Transcript: The normal rate of evolution via natural selection is slow at a genetic level. There typically is one mutation for every 100,000 divisions of a cell. However, when brains form, the acceleration of evolution is possible, because brains enable the manipulation of the environment. On Earth, brain size tends to scale with the size of the organism. Humans have a relatively large brain for their size, but similar ratios are seen in orcas and other marine mammals, and elephants are not that dissimilar. We believe the complexity of the human brain outstrips the complexity of any other brain of an animal on Earth, but evaluating the intelligence of other animals that we share the planet with is difficult. Orcas, for example, are top of the food chain in the oceans, have a large and complex brain surface, have a complex language, as yet not understood by us, engage in play, have complex social networks, hunt in groups, and do other sophisticated behaviors. How intelligent are orcas, or even other great apes? It is hard to say. We share 95% or our DNA with the great apes, and some species of chimpanzees are so similar to us that they’re barely a different species, and yet humans are special in their high capacity for abstract thought.