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The culture’s killing implements abruptly appeared in the archeological record and disappeared just as fast, after the easily killable megafauna went extinct. Today’s North American megafauna are , not North American megafauna that learned to avoid humans. Bison are the only significant exception, although they came from Asia, too, and explaining their survival remains a minor curiosity, but is about the only circumstance not neatly aligned with the overkill scenario. The “” paper concluded that although the South American extinction was the greatest of all, it is the most poorly investigated and that the overkill hypothesis cannot yet be attached to South American extinctions. That may be a prudent position for a specialist who pronounces judgment only when all the evidence is in, but I will be among the most surprised people on Earth if the pattern of 50 thousand years did not continue there, especially since it had no ice sheets. There can be no more pertinent example than comparing Africa to South America. They inhabited the same latitudes and have similar climates, separated by the Atlantic Ocean. Africa was the home of humanity, where its animals had millions of years to adapt to the human presence, and Africa only lost about 10% of its megafauna (probably to human hunters with their advanced weaponry) while South America lost nearly all of its megafauna, and quickly. Climate change did it? How could it have even contributed?

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and noted that Eurasia was spread along an east-west axis, while Africa and the Americas were north-south, which made Old World diffusion easier, but that idea also has problems, as Fertile Crescent crops did not spread to East Asia due to rainfall timing differences (winter rains in the west and summer monsoons in the east). Mesoamerican and Andean civilizations had dramatic geographic limitations, which was their greatest contrast with Eurasian civilizations. However, like the migration of or the exchange when , it was easier for cultural innovations to spread along the same latitude, as they would move through similar biomes. North-south diffusion is far more difficult, as it moves through different biomes, such as tropical forests and . Eurasia's geography was more conducive to communicating innovations, which made it more cosmopolitan than sub-Saharan Africa or the Americas, which helped them technologically advance at a faster pace. Isolated peoples are usually culturally and technologically backward compared to nearby peoples who are more cosmopolitan, and people isolated by mountainous geography, such as those of the Scottish Highlands, Balkans, Appalachia, and Southeast Asia were relatively primitive compared to those around them. and are classic instances of isolated peoples keeping their cultures intact, which provided a window into the human past, but their cultures also did not "progress," which included their technology.

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The (c. 5.3 to 2.6 mya) began warmer than , but was the prelude to today’s ice age, as temperatures steadily declined. An epoch of less than three million years reflects human interest in the recent past. Geologically and climatically, there was little noteworthy about the Pliocene (although the was created then), although two related events made for one of the most interesting evolutionary events yet studied. South America kept moving northward, and the currents that once in the Tethyan heyday were finally closed. The gap between North America and South America began to close about 3.5 mya, and by 2.7 mya the current land bridge had developed. Around three mya, the began, when fauna from each continent could raft or swim to the other side. South America had been isolated for 60 million years and only received the stray migrant, such as rodents and New World monkeys. North America, however, received repeated invasions from Asia and had exchanges with Europe and Greenland. North America also had much more diverse biomes than South America's, even though it had nothing like the Amazon rainforest. The ending of South America’s isolation provided the closest thing to a controlled experiment that paleobiologists would ever have. South America's fauna was devastated, far worse than European and African fauna were when Asia finally connected with them. More than 80% of all South American mammalian families and genera existing before the Oligocene were extinct by the Pleistocene. Proboscideans continued their spectacular success after leaving Africa, and species inhabited the warm, moist Amazonian biome, as well as the Andean mountainous terrain and pampas. The also invaded and thrived as a mixed feeder, grazing or browsing as conditions permitted. In came cats, dogs, camels (which became the ), horses, pigs, rabbits, raccoons, squirrels, deer, bears, tapirs, and others. They displaced virtually all species inhabiting the same niches on the South American side. All large South American predators were driven to extinction, as well as almost all browsers and grazers of the grasslands. The South American animals that migrated northward and survived in North America were almost always those that inhabited niches that no North American animal did, such as monkeys, (which survived because of their claws), and their small cousins (which survived because of their armor), , and (which survived because of their quills). The opossum was nearly eradicated by North American competition but survived and is the only marsupial that made it to North America and exists today. One large-hoofed herbivore survived: the . The (it weighed one metric ton!) survived for a million years after the interchange. , that , also survived and migrated to North America and lasted about a million years before dying out. In general, North American mammals were , which resulted from evolutionary pressures that South America had less of, in its isolation. They were able to outrun and outthink their South American competitors. South American animals made it past South America, but none of them drove any northern indigenous species of note to extinction.

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What seems to explain invader and endemic success with those migrations is what kind of continent the invaders came from, what kind of continent they invaded, and the invasion route. Asia contains large arctic and tropical biomes, unlike any other continent. North America barely reaches the tropics and only a finger of South America reaches high latitudes, and well short of what would be called arctic latitudes in North America. Africa’s biomes were all tropical and near-tropical. The route to was straight across at the same latitude, so the biomes were similar. About the same is true of the route to Africa from Asia. Asian immigrants were not migrating to climates much different from what they left. But the route to North America was via , which was an Arctic route. Primates and other tropical animals could not migrate from Asia to North America via Beringia, and even fauna from temperate climates were not going to make that journey, not in Icehouse Earth conditions. Oligocene North America was geographically protected in ways that Oligocene Europe and Africa were not, and it already had substantial exchanges with Asia before and was a big continent with diverse biomes in its own right. It was not nearly as isolated as Africa, South America, and Australia were.

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