Polar bears are such massive, popular and iconic animals that you'd think we'd have long understood whence they came.

However, establishing the origin of polar bears has proved difficult.

We've struggled to reveal when they became the instantly recognisable white bear we know today, and still know relatively little about what happens when polar bears and their darker-coloured cousins come together, and perhaps even mate.

It has been long known that polar bears are indeed bears, belonging to the Ursids, the family of mammals that include brown and black bears, as well as others such as sloth and spectacled bears.

That may seem obvious. But there was, until quite recently, a long-standing confusion around the origin and relationship of one the polar bear's closest cousins; the panda, with scientists debating whether that species was a true bear at all, until genetic studies confirmed it was.

It has been difficult to determine the origins of polar bears in part because few preserved ancient polar bear remains have been discovered.

So scientists have turned to studying the genetics of bears to establish when they diverged from each other.

One study published in 2013 suggests that pandas split from the bears anything from 8 to 38 million years ago.

Black and brown bears then split into unique lineages between 1.5 and 6.5 million years ago.

And polar bears diverged from brown bears between 130,000 and 650,000 years ago, with the general consensus that they first appeared in the Pleistocene, and must be at least 115,000 years, the date of the oldest known polar bear fossil.

However, that is not the end of the story. Discrete populations of polar bears are thought to have mated and bred with populations of brown bears since.

Now-extinct brown bears that once lived in Ireland, for example, had polar bear ancestry, perhaps because past changes in the distribution of polar ice, for example, stranded polar bears or hybrids on the island.

Brown bears living on islands off the coast of Alaska also appear to have polar bear ancestry.

A study published in 2014 also found tantalising genetic evidence that bears living in the Himalaya mountains, a vast distance from the Arctic, may have derived from polar bears. This unique heritage could have produced bears that look and behave slightly different from the brown bears that usually live in the region, albeit at lower altitudes. And these odd, high-altitude bears may be the origin of the Yeti legend, speculate the scientists who conducted the study.

A meeting of bears

What happens when white and brown bears come together is difficult to answer, as it occurs rarely.

Polar bears do come ashore in summer, and in some regions of the Canadian Arctic brown bears have been observed wandering around on the pack ice. But generally, they live in separate habitats, and there are few recorded instances of polar bears and brown bears mating.

However, that may happen more often if the sea ice in the polar bear's natural habitat melts, forcing more bears ashore and for longer periods.

So far, there is just a single hybrid polar and brown bear known from the wild. In April 2006 a strange-looking bear was shot by a hunter in Nelson Head, on the southern part of Banks Island in the southern Canadian archipelago. A study of its genes revealed it to be a brown-polar bear-hybrid.

Precisely 17 more are known from zoos, the result of bears born to polar bears and brown bears kept together in enclosures, which subsequently mated. And scientists have studied these hybrid bears' features.

They found that polar-brown bear hybrids inherited traits from both parents. Hybrids have visible tails, like polar bears, whereas those of brown bears are barely apparent. They have longer necks more typical of polar bears, but also display small shoulder humps reminiscent of brown bears.

They also inherited blended traits. For example, in terms of overall size, they fall between the larger polar bear and smaller brown bear. The size and shape of their heads is intermediate between the thicker-set brown bear and more slender-headed polar bear.

The bears' feet are also an intriguing blend. The soles of the hybrids' feet are partially covered in hair. Polar bear feet are covered in hair to insulate them from the ice, whereas brown bears have hairless soles and clearly visible toes.

But most intriguing is the bears' hair.

When viewed as a cross section, the shaft of a brown bear's hair is either solid or full of small hollow regions, depending on where the hair is on the bear's body.

The hair of a polar bear is almost completely hollow, with large empty regions within its core. The hybrids' hair was partially hollow.

Behaviourally, the two hybrids have much in common with polar bears.

The existence of these bears proves that polar bears and brown bears in close proximity can and do mate.

What is more, as well as having a combination of features, they are also fertile.

The odds of it occurring may be low, but it raises the possibility that, in a future, warmer world with less sea ice, polar and brown bear may yet consistently breed.

That may either create a new hybrid species, or polar bears and brown bears may merge once more, repairing the split that led to the origin of polar bears in the first place.