective
advantages of doing so, and despite the fact that plants and a vast range of
competent bacteria managed to do it?
Plants themselves have similar hang-ups. Many bacteria can get all their
nitrogen from the air, including many of the cyanobacteria that were the
presumed ancestors of all plant chloroplasts. But plants can only fix nitrogen
with the help of microbes, such as the root nodule bacteria of legumes. Why did
they lose such an apparently enormous nutritional advantage? Was Gaia looking
the other way?
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Hopetoun, Western Australia
