Here's what popular dog breeds looked like before and after 100 years of breeding

Dogs have been our furry companions for thousands of years, but they didn't always look the way they do today.

Many well-known dog breeds have changed a lot physically in the last century, thanks to humans.

By identifying specific traits - such as size, coat color, and demeanor - and allowing only those animals to mate, we've created at least 167 different "breeds," or groups of dogs with unique physical and mental characteristics (still, they're all part of the same species).

The "Science of Dogs" blog put together a side-by-side comparison of several popular dog breeds from the 1915 book "Dogs of All Nations" by Walter Esplin Mason showing what they look like today.

Here are some of the dogs from that list, plus a couple more we found ourselves: