“GRAZING IN THE GRASS”
THE SPREAD OF GRASSLANDS AND GRAZING ANIMALS
Beginning 25 million years ago
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| This is a general picture of grassland at Oglala National Grassland, Nebraska. Credit: Photo by Brian Kell, courtesy of Wikimedia. |
We tend to take grasses for granted. We mow them in our front yards, we eat them in the form of wheat, rye barley, rice, and corn, and their pollen can give us sneezing fits from hay fever. Yet grasses only started becoming common on earth starting about 25 million years ago and especially widespread in the form of prairies starting about 7 million years ago.
Prior to that, most of our planet’s vegetation consisted of trees, bushes, and shrubs. And most of the plant-eating mammals of the world were browsers, having tooth chewing surfaces with high peaks and low valleys (“high crowned”) to tear up tough leafy vegetation. Modern browsers include most deer, goats, kudu, dik-dik, giraffe, camel, the Black rhino, as well as prehistoric forms such as mastodon. With the spread of grasslands, some mammals shifted their diets over time to grass-eating or grazing, chewing hard grass seeds and stalks (hay), and evolved tooth chewing surfaces that were flatter (“low crowned”). Modern examples of grazers include elk, horses, cattle, sheep, antelope, hippos, and elephants (as well as the prehistoric mammoths). Some species, such as eland and impala, can eat both types of vegetation.
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| Merychippus is an extinct horse genus found during the Miocene, about 20 million years ago. Based on teeth morphology, it is the first horse known to have grazed. From left to right are the cranium, complete skeleton, left forefoot front view, left forefoot side view. It is currently housed at the Staatliches Museum für Naturkunde Karlsruhe, Germany. Credit: Photo by H. Zell, courtesy of Wikipedia. |
HOW DO WE KNOW?
We can see a change in tooth morphology towards flatter-crowned grazers over time, and prehistoric pollen analysis shows more grass pollen through time in many areas. In addition, isotope studies of ancient soils (tropical grasslands are characterized by having higher carbon 12 to carbon 13 ratios, while shrublands, woodlands, and forests are characterized by the reverse) also indicate increases in grasslands. Thus, all three of these lines of evidence confirm that open grassland were becoming more and more prevalent over time in many areas of the world
WHY SHOULD I CARE?
As we will see, ancient humans co-evolved with other animals in a world that was becoming increasingly grass-dominated, especially in Africa, the Cradle of Humankind. So in a very real sense we evolved as grass-dwellers, and our more recent evolution, especially in the last five million years, is intimately linked to these grasslands.
WEB RESOURCES
This is a website about grasslands and grassland animals.
https://www.mbgnet.net/sets/grasslnd/animals/index.htm
This University of California at Berkeley Paleontology website is about grasslands.
https://www.ucmp.berkeley.edu/exhibits/biomes/grasslands.php
This is a comprehensive website about the various grassland systems all over the Earth.
https://en.wikipedia.org/wiki/Grassland
This is a New York Times article, despite being an opinion article, that is very informative and well researched, about the impact of grasses on earth’s ecology and the rise of human civilizations.
https://opinionator.blogs.nytimes.com/2010/03/02/evolution-by-the-grassroots/
