It's partly because of a more complex version of that, partly because of evolutionary drift. Mammals can still be incredibly distinct when you get down to spec

It's partly because of a more complex version of that, partly because of evolutionary drift. Mammals can still be incredibly distinct when you get down to specific biological pathway behavior. Or they can be exactly the same. Depends on how the mutation dice came up. For example, the different ways various mammal species metabolize theobromine [1] (aka, the "dangerous" thing in chocolate). Dogs and humans are similarly allergic, but because we metabolize it differently it's eliminated from our bodies much more quickly, which keeps it from building up to dangerous levels. As my dad (vet / path / drug development) would say, 'Mother Nature is lazy before anything else. She'd rather repurpose an already evolved system than create something new in almost every circumstance.' Multiply that by ~95 million years (last common dog-human ancestor), and you get a lot of quirky genetic code debt. ;) [1] https://www.ncbi.nlm.nih.gov/m/pubmed/6144479/ https://www.ncbi.nlm.nih.gov/m/pubmed/6144479/