Over the course of the Earth's history, unthinkable numbers of organisms have disappeared forever, and, to some extent, that's just the way the world works. But some scientists think that today we are living through a mass extinction event that could rival some of the major wipe-outs in the Earth's history.

For reasons ranging from habitat destruction to climate change, experts estimate that 27,000 species vanish from the Earth every year, never to be seen again.

Just one of them was Lonesome George, the last of the Pinta Island Tortoises in the Galapagos, who died in 2012 after most of his brethren had been hunted. Now, there are not any Pinta Island Tortoises.

These days, lots of species are facing the same fate, and we can't do justice to all of them. But in honor of Lonesome George, today we are taking a look at what may be five coolest, rarest animals on Earth.

Number Five - the Iberian Lynx. It's hard to pin down the rarest animals in the world, because how do you count what might not exist anymore, but the Iberian Lynx is one of the rarest cat species out there. 

There are only about 100 left, native to Spain and Portugal, it's lost most of its habitat to human development, and its chief prey, rabbits, have been dwindling due to disease.

If the Iberian Lynx disappears, it will be the first known feline extinction since saber-tooth cats disappeared 10,000 years ago. Also, it's gorgeous. Just look at those adorable, crazy eyes.

Speaking of crazy eyes, number four is the Pygmy Tarsier. This primate was thought to be extinct for 85 years until 2008 when an expedition found a handful of them in the mountains of Indonesia. 

Pygmy Tarsiers live in trees, and the aggressive logging of their habitat has led to a steep decline in their numbers. If the Pygmy Tarsier goes extinct, we will have lost the only carnivorous primate in the world, because they only eat insects.

Number Three - the Javan Rhino. Once widespread throughout Asia, there are now only about 40 Javan Rhinoceroses in the wild. Sadly, this has a lot to do with greedy, crazy people. In the 1930s, Javan Rhinos were nearly hunted to extinction in Malaysia, India, Burma, and Sumatra because people believed their horn had medicinal properties.

Today, this rhino is only found on the extreme western end of the Indonesian Island of Java, making it likely the rarest, large mammal in the world.

Number Two - the Hawaiian Crow. Hawaii is one of those places where until Europeans moved in, biodiversity was through the roof. The white folks, we brought cats and rats and snakes and bird malaria and feral pigs, and these days, Hawaii is known as the endangered species capital of the world.

Since Captain Cook first set his buckled shoe on Hawaii, 28 native bird species have gone extinct, and the Hawaiian Crow, or 'Ê»Alalā, is one of the newest. There are fewer than 100 'Ê»Alalā alive today and they are all in captivity, making them totally extinct in the wild.

And Number One - the Baiji. A freshwater dolphin found only in China's Yangtze River, the Baiji is functionally extinct, meaning that there are so few of them that they're probably not reproducing anymore.

Nobody knows for sure whether the last of these small, white dolphins has already succumb to over fishing, habitat loss, and pollution, but a 2006 search of the entire river yielded no sightings. A year later, a fisherman took a video of what looked to be a Baiji, but the tragic fact is, one individual isn't enough to bring a species back from extinction. Just ask Lonesome George.

Thank you for watching this extraordinarily depressing SciShow dose. If you have any comments or questions or ideas for us we're on Facebook and Twitter and, of course, down in the comments below. We'll see you next time.