21 March 2010
Imagine the Earth without grasses. There would be no lawns or meadows. No prairies. No savannahs or steppes. No wheat fields or rice paddies. No sugar cane.
No sheep, elephants or horses.
We live in the age of grass. Indeed, from our point of view, the evolution of grasses was one of the most momentous events in the history of the Earth. Which is why I’m nominating them for Life-form of the Month: March. Let’s limber up with a few facts. In general, grasses spread their pollen by wind, so they are not dependent on bees or other insects. Grasses also grow fast, and can easily colonize patches of bare ground, or move into a landscape after a fire. They can withstand being eaten (or mown) better than most other plants, because their leaves grow from the base, not the top.
Like all plants, they make energy from the sun by means of photosynthesis. However, grasses have repeatedly evolved a variation — known as C4 photosynthesis — that uses less water, and is thus a particular advantage in hot, dry places, or when carbon dioxide levels are low. This has allowed them to flourish in difficult habitats, like rocky outcrops and dry soils. One other detail: grasses fill their leaves with silica. That is, they are factories for tiny opals.
As a group, grasses have been wildly successful. Today, the grass family contains more than 10,000 species — that’s more species of grass than species of bird — and grasslands cover about a third of the planet’s landmasses. (“Grassland” refers to an ecosystem, like prairie, where grasses dominate; it doesn’t mean they are the only plants there.) Grasses can be tall (think bamboo) or short (think lawns), and they include our most important crops. Rice, wheat, rye, oats, maize, millet, barley, sorghum and sugar cane are all grasses.
We humans are dependent on grasses: we get more than half our calories directly from the tetrad of rice, wheat, maize and sugar cane, and we feed grasses to our sheep, goats, horses and cows.
But I’m getting ahead of myself.
The early history of grasses is obscure. However, we do know that they blew onto the scene relatively late — around 80 million years ago, shortly before the dinosaurs went extinct. In evolutionary terms, that’s yesterday.
And having arrived late, their rise to prominence got off to a slow start. If you climbed into your time machine and set the dial for 55 million years ago, you wouldn’t find much in the way of grasses when you got there: at that point, they were still minor players on the Earth’s stage. But by 15 million years ago, that had changed. Grasslands had become abundant.
Exactly why this happened is a matter of debate. But whatever the reasons, the effect on other life forms has been profound.
Grasses affect the landscape both above and below the surface of the Earth. Below: they alter the texture of the soil. Grassland soil is typically characterized by small crumbs that are rich in organic matter. This is partly due to the way that grass roots grow, and partly due to the animals that grasses encourage — like earthworms and insects. Many of our richest agricultural lands were made so by grasses.
Above ground, grasslands create wide open spaces where large animals can run fast and go about in big herds. Hence, the spread of grasses triggered the evolution of big, herding mammals with long legs and hooves — horses and antelopes, for example. Moreover, all those opals are hard to eat: they wear down teeth. So the rise of grasses was also met with the evolution of “hypsodonty” — long teeth.
(Just as grasslands sculpted the evolution of certain mammals, so too mammals sculpted the evolution of grasslands. Many mammals eat young trees — and thus prevent trees from invading a grassy area. Elephants can — and sometimes do — uproot big trees. The high opal-content of grasses is, in part, an evolved response to being eaten.)
Now pause for a moment to imagine these savannahs with their big herds of galloping mammals. For these are also the landscapes in which, perhaps, our earliest ancestors stood on two legs and learned to hunt — though again, this is a matter of debate.
Yet regardless of how much grasses shaped our earliest evolution, in the recent past they have transformed us. We usually talk of our domestication of grasses, and the ways in which we have evolved them: we have made plants with bigger, more nutritious seeds that don’t fall to the ground, for example.
But their effect on us has been far more profound. Our domestication of grasses, 10,000 years ago or so, allowed the building of the first cities, and marks the start of civilization as we know it. Grasses thus enabled the flowering of a new kind of evolution, a kind not seen before in the history of life: the evolution of human culture.
The New York Times