When the last members of a species are killed, it becomes extinct.  Natural causes have always been a source of extinction, but many human processes have intensified the rate at which extinction occurs.

Mass extinctions have happened about every 50 million years.  250 million years ago, as many as 96% of all species inhabiting the earth died off, the greatest mass extinction ever.  Of all the creatures that have inhabited the earth throughout history, 99.9% have gone extinct.

Long ago, species went extinct due to volcanic eruptions, habitat destruction, food supply loss, or asteroids and meteors that hit the earth.

Though extinctions have always occurred, the rate at which the process is occurring in modern times is astounding.  According to estimates by James Fish, in the last 400 years the earth has lost 100 types of mammals and 258 bird species.

In the course of history, many animal species have been killed off by humans. In 80 AD, the European lion became the first species in recorded history to become extinct. 

A Passenger Pigeon.  Photo Credit: Chicago Academy of Sciences.

Early North American settlers wiped out tens of millions of passenger pigeons, destroying the species by 1914.  These settlers also wiped out species of grizzly bears and wolves, and one type of fox and cougar.  Since the arrival of the Pilgrims, more than 500 plant and animals species in North America have become extinct.

The extinction problem remains severe today.  In 1988, Harvard professor Edward O. Wilson estimated that 17,500 species were going extinct annually.

It is hard to know exactly how extinction will affect humans.  Different scientists and environmental activists have different opinions on the issue.