At least five types of dog existed by the end of the Ice Age, 11,000 years ago - CNN

"In many cases humans would simply bring their dogs with them as they migrated and moved across the world," he added.

"But in some cases dogs display different history, suggesting there were more complex factors at play: Perhaps sometimes people moved without bringing their dogs, or perhaps sometimes dogs were traded between human groups," he said.

European dogs have lost their diversity -- but no one knows why

And while modern European dogs appear to come in such different shapes and sizes, genetically, they share a recent common ancestor and are not as genetically diverse as they once were.

Bergstrom told CNN that before 4,000-5,000 years ago, Europe had a greater diversity of dogs, with some animals similar to Middle Eastern, Siberian and "everything in-between" dogs.

At some stage, however, this diversity was lost, and is not found in today's European dogs.

"If we look at present-day (European) dogs, despite their enormous variation in terms of what they look like they actually derive from just a small subset of the diversity that used to exist in Europe," he said.

"It's a bit of a mystery how this happened," Bergstrom told CNN, "but likely at some point a single population of dogs banded across Europe and replaced everyone else."