The existence a common feature between animals, or between animals and humans tells you nothing about their origins. The great philosophers, such as Descartes, used their belief that animals cannot think as a justification for arguing that they do not have moral rights . It is one thing to treat animals as mere resources if they are presumed to be little more than living robots, but it is entirely different if they are recognized as fellow sentient beings . Working out the moral implications makes a perfect puzzle for a large-brained, highly social species like our own. The existence of a brain in animals does not alter the case for animal rights, any more than does the existence of legs.
Chickens have two legs and walk upright.
They seem to be leaning toward apes, dogs, dolphins, etc. Our Declaration of Independence says that our rights come from our Creator. Animal rights require a source, just as human rights do. Our Constitution extends this notion. Based on the belief in human rights from a Creator, the Constitution develops the idea that government does not dispense rights, but has only the rights delegated to it by people who previously possessed them.