For one thing, if there were 2 smart species or even 20 at any point in our history, only 1 would survive. We are very territorial, belligerent, greedy..etc and let's face it, our ancestors were just plain nuts. They would have killed off anything remotely displaying intelligence long ago.
Look at Big Foot, we don't even know if they exist yet there are many that try to hunt them down to this day. If found they wouldn't survive long without govt intervention and zoo's or reservations for them. We are the most dangerous species to ever walk the earth.
We can thank our natural war like behaviors to drive our need for better technology which requires smarter brains. Octopuses are also pretty darn smart but there is nothing in their environment that requires a necessity to push for more brain power.
Historians believe there may have been as many as 5 species of humans living alongside at one point but apparently our genes, being a mix of the bunch it appears, won out. If anything else was ever intelligent to much degree, they got murdered out as well. If not they probably would us. "There can be only one!!"