He walks through the numbers with the precision of someone who has spent years in the data. Between 14,000 and 11,600 years ago, half of all megafauna species on Earth disappeared. Not small animals. Not birds. Mammals over 100 pounds in body weight. Today, if you census every mammal on the planet above that threshold, you are looking at roughly 100 to 120 species total. In North America alone, that includes your mountain lions, your moose, your bears, and a handful of others. The question he is building toward is simple: what could cause that kind of selective extinction across an entire hemisphere in such a narrow window of time? The size threshold matters because it tells you something about the mechanism.