Question

### Late-Pleistocene Megafauna Extinction

During the late Pleistocene epoch, approximately $12,000$ years ago, many species of large mammals (megafauna) in North America, such as mammoths and saber-toothed cats, went extinct. Two scientists present different viewpoints regarding the primary cause of these extinctions.

**Scientist $1$**
The extinctions were primarily caused by rapid climate change at the end of the last glacial period. As temperatures rose and glaciers retreated, vegetation patterns shifted dramatically. Megafauna species, adapted to specific cooler habitats and plants, could not migrate or adapt quickly enough to the new ecological conditions, leading to starvation and population collapse. Humans played a negligible role, as megafaunal populations were already in steep decline before human populations became widespread.

**Scientist $2$**
The extinctions were primarily caused by human overhunting, a phenomenon known as the 'blitzkrieg' hypothesis. As humans migrated into North America via the Bering land bridge, they encountered large mammals that had no evolutionary fear of humans. Using advanced projectile weapons, human hunters rapidly depleted megafaunal populations. The extinctions occurred within a few centuries of human arrival, too quickly to be explained by gradual climate shifts, which these species had survived during previous interglacial periods.

Based on Scientist $1$'s viewpoint, which of the following changes was the primary cause of the megafaunal extinctions?