Stanley Applies Horse
Johns Hopkins paleobiologist
Steven Stanley has sleuthed
clues to the evolution of horses, coming up with a new solution
for an enduring mystery: What caused the extinction of many
equine species and other mammals 6 million years ago?
Like the protagonist in an evolutionary detective thriller, Stanley pursued a hunch that apparently had never occurred to other scientists. His long shot hit a bull's-eye, enabling Stanley to learn how shifting climate and changing vegetation likely altered the fate of horses in North America millions of years ago.
Stanley, a professor in the Department of Earth and Planetary Sciences, pieced together the findings of other scientists, connecting those data in a way no other researchers had done previously. The result was evidence for his theory that the Earth's changing climate and vegetation were directly involved in the dramatic evolutionary trends of horses and other animals.
He will touch on his theory during a 1:35 p.m. talk on Sunday, Oct. 19, at the Geological Society of America's annual meeting in Salt Lake City.
Contrary to the popular belief that horses were foreign to the New World until they were brought here by the Spaniards, the animals actually evolved in North America, spreading to Europe by crossing the Bering land bridge that once connected Alaska and Siberia. Only when the influential English evolutionist Thomas Huxley, known as "Darwin's bulldog," ventured across the Atlantic to present the opening address at the founding of Johns Hopkins in 1876 did he become convinced that horses had evolved in North America. During a visit to Yale, Huxley was shown newly discovered fossils that represented a primitive, four-toed horse the size of a small dog, Stanley said.
But horses later died out in North America during the end of the Ice Age. Well before their extinction, however, their life history took an abrupt turn that killed off all but those with the longest teeth. In fact, numerous other mammals, including camels, suffered the same fate in North America.
Scientists have known that the extinctions were somehow related to expanding grasslands and shrinking forests. Grasses possess a gritty compound called silica, which is contained in sand and is used to make glass. As animals chew grass, the silica wears down their teeth. Therefore, animals with longer teeth live longer because their teeth don't wear down as fast, and they can continue to feed.
For tens of millions of years, as the Earth's climate became cooler and drier, the trend toward expanding grasslands and receding forests continued in North America. About 13 million years ago, the 15 or so species of horses in North America were split between those with long teeth and those with average-length teeth. Also at that time, a few new species emerged that had very long teeth.
As grasslands expanded, the horses with long teeth lived longer because they were best adapted to eating grasses instead of leaves. Living longer enabled them to produce enough offspring to guarantee survival of their species and the evolution of new species.
By 11 million years ago, only the horses especially adapted to eating grasses--those with the long and very long teeth--were living in North America.
"Then, there is this sudden event, 6 million years ago, more or less, and what you see is a big extinction pulse, a big drop in total diversity, and the survivors are all the ones with very long teeth," Stanley said.
The conventional wisdom has suggested that the long-toothed horses disappeared because of expanding grasses. But that just didn't make sense, Stanley said, because the horses with long teeth were especially adapted to eating grasses.
"So, why would more grass be a problem for them?" he asked.
Somehow, something about the grasses must have changed, he reasoned.
Meanwhile, other scientists had discovered that, as the climate became drier and cooler, a different type of grasses began to dominate North America. Those grasses, known as C4 grasses, which thrived in hot, dry climates, replaced many of the previously dominant grasses, known as C3 grasses.
"I thought, well, this seems like a long shot, but I wonder if there are on average more silica bodies in the C4 grasses than C3 grasses," said Stanley, a professor in the Department of Earth and Planetary Sciences.
His hunch proved correct. Stanley found that, on average, C4 grasses contain about three times as many of the silica particles as do C3 grasses.
"Think about a species that was doing alright eating C3 grasses. Maybe it lived 10 years on average and produced enough colts to reproduce the species. Well, what happens if that horse is suddenly only living seven years, or six years? It may not produce enough colts to perpetuate its species.
"I think that's what happened. I think there was a big grind-down."
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