Rainforests, which now occupy only 6 or 7 percent of Earth’s land area, hold 50% to 90% of the diversity of life on Earth. Most areas of tropical rainforests have never been covered by glaciers, so evolution has continued there uninterrupted for as long as 65 million years.

In that time, the planet has undergone numerous periods of expansion and contraction of life, as the world’s natural path around the solar system has generated a pattern of glacial retreat and advance. These glacial periods have scoured life from temperate and boreal zones, but left the tropics intact.

Also, other "natural" calamities, such as strikes by asteroids, have, in their occurrance, wiped out large amounts of Earth's biodiversity.

These events have allowed evolution to have its way, giving birth to the extreme levels of diversity we’re just now discovering in tropical forests.

And rainforests are a special case even beyond that. Rainforests are classified as forests that receive two-hundred or more centimeters of rainfall per year. That's about 80 inches — twice as much as New York City receives annually. (We've seen varying classifications with minima as low as 150 centimeters or as high as 160 inches per year.)

It is well accepted that 65 million years ago, an asteroid struck our planet. The explosion generated heightened earthquake and volcanic activity. The impact, apparently in the region of the Yucatan Peninsula of Mexico, pulverized soil and rock and water and life and sent them into the atmosphere in enormous quantities. The ensuing cloud of dust circled the globe, causing what scientists have dubbed a “nuclear winter.” This blocked out the sun for a number of years, shutting down photosynthesis and much of the bases of the food chains around the globe. This led to the extinction of about 70% of life on Earth (for more on this asteroid impact, click here).

For the 65 million years since then, rainforests have been Earth's greatest strongholds of evolution — engines of diversity generating unimaginable levels of biotic variety. It has largely been in the rainforests that Earth’s evolution has been expressed in the current geological age.

Until now.