There are hundreds of thousands of species of beetle, but only one species of human, and koala, and aardvark.

Homo sapiens evolved around 300,000 years ago, when there were at least three other species of the genus Homo on Earth. By 40,000 years ago, we were the only one left.

But at least we have great ape cousins in the hominid family. There are other species on Earth that are even lonelier, the last branches of their family trees.

Koalas live only in Eastern Australia—the only species and the only genus in their families. Their closest living relatives are another unusual Australian, the wombat, which they diverged from 35 million years ago.

It’s thought that koalas’ highly specialized diet is the reason they branched off. They’re adapted to eat eucalyptus leaves—and only eucalyptus leaves. Which happen to be high in toxins, high in fiber, and low in calories.

To handle this poorly nutritious diet, they developed special teeth, special stomachs, and special livers. And sleep nearly 20 hours a day to process it through their systems.

Aardvarks have been lonelier longer. Their closest living relative is the African elephant, separated by 80 million years. They may look like South American anteaters, but that’s because they evolved to eat the same diet of ants and termites.

There’s one animal even lonelier, surviving almost unchanged since the dawn of the dinosaurs. But we’ll save that for another EarthDate.