A sleeping koala. (Rebecca Johnson/Australian Museum Research Institute)

Before we get to the science, let's all just admit we want a koala. So cute!

The koala challenges the kangaroo as the most iconic Australian marsupial. It is a very peculiar animal, spectacularly specialized, living in eucalyptus trees and surviving almost entirely on their leaves, which are highly toxic to most organisms.

But not just any eucalyptus trees: There are about 600 species of eucalyptus, and koalas can be found in just 120 of them, of which only 20 species provide the bulk of the koala diet. And koalas are fanatically choosy about their leafy greens, favoring the ones high in nutrition and water content and pausing to bury their adorable Yoda-like faces in the leaves for a big sniff before nibbling.

So they're definitely not generalists.

"You're talking about a niche within a niche," says Rebecca Johnson, director of the Australian Museum Research Institute.

Johnson and dozens of Australian colleagues on Monday published the full koala genome in the journal Nature Genetics.

The biological instructions for the making of a koala shed light on how this animal survives. One finding is that the koala has an unusual number of genes that encode for enzymes that can break down toxins in leaves.

The genetic information could prove crucial in conserving koalas, which are a threatened species because of disease and habitat fragmentation. One immediate goal for conservationists would be to use the genetic data to improve vaccines already being deployed to prevent the spread of chlamydia, a bacterium that causes blindness and urinary tract inflammation among koalas, said study co-author Katherine Belov, a University of Sydney professor of comparative genomics.

They're big sleepers. A koala sleeps about 14½ hours a day. The remainder of its day is devoting to eating and resting. It moves, on average, four minutes a day.

The koalas, Johnson said, "are basically wombats that decided to go into the trees." They must have faced less competition for food in the trees as they evolved.

Koalas endured a die-off roughly 30,000 years ago for unknown reasons and then took another hit when Europeans arrived in Australia more than two centuries ago and used koala fur to make hats.

Koalas live all along the east coast of Australia, where most of the human population is, but they form two distinct genetic populations. In the north, they are genetically diverse but falling in number because of urbanization and deforestation.

In the south, they're all over the place, and indeed in many cases are starving as they strip trees bare. But they descend from a tiny founder population, probably established from captive animals two centuries ago after European colonists wiped out most of the wild population. They're inbred.

"When you have very low genetic diversity, the animals become essentially immunological clones. So any change in disease, change in temperature, change in climate, can potentially affect the entire population," Belov told The Washington Post. "You can say there’s a lot of animals but that doesn’t mean there’s a genetically healthy population."

A niche existence, especially an inbred one, is precarious on a rapidly changing planet experiencing global warming and the direct impacts of human civilization.

Koalas experienced a precipitous population decline about the same time that much of Australia's megafauna vanished more than 30,000 years ago. Human predation seems unlikely, Johnson said. According to traditional knowledge among Aborigines, koalas were considered sacred in many communities, Johnson said. Archaeologists have not found koala remains in human middens, she said.

So it's unclear what drove the collapse of the prehistoric population.

“It's very likely to be some large-scale environmental change," Johnson said.

Read more:

Why Australia euthanized hundreds of koalas 

Human hunters and climate change may have killed off the Ice Age giants