Fossils show us that around 21 million years ago the early forests of North America’s Great Plains region began to die away due to several cooling periods. During such events, there was less moisture in the air with much of it locked up at the poles. This made the world not just colder but also drier and far more open. Grasses were able to spread due to their ability to dominate arid or drought-prone areas. They can do this because they evolved a more efficient method of photosynthesis that loses less waterthan many other plants.
Six million years later, a sudden climatic shift called the Middle Miocene Disruption began when the Antarctic ice sheets began to grow, and shifting ocean currents caused global temperatures to drop. As the world grew colder, grass easily spread in these now dry and cooler plains and so did herbivores that fed on them. In response, grass evolved various methods of defense. Their leaves are covered with little hairs on their surface that make them waterproof and highly unpalatable to many creatures. They are also filled to the brim with phytoliths, bits of stone-like silica that they take up through their roots from the soil. Not only are they already distasteful to the tongue and tough to digest, they literally have rocks inside their leaves. One might imagine that nothing would eat grass and that it would be quite safe from consumption by animals. Of course, we all know that this is far from the truth.
The mammals were responding to their food with relative efficiency. The phytoliths wore down the tooth enamel of those mammals that ate them, and several different lineages evolved tall teeth with thick enamel as a response. These tall-crowned – or hypsodont – teeth had a great deal of enamel in the tooth, and a number of tightly-packed ridges crisscross the tooth’s surface. This prevents wear after an animal consumes a lot of gritty soil with its plant meal. They first evolved in early rodents in what is now Chile and soon became beneficial for eating grass and for avoiding wear and the destruction of enamel while chewing.
Once global cooling opened up forested habitats, the bodies of grazing mammals grew larger. They became leaner with long muscular limbs, and began to diversify into myriad new forms, including giraffes, horses, camels and antelopes. The toes of horses were reduced from three to just one weight-bearing hoof for running. Antelope, deer, goats and the like became cloven-hooved. Elephants became immense beasts able to engineer their ecosystems. And under the grazers' feet the rabbits and rodents remained as tiny burrowers. And after these ran all manner of predators, filling the Miocene plains with bone-crushing dogs and saber-toothed cats.
Interestingly, grasses often thrive on being eaten and constantly grazed upon. The meristem, the part of the plant responsible for the growth of its organs and leaves, is closer to the bottom of the plant. This allows them to recover quickly from almost any attacks by herbivorous animals, as large grazers are unable to destroy the roots of a grass, which lay comfortably beneath the soil. What's more, the hooves of these very same grazers trample saplings and preclude the growth of trees, keeping the grasslands open.
The primate's closeness to grass is one that stretches far beyond written history, back to the very end of the Miocene, when our lineage split from the common ancestor with chimpanzees in eastern Africa. For a long time even these savannah apes were very much like their predecessors. This was soon to change, for something better than a four-footed stance was needed to see over tall grasses.
An upright stance meant that the head was located at a high level, which when combined with the superb primate color vision meant that they were able to pick out any predators at a distance. It also freed their hands from having to bear any weight. The opposable thumbs used to grab branches would now become useful to shape their world and these hominids would rule the African grasslands for a few more million years. Thus we partly owe our rise to the simple grass and its closest relatives.
The story of plants of course, is not yet over and they will continue to blanket the Earth for hundreds of millions of years to come. Even though many see plants as a passive feature of our landscapes, they are living organisms that evolve and adapt much like animals – sometimes shaping the history of those around them. By pumping oxygen into the early atmosphere, creating a green carpet on which the first land animals could tread on, and using them to spread far and wide via their seeds, plants have shown they impact the entire world. They were some of the first living things to conquer the land, and may well be some of the last to stand.
This is part five of a five-part series on the evolution of plants.
1: The first conquerors of land
2: Birth of the forests
3: The Age of Coal
4: A tale of flowers and seeds
5: The grassland empire