Monday, August 12, 2013

Human and plant evolution

I had a couple of thoughts when I heard this show, related to human evolution related to plants.

(1) Humans evolved along with the plants we eat, including the toxins they contain. If we are exposed to fewer of these toxins, it may be harmful in some ways, similarly to the apparent rise in allergies due to having less contact with germs. Eg., if we make extra amounts of some substance to detoxify a toxin, when the toxin is decreased, it might have harmful effects. If we make extra amounts of a substance to make up for some that the toxin causes to be decreased, when the toxin is decreased, we may end up with too much of that substance.

(2) It has been suggested that cooking helped us develop bigger brains, by making more energy available from a given amount of food. Cooking destroys some plant toxins, so that might have had effects on human health. I would guess that we have been eating cooked food long enough to have reduced any possible negative effects. And if the body no longer needed to use as many resources to deal with toxins in our food, more resources would be available to develop a bigger brain.

How Insects Influence Plant Evolution
Air Date: Week of August 9, 2013

From the spiciness of the chili pepper to horseradish’s bitter bite, many plant traits are evolutionary adaptations to insect attacks. Cornell biologist Anurag Agrawal explains to host Steve Curwood just how speedily plants can evolve and adapt when insect populations change.


And for millenia, insects - such as the silk worm – have in turn helped guide the evolution and defenses of plants.
A new 5 year study from researchers at Cornell University in Science Magazine focuses on the critical role insects can play in plant evolution, and how speedy that evolution can be.
The lead author is Anurag Agrawal, Professor of Ecology and Evolutionary Biology at Cornell.


AGRAWAL: Yeah, there’s been a long history of circumstantial evidence that many of the traits plants have are adaptations to insect herbivores. If you think about the things we eat, what makes horseradish spicy? Horseradish is spicy because of a series of compounds called glucosinolates that we believe have evolved by natural selection to ward off insects. You know, the poisons that make milkweed so famous? - you know, same expectation. What’s nicotine? Nicotine is a neurotoxin of tobacco that has evolved by natural selection to ward off insects.


CURWOOD: Please describe the specifics of your study for us – what were you looking at?

AGRAWAL: Well, what we did is we set out experimental plots here at Ithaca, New York to examine the impact insect herbivores have on the plants, and removed insects from half of the plots using an insecticide treatment, and over a very quick period of time – about five years, five generations of the plants we were studying, the common Evening Primrose, it’s a wildflower – removal of insects resulted in the evolution of two very critical plant traits: How early the plants flower and the production of a toxin in the fruits of those flowers. When we removed insects, the plants were able to relax those defenses. They flowered earlier and they produced less of those toxins in their fruits.


AGRAWAL: Well, it’s complicated, and I appreciate the question. Insecticides certainly have been a very valuable tool in the production of agricultural crops, and I think that what it tells us is that when we take insects out of the picture – using things like insecticides – we are encouraging plants throughout the evolutionary process to relax their defenses.

And, in fact, this is a story that I think is unfortunately really a big part of worldwide agriculture, and that is that we tend to select varieties of plants to grow that are diminished in their natural defensive capacities. If you take a wild plant that has survived out there for millions of years, it typically has a remarkable array of toxins and defense tactics to ward off pests.

About 10 percent of all plants produce hydrogen cyanide. We know hydrogen cyanide as a very general toxin – an anti-life compound – and it’s hydrogen cyanide that’s in 10 percent of the plants. It has no primary function, it doesn’t help the plant capture sunlight or produce seeds directly, what hydrogen cyanide does is it poisons insects that are trying to eat those plants.

One of the things that I think we do a little too well is we – either on purpose or inadvertently – we breed crops so that they have relaxed defenses and that increases our needs and our usage of pesticides, which I think we can all agree is problematic for the environment.

CURWOOD: Anurag Agrawal is Professor of Ecology and Evolutionary Biology at Cornell University. Thanks so much for taking the time today!

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