During the Ice Ages, the last of which occurred about 10,000 years ago,
the frozen areas of the North and South Poles spread over much of the earth,
causing huge numbers of extinctions. But the giant freeze did not reach many
tropical rainforests. Therefore, these plants and animals could continue to
evolve, developing into the most diverse and complex ecosystems on earth.
The nearly perfect conditions for life also help contribute to the great
number of species. With temperatures constant at 75 -80 degrees F. the whole
year, animals don't have to worry about freezing during cold winters or finding
shade in the hot summers. They rarely have to search for water, as rain falls
almost every day in tropical rainforests.
Some rainforest species have populations that number in the millions. Other
species consist of only a few dozen individuals. Living in limited areas, most
of these species are endemic, or found nowhere else on earth. The maues
marmoset, a species of monkey, wasn't discovered until recently. Its
entire tiny population lives within a few square miles in the Amazon rainforest.
It is so small, it could sit in a person's hand!