The document discusses several hypotheses for what caused the mass extinction of megafauna (large animal species) at the end of the Little Ice Age period between 1300-1850 AD. The main hypotheses presented are: 1) "Overkill" - Early human hunters in North America over-hunted megafauna species when they first arrived, driving many to extinction. 2) "Overchill" - Rapid climate changes at the end of the ice age, particularly a cold snap between 12,900-11,500 years ago, stressed megafauna populations and habitats leading to widespread die-offs. 3) "Overill" - A virulent "hyperdisease" brought by the first humans