The last known passenger pigeon, named Martha, died 100 years ago today at the Cincinnati Zoo. She was 29.
The passenger pigeon, which once numbered in the billions in Norther America, has become a symbol of extinction. It was wiped out in the 19th century due to overhunting.
Animals face many natural threats, including changing temperatures, predators, and unexpected disasters. But no external stresses have proved more destructive to the survival of other living things than people.
The following gallery includes other animals that have gone extinct almost directly by the hands of humans.
Tasmanian tiger (Extinct since 1936)

The Tasmanian tiger, also called the thylacine, was a marsupial native to Australia and the island of Tasmania.
The carnivore was seen by farmers as a threat to sheep and therefore hunted, trapped and poisoned for government bounties.
"Many people, however, believe that bounty hunting alone could not have driven the thylacine extinct and therefore claim that an unknown disease epidemic must have been responsible," researcher Thomas Prowse, of Australia's University of Adelaide, said in a statement.
Using population models to simulate the direct effects of bounty hunting and habitat loss, the new study found that humans alone were responsible for the animal's doom.
The last wild Tasmanian tiger was captured in 1933 and taken to the Hobart Zoo, where it died three years later.
Woolly Mammoth (Extinct for ~10,000 years)

The woolly mammoth disappeared about 10,000 years ago, after roaming Siberia and North America for around 250,000 years.
Although there's been some disagreement about what delivered the final blow, a recent study found that hunting by humans, on top of environmental stresses like climate and habitat change, spelled the end for the furry beast.
Dodo Bird (Extinct since ~1681)

For centuries, the flightless Dodo bird lived undisturbed on the island of Mauritius off the coast of Africa. Because they had no enemies on the island, the wingless birds were easy prey when humans arrived in the early 16th century.
Although the exact date is uncertain, people believe the last dodo bird was killed in 1681.
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