Wolves. The great ancestor to all dogs, big and small. Some are the spitting image, such as huskies or German Shepards. Even chihuahuas still have the characteristic head shape. Humans, however, have designed whatever type of dog they desire, despite health risks it may cause. One breed in specific, the bulldog, is particularly terrible as far as genetics.
Most people know the pushed-in-nose style of some dogs that is oh so cute to many people. Unfortunately, dogs with a pushed in muzzle have a terrible time breathing. They typically snort when they get excited, trying to clear their airways while they struggle to breathe. Even worse, dogs pant to cool down. When a dog can't breathe well, they can't cool off properly, making them prone to overheating.
Another problem stemming from the pushed in face comes from all the wrinkles and folds around their faces. Bacteria, moisture, dirt, hair, dust, oils, and anything else in their environment can get in the wrinkles and must be cleaned out at least once a day to prevent infection. Some people, however, don't have time for this and their dogs spend a good portion of their time uncomfortable (to say the least) with infection in their eyes or skin.
Did you know a bulldog can't give birth? That's right, bulldogs are physically unable to perform a natural birth. Instead, every single litter of puppies must be delivered via C-section. This doesn't seem like a big deal until you think about the huge amount of bulldogs that exist and how many mamas that had to be put under and cut open. Even worse, I don't even want to think about what puppy mills do when they're breeding bulldogs...
Other issues with their bones include bone deformities common in the elbows and knees. A dog that is in pain when it walks and can't breathe properly is a dog who is not going to be running around a whole lot. This means that a bulldog may be more prone to becoming overweight and succumb to illnesses related to obesity and inactivity.
With all the issues bulldogs suffer from, there's no doubt that they should've never existed. As someone who loves dogs, I can recognize when humans have messed up. If you have a bulldog, love your bulldog, give them a great life. But don't breed them. Discourage breeders you know from breeding bulldogs. These animals were designed poorly to the extreme and I think it's important to make it stop.