Ask the Experts: Why are some animals extinct and others arent?

While it may not seem like it on the scale of a human lifetime, the Earth is constantly changing over thousands or millions of years. And, likewise, all the lifeforms that inhabit Earth must constantly change to adapt to it. The animals we see alive today are the ones that are best adapted to the environment and ecology of the planet, the way it is right now.

There are several ways that species can become extinct. The most obvious one is when all the members of a species die out. This can happen for a lot of reasons: there can be a catastrophe, like when a meteor hit the Earth 65 million years ago and killed off the dinosaurs. There have been 5 times in the course of Earth's history when something major happened that killed off many, many species all around the planet. The biggest, called the end-Permian extinction, caused 95% of all species in the oceans to become extinct.

Species can also die out when other species compete with them for the same resources, and they lose, or when the environment changes and they don't adapt fast enough.

Its very,very rare, though, for a species to become extinct this way. The vast majority of extinct species we see in the fossil record became extinct when all of their members evolved into something new. All the species we have on Earth today evolved from similar-looking but different species that aren't around anymore. Different types of animals can evolve at different rates...for mammals, the average time a species can exist before it evolves into something else is about 1 million years. That means, in a million years, modern squirrels or rabbits may be extinct, but we may have another or even several new species of squirrel- or rabbit-like animals that descended from them.

In the museum, you can see many extinct species that are related to modern species. Mammoths and mastodons are similar to modern elephants, and all of them evolved from a common elephant-like ancestor. The giant Shasta ground sloth you can see is related to the smaller modern tree sloths, as well as two other types of giant ground sloths that were all adapted for living in different types of environments - forests, grasslands, and arid plains. Unfortunately for all of them, when the Ice Age ended and the environment changed, they couldnt adapt fast enough, and so went extinct.

DJ