Since different organisms adapt to different environments and receive different mutations, they adapt differently and slowly change. That is why there are different species of birds, just look at the galapagos islands, there are thousands of species of finches that changed to survive on each island. The same could be said for every organism since the early organisms that we evolved from were widespread, some evolved differently than others, leading to different fish, different lizards, different early mammals, and so on. That is why humans look different than dogs or cats, we evolved best to our environment so we could survive.
