FireHawk Hulin / Flickr Over the millennia, animals have gone extinct on Earth for many different reasons. Sometimes it's because of a dramatic shift in the climate. Other times it was because of human intervention.

Advances in science, specifically biotechnology, could enable scientists to bring some of these animals "back" from extinction, and there are a few already on the list.

Generally, it helps if there is a species still alive today that is genetically similar to the extinct animal, like elephants for woolly mammoths or cows for aurochs.

There are also certain criteria to consider, as bringing an animal back from the grave has a lot of biological and ecological implications.

Scientists must be able to show that the species is desirable, such as having an important ecological function or being beloved by humans. And they also must consider practical matters, such as whether we have access to tissue that could give us good quality DNA samples. Most importantly, though, the animals must also be able to be reintroduced into the wild in the first place, so sufficient habitats, food, and limited contact with humans are pretty important.

Unfortunately, dinosaurs score badly on all of these points, so there probably isn't ever going to be a real Jurassic Park. However, plenty of animals are still on the table.

Here are some of them from the list of candidate species for de-extinction from The Long Now Foundation, which was founded by biologist and writer Stewart Brand, plus some others added from our own research.

Jennifer Welsh contributed reporting on a previous version of this article.