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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Sunday 13th November 2011

I’ve been reading a paper, from 2008, written by a trio of evolutionary biologists from America, Canada and Germany. I’ve written before 29th Oct that I’m not always happy with the way people describe the process of evolution when referring to the properties plants currently have.

It seems to me that evolutionary biologists are keen to explain absolutely every interaction between different species in terms of evolution but I don’t believe that to be the case. I’ll just repeat that I don’t for a moment doubt that evolution exists and works. I just don’t think there is an evolutionary explanation for everything.

Take Heracleum mantegazzianum, giant hogweed, as an example. As far as has been established, the production of furocoumarins by plants is a response to the presence of a root fungus. The plant produces the furocoumarins to prevent the root fungus from doing so much damage that the plant dies. In evolutionary terms, the two organisms have arrived at a balance where all the versions of the plants that didn’t produce enough furocoumarins have died out and all the versions of the fungus that weren’t put off by the furocoumarins killed their host and, thus, died themselves.

But, furocoumarins act on the skin of humans to make it sensitive to light resulting in burns. The burning doesn’t happen for up to 24-48 hours after the contact so it doesn’t stop a human from completely cutting down a giant hogweed. There’s no way to claim that the plant benefits from making human skin so light sensitive as to cause pain and harm.

There’s no evolutionary mechanism in this. The evolution is going on between the plant and the root fungus. What happens to humans is just collateral damage, in today’s jargon. But, going from the paper I referred to above, I would expect evolutionary biologists to want to prove there was some purpose in this interaction.

Not that the paper had anything to do with phytophototoxicity. What it was looking at was the effect of psychoactive substances on humans. The authors acknowledged that current theories of what psychoactive substances do create a paradox in terms of evolutionary interaction between plant and humankind but, rather than say there is no evolutionary interaction, they proceed to try and unravel the paradox.

Back in June 19th, I briefly explained what psychoactive substances do for humans;

‘To survive, human beings have to do three things; eat, reproduce and protect and provide for our dependants. In order to encourage us to undertake these activities we have a reward system that makes us feel good when we eat a satisfying meal, engage in the sex act or apply effort to achieve some end like providing a shelter. Like everything going on in our bodies, this ‘feeling good’ response is the result of complex chemical reactions. Psychoactive substances reproduce those chemical reactions so that it is possible to achieve the feeling of reward without engaging in the activity.’

In June, I was interested in why this mechanism means you can never stop some people from wanting to use psychoactive substances but, today, I want to look at the evolutionary aspects. We’ve evolved to have these reward systems as a way of ensuring we reproduce. Something that means we don’t need to make the efforts to advance our species in order to achieve the reward sensation runs counter to evolution.

Look at it from the plant’s point of view. Plants have evolved to have substances that help them stay alive to the next generation. Some of those substances are toxic to herbivores so that a balance has been reached deterring herbivores from consuming so much of a plant as to prevent its bearing fruit and reproducing. Substances that are psychoactive to humans encourage consumption of the plant and run counter to the whole idea of plant components being part of the plant’s defence against getting consumed. Again, the normal ideas about how evolution works and what it achieves are stood on their head.

But, rather than say, evolution isn’t involved in the use of psychoactive plant-derived substances, the evolutionary biologists who authored the paper I was reading offer some bizarre ways in which evolution can be made to fit the relationship.

I’ve read elsewhere, the suggestion that plants make substances that humans like so that we will take more care of them and increase their population. This has been said about Cannabis sativa and on 29th June I wrote about the flaws in that argument. Cannabis produces its sticky resin as an aid to pollination not because it wants to be loved by man.

The case is made that finding the evolutionary drivers that make these plant-derived substances psychoactive and lead to addiction in at least some of the people who use them will give us help in determining the causes of addiction.

But I don’t see that searching for something that isn’t there can be helpful. 


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