It's their ability to smell, in particular to smell approaching humans, that make them so useful. If you've ever walked past a homeless person that hasn't showered for a few days, you know that humans are particularly smelly in their natural state. For most hunting animals, this would be a negative because it alerts prey. For humans, it's like the rattle on a rattlesnake. It's a warning to get out of the way of Earth's most deadly killer. Dogs serve as first alerts in tribal warfare of an impending surprise attack. Inferior skilled dogs were killed off along with the tribes that owned them.
When my Belgian Sheepdog (Groenendael) boy was a puppy- first day home- I took him into the woods to go potty. He walked around a bit, then suddenly, took that cute, needle nose of his and plunged it into some pine needles and pulled out a baby rabbit. I would have NEVER have guessed a bunny to be there. But he did, right away. Bunny was dead instantly.