Friday, September 9, 2011

Let the Earth Bring Forth Plants

The first chapter of Genesis in the Old Testament is a hymn to an orderly and bountiful cosmos. Surprisingly enough, it begins by describing a universe that existed before God began to create. This universe had two problems: it was chaotic and empty (the old King James English says “formless and void”). During the six days of creation, according to this hymn, God solved these two problems. On the first three days, he brought order into the heavenly, the fluid, and the earthly realms: making light separate from darkness, air separate from water, and land separate from sea. On the last three days, he filled the heavenly, fluid, and earthly realms: filling the heavens with stars, the water and air with fish and birds, and the earth with “creeping things.” It is obvious to everyone except creationists that this chapter was never, even when it was first written, intended as an historical description of the history of the cosmos. It described the work of creation as categories, not as a timeline. Astronomy, geology, and biology now provide explanations for the processes of creation described poetically in Genesis.

As beautiful as it is, however, the first chapter of Genesis reflects the human-centered viewpoint of the time in which it was written (about 500 BCE), a viewpoint to which most modern people still cling, even if they are not particularly religious. This viewpoint puts humans at the pinnacle of creation, something that most people (including those who think they understand evolution) still do. The writer of Genesis did not praise humans as much as you might think, however: humans were created on the same “day” as all of the creeping things. Humans did not even get their own special day of creation. But Genesis clearly states that humans rule all of the creation, and uses terms that imply that humans have the right to conquer and rape the creation.

One example of this domination is the creation of plants. Plants appear on Day 3, along with the dry land. That is, in this hymn, plants are not considered to be living organisms; they are just a part of the landscape. In a sense this is true: any landscape without plants is quickly ravaged by erosion, as I have explained in my book Green Planet. But the God of Genesis does not give plants any role of their own other than to be eaten by animals. Behold, he says on Day 6, I have given you every green plant for food.

But plants are organisms. They’ve got to make a living like everybody else. Plants have populations that evolve. It is not in the interests of the plants to just sit there and allow themselves to be eaten by animals. The plants that natural selection favors are those that can avoid getting eaten by animals.

As a result, the world is not just a big salad bowl. Nearly every wild plant has several or many characteristics that make it unpalatable to animals. In some cases, plant defenses are clearly visible: spines on cactuses, stinging hairs on nettles. In other cases, the defenses are invisible but even more effective. Most plants have leaves that are at least mildly toxic. Plants produce thousands of different kinds of chemicals that are toxic to animals. Some of them accumulate poisonous mineral ions out of the soil and put them in little sacs (vacuoles) in the leaf cells, where the poisons will not harm the leaves but will harm any animal that chews the leaf, breaking open the sacs of poison.

Humans have bred crop and garden plants that are palatable, and this has created for most of us the illusion that plants are just waiting to be eaten. They are like Al Capp’s cartoon shmoos, which fall all over themselves in an attempt to get eaten, with stupid grins on their faces. Be warned: wild plants do not have brains, they are in fact stupid, but evolution has produced in them many defenses that appear to those who study them to be very clever. Wild relatives of crop plants, such as beans, tomatoes, and potatoes, are poisonous. Some tribal peoples subsist on wild cassava. But they have to boil and squeeze the cassava pulp to remove poisons first.

Even the parts of plants that are not poisonous are not simple gifts to feed animals. Many species of plants produce soft, colorful, sweet, fragrant fruits. These fruits make themselves very visible to animals, and reward the animals for eating them. Such fruits, abundant and hanging low, are in fact part of our image of paradise. (The word paradise itself comes from the Persian word pairidaeza, a walled garden.) But the plants have something to gain from this arrangement. The fruits have seeds in them. Animals usually eat the entire fruit, swallowing the seeds. It is just too much work for the animal to pick out the seeds, especially since the pulp clings closely to them. Plants cannot move around and carry their seeds to new locations, nor can the seeds move under their own power. Many plants therefore get animals to carry their seeds to new places by getting the animals to eat them. They pay the animals by feeding them the sweet and nourishing pulp of the fruit. The animals deposit the seeds, along with a pile of fertilizer, far away from the parent tree. Both the plants and the animals benefit. In other cases, plants take advantage of the animals. Seeds with spiky burs entangle the fur of mammals, which carry the seeds to a new location while they are trying to scrape the burs from their fur.

Plants and herbivores are involved in what has been called an arms race. Plants evolve defenses, and herbivores evolve ways of circumventing the defenses. Plants then evolve new defenses, and herbivores evolve ways of circumventing these as well. For both the plants and the animals, this interaction is expensive. The plants have to use their energy and molecules to make poisons rather than using them to grow new leaves and roots. Animals have to use energy and molecules to digest the plants despite their defenses. In human societies, spending more money on defense means spending less on growth (such as infrastructure and education). Plants and animals find themselves in the same predicament as humans do.

This is coevolution: natural selection favors plants that are prickly or poisonous, but not so much that they cannot grow, and animals that can tolerate or avoid the poisons, but not so much that they divert too much energy away from their own metabolism, movement, and growth.

One you have learned about the coevolutionary relationship between herbivores and plants, you can never again look at a serene natural landscape the same way as before. The Lord is my shepherd, said the psalmist, leading sheep into green pastures. The psalmist also said that God prepared a table for him in the presence of his enemies. But the psalmist could not have guessed that, to a certain extent, the green pastures themselves were one of the enemies. There never was a natural Garden of Eden in which all the plants were uncomplicatedly nutritious.

This entry was part of my book Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, published earlier this year by Prometheus Books.

1 comment:

  1. " it begins by describing a universe that existed before God began to create." (honest Abs words)

    Honestly Ab?
    Actual words of the Bible...."In the beginning God created the heavens and the earth" Genesis 1:1.

    ReplyDelete