Although a fossil record can be somewhat vague concerning when a species comes into existence for the first time, it is usually quite clear concerning when it ends. One repeating pattern in all phylogenetic diagrams is extinction.
Modern humans are massive agents of species destruction. In every major habitat around the world, animals and plants are vanishing faster than we can count them. But humans are only the latest force to upset the equilibrium and wipe out hundreds of thousands of species.
Extinction is as much a part of the web of life as speciation.
Successful phyla, such as the dinosaurs, suffered one of the most dramatic shifts in fortune at the end of the Cretaceous period. After 160 million years of existence, they vanished almost overnight.
There is still a lot of doubt about what caused such a major extinction (which took out many other groups of plants and animals), but evidence is strongly growing to support the Alvarez hypothesis.
According to this idea, two asteroids, one large and another a bit smaller, smashed into earth 65 million years ago. The largest struck in what is now the Caribbean Sea, causing unimaginable tidal waves that rushed inland for many miles. It vaporized hundreds and thousands of tons of rock that mushroomed into the sky.
Blanketed by vaporized rock debris, the earth cooled because it received no sunlight. Many species, including most dinosaurs, could not survive such a "nuclear winter" and they perished. To seal their fate, the smaller asteroid hit earth in the middle of the United States, either at the same time or shortly after, adding its own cloud of debris to that of the first.
As the dust slowly fell back to a vastly depleted earth, it carried iridium, a common element in asteroids but a rare element on earth. The fine rain of dust covered the bones of the last "terrible lizards" and got into the eyes of a tiny mouselike, furry animal that spent most of its life hiding in trees.
It came out only at night so as to avoid being caught by the reptiles. Blinking in the dawn of a new era, this mammal found itself in a different world, a world wiped clean of its major predators and filled with new opportunities.
Hesitating for moment, the mouselike animal slowly came out into the returning sunshine, paused for a moment, and then ran off onto the plains of Africa. Very soon, it was exploiting the surrounding new opportunities and undergoing adaptive radiation.
The evolutionary path that would eventually one day end up with humans had just started.