Besides a large complex brain and similar chromosomal structure we know that dolphins share our common ability to feel pleasure and pain, and to form complex, lasting emotional bonds. These are the things that make human life meaningful and valuable. Because these capacities are shared by dolphins, and sometimes had to a greater degree, we must recognize that the same things make their lives meaningful and valuable.
Dolphins have evolved highly developed nervous systems and demonstrate a capacity for intelligence, emotion, and self-awareness that is rivaled by few other species on the planet. Their hearing is at least 10 times more sensitive than that of humans and they have evolved an extra sense, an echolocation system to image their environment in three dimensions. Imaging studies have also shown that the dolphin brain contains up to three times more specialized "spindle" neurons than the human brain. These neurons are thought to function in the processing of emotions and awareness, and have thus been found only in the large brains of higher vertebrates.