Carriers of Extinction  By Carl Zimmer|Saturday, July 01, 1995
When Ross MacPhee walks in the woods around his house in New
Jersey, he is sometimes beset by ghosts. He sees deer and
groundhogs, and as a paleontologist, he can’t help thinking
about what lived in North America 12,000 years ago. Sloths the
size of dump trucks, teratorn birds with 25-foot wingspans,
shaggy mastodons, herds of horses pursued by cheetahs, lions, or
saber-toothed tigers--all in all, a bestiary that would have put
the Serengeti Plain of East Africa to shame. Yet in just a few
centuries, between 11,000 and 10,500 years ago, all this life
disappeared. No one is sure why.  North America is not alone:
between 40,000 and 1,000 years ago, Earth was visited by the
biggest wave of large-animal extinctions since mammals took over
from the dinosaurs 65 million years ago. North and South America
lost 135 species, including almost all their animals over 100
pounds, but extinctions happened elsewhere as well--in Australia
and New Zealand, for ins