Listen to part of a lecture in an ecology class.

The polar bear lives in one of the coldest, harshest environments on Earth, the Arctic, but over the past 100 years, average temperatures have risen three to five times faster in the Arctic compared to average global temperatures. What happens if all the Arctic ice melts? Well, if the polar bear becomes extinct, that could trigger a chain reaction on the rest of the Arctic ecosystem. It could set in motion a trophic cascade.

A trophic cascade occurs when a top predator is added to or removed from an ecological system. An ecosystem's food chain, the transfer of energy from organism to organism, is highly interlinked. In the Arctic, the seal is the polar bear's primary food source, but the arctic fox eats the leftovers from the polar bear's hunt and depends on that for survival. The fox would have to replace that food source, causing another level of disruption, and this effect would expand to other levels as well.

Okay, before we can talk about what the future of the polar bear might look like, it would help to understand its complex history. Now we know that the polar bear and the brown bear diverged from a common ancestor and evolved into two distinct species with DNA unique to each species, but if you ask me when, well, that's another matter.

Part of the problem is that some researchers have based their conclusions on mitochondrial DNA testing, while others have based their conclusions on nuclear DNA testing. Now the various mitochondrial and nuclear DNA studies on modern bear populations have provided valuable information that can help us make inferences about when polar bears and brown bears diverged. But remember, they are only inferences, and mind you, each study has led to different conclusions.

So despite multiple studies, we still don't know if the split happened 100,000, 600,000, 1 million, or as many as 5 million years ago. Each time a new study is done, previous researchers acknowledge that certain conclusions they came to were probably wrong, and unfortunately, polar bear bones are an uncommon find. Polar bears live their lives on ice, and when they die, their bones typically sink down into the depths of the ocean. One of the oldest bones is dated to 120,000 years ago.

So what can we really say at this point? Anyhow, here's what scientists believe led to the split. At some point in the bear history, the climate warmed during an Ice Age and massive continental ice sheets shrank back to the poles. This would have allowed some bears from an ancestral bear lineage to extend their territory much farther north, scavenging for the plants and small animals that made up most of their diet, but when cold temperatures returned, these bears were cut off from warmer climates by massive glaciers and eventually evolved key genetic differences that made them uniquely suited to hunting in the Arctic.

For example, the shape of their teeth changed, their fur color became very light, and there's also the gene APOB. APOB affects the metabolism of fat. Changes in the sequence within this gene enable the polar bear to switch to a high-fat food source and survive. So far, so good. But here's where things get particularly interesting. Let's look at a group of modern-day bears in Arctic Alaska, known as the ABC Islands bears. The ABC Islands bears have brown bear DNA and look like brown bears, but they also have some polar bear DNA. I mean, if the polar bear and the brown bear evolved separately from an ancient bear, what happened here? Well, one hypothesis is that polar bears had been stranded on the ABC Islands due to receding glacial ice during a relatively recent warming period.

Brown bears swam over to the islands from the Alaskan mainland and mated with the polar bears, producing fertile offspring. This interspecies mating happened repeatedly over generations until the offspring ultimately resembled brown bears, which are better suited to life in the forest. So we have evidence that the two bear species have created fertile offspring, and although it's still a rare occurrence, there have also been documented instances of the two species mating to produce offspring in other places as well. Ultimately, then, this appears to open up an interesting possibility for the future fate of the polar bear, doesn't it? With the Arctic climate getting warmer, similar scenarios may repeat themselves with greater frequency.