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Assignment task: A Brief History of Equus (modern horses)

One reason scientist previously thought horse evolution was fairly linear was the limited diversity of living Equidae today. How did this come to be?

Question 1: What happens to the number of horse genera from the late Miocene through the present day in North America?

Question 2: While much of the evolutionary history of horses took place in North America, horses went extinct here and in South America, leaving Equus in Eurasia as the last surviving genus Equidae. According to Figure, approximately when does this occur?

This timing corresponds to the retreat of the most recent glacial advance as well as the arrival in North American of humans. The glacial-interglacial cycles can be seen in the saw-tooth temperature pattern in Figure. Equidae persisted throughout many previous glacial-interglacial cycles, so although it may have played a role, climate alone is unlikely to have caused the extinction without the new player in the game-human hunters. Overhunting by humans is thought to have played a key role in this extinction, which included 73% of megafauna in North America. It was not until Columbus that horses were reintroduced to North America from surviving Equus in Eurasia.

The fact that horses went extinct in North and South America and were absent until reintroduced by Spanish explorers is important because wild horses, which descended from these reintroduced domestic horses, are generally considered non-native by most state and federal wildlife agencies in the United States. Today there are more than 40,000 wild horses on public lands in the western United States. Because they are not considered "native," they are not protected in the same way as native species and are subject to roundups and removal.

The Bureau of Land Management argues that there are more horses on rangelands than the land can support, while animal rights activists argue they should be protected like native species. What do you think?

Question 3: List five things you learned about horses today.