Grasses and the evolution of grazing mammals

Introduction

Grasses are eaten by many animals, including insects, birds, and especially grazing hoofed mammals called ungulates. Grazing ungulates include perissodactyls (odd-toed ungulates), like horses and rhinos, and artiodactyls (even-toed ungulates), like bison, cows, and sheep. Biologists and paleontologists have long speculated about how the evolution of grasses and the expansion of grasslands affected the evolution of these animals, particularly their teeth.

One of the most widely discussed examples of long-term evolutionary change as illustrated in the fossil record—the evolution of horses—focuses on the role of grasses in shaping the teeth and other characters of these mammals. The story of grasses and horse evolution has been part of the literature of paleontology and evolution for more than 150 years, but recent research suggests that it is more complicated than presented in most textbooks and museum exhibits.

Grasses, grit, and grazers

During the Miocene epoch, about 20 million years ago, global temperatures cooled and continental interiors dried. These climatic changes caused forests to shrink and grasslands to expand. Grasslands eventually spread to cover huge areas of Africa, Asia, and North and South America. In response to the expansion of grasslands, some ungulates switched from browsing (eating mostly leaves from trees and shrubs) to grazing (eating a diet rich in grasses). This change in diet required modifications to their teeth because the diet of grazers is highly abrasive and wears down teeth faster than browsing.

The abrasiveness of a grazer's diet is a combination of two factors. First, because grasses grow close to the ground, eating them frequently involves consuming a lot of gritty material (like sand) from the soil. Second, grass tissues contain tiny particles of silica (SiO₂) called phytoliths. While many types of plants produce phytoliths, grasses are especially known for producing abundant phytoliths in their epidermises (the epidermis is the outer layer of tissue covering the plant body). Scientists have proposed many possible purposes for phytoliths, such as limiting radiation damage, providing structural support to plant tissues, contributing to drought tolerance, and deterring grazers. 

As an adaptation to their abrasive diet, grazing mammals evolved high-crowned (hypsodont) teeth, which last longer because they take longer to wear down. One debate about the evolution of grazing mammals is whether the source of abrasive material in their diet—and thus the driving force behind the evolution of high-crowed teeth—is mostly from eating soil or mostly from eating phytoliths contained in grass tissues. 

Research on the topic of grasslands and evolutionary changes in grazing mammals goes back to Russian paleontologist Vladimir Kovalevsky. In 1874, Kovalevsky suggested that ungulate mammals became increasingly adapted to open grassland habitats and a grazing diet over time. He emphasized the role of grit in this transition. Grazers would ingest a large amount of abrasive grit, wearing down the molar teeth of grazing mammals. Therefore, natural selection favored individuals with high-crowned teeth.

Later scientists agreed that there was an evolutionary correlation of hypsodonty (high-crowned teeth) and grasses. Most of them, however, thought that phytoliths rather than grit from the soil were the cause of tooth wear. This view eventually became the mainstream view in evolutionary biology. Some scientists theorized that the evolution of grasses and grazers was an excellent example of an evolutionary “arms race”: grasses evolved phytoliths as a response to pressure from grazers, and grazers gradually evolved higher and higher crowned teeth in response to eating phytoliths.

The traditional evolutionary story of an arms race between grasses and grazers is now known to be incorrect. The fossil record shows that grasses evolved during the Cretaceous period, when dinosaurs still roamed the Earth, and that they began diversifying many millions years before the radiation of grazing mammals. Grass phytoliths do not deter feeding by large mammals, and phytoliths are not be hard enough to abrade tooth enamel. Nevertheless, the expansion of grasslands in the Miocene did influence the evolution of grazers, which took advantage of this enlarging habitat with abundant food. Grazing involves ingestion of grit, which appears to be the driving force behind the evolution of high-crowned teeth in large grazing mammals.

Grasslands and the evolution of horses

Historical ideas about horse evolution

Many textbooks and museum exhibits tell a straightforward story about the relationship between the evolution of grasses and the mammals that ate them. Historically, changes were commonly presented as though they were gradual, progressive, and occurred in a straight evolutionary line. Thus, teeth gradually got taller and grazing mammals gradually became larger as they evolved from forest-dwelling, browsing ancestors.

In horses, the increase in tooth height and body size was accompanied by a reduction in the number of toes on each foot and other modifications to the leg bones that are associated with living in open grasslands. Evolutionary biologist and paleontologist George Gaylord Simpson (1902-1984) called these evolutionary changes in horses, which first appeared during the early Miocene epoch (about 23 to 16 million years ago), “The Great Transformation.” In fact, horses, which have an excellent fossil record beginning in the Eocene epoch (about 55 to 34 million years ago), have long been the main example used to illustrate how the expansion of grasslands drove the supposedly linear evolution of grazers from browsers in hoofed mammals. 

Modern ideas about horse evolution

The genealogy of horses is now known not to be a single line, but rather a tree with many branches. Whereas today there is a single genus of horses (Equus), at some points in the geological past, horse diversity was much greater.

Some ancient horse species were large animals with high-crowned teeth, but others were small animals with low-crowned teeth. In the branches of the horse family tree that did evolve larger bodies, the rate of size increase was not continuous. Rather, it sped up dramatically between 10 and 20 million years ago. Thus, while horse evolution did accelerate with the expansion of grasslands in the early Miocene, that acceleration was complex. It may have begun with minor increases in hypsodonty (tooth crown height), followed by an increase in species diversity as the spread of grasslands isolated patches of habitat and separated populations, leading to their genetic divergence.

The branch of the mammalian family tree leading to modern horses began with small (3 to 35 kilograms or 6.6 to 77 pounds) ancestors known as Hyracotherium. Hyracotherium lived in forests of Europe and North America beginning in the early Eocene (about 55 million years ago). It had four toes on its front feet and three on its hind feet. These animals had very low-crowned teeth, which were suitable for eating the broadleaf vegetation that grew in those forests.

The descendants of Hyracotherium did not evolve in a continuous direction toward modern horses (Equus). Instead, many different types of horses evolved. By the early Miocene (about 23 to 16 million years ago), there were at least a half dozen horse genera and many species that varied in size.

Around this time, approximately 22 million years ago, grasslands began a rapid expansion in the Great Plains region of North America. Then, around 17.5 million years ago, the first horse with hypsodont teeth, called Merychippus, appeared. Horses with very high-crowned teeth appeared about 14 million years ago. Nevertheless, many other Miocene horses had low-crowned teeth and lived in forested habitats.

The diversity of horses remained high until relatively recently in geological time. Extinction whittled horse diversity down to the single modern genus Equus by the Pleistocene "Ice Ages" (about 2.6 million to 11,700 years ago), giving us the misleading impression that the modern horse was the result of an evolutionary trend with a single direction.