A Deepness of Thought
A study was done a few years ago in which bottlenose dolphins were given a mirror in their tank then marked on their bodies. They immediately went to the mirror to see the mark. This awareness that the reflection in the mirror is themselves is rare. Primates have it. Monkeys do not. What I find so interesting about dolphin self awareness is that it means this self-awareness has evolved independently at least twice and is therefore probably not a random event.
Dolphins also easily understand when a human points. They have complex vocal communication that seems to have grammar. They naturally mimic. These are all things chimpanzees have difficulty with. They are also not trivia. They are the basics of learning.
The brain size of bottlenose dolphins is the closest to humans. The encephalization quotient (EQ) is a measure of an animal's brain size relative to body size for animals of similar size. Humans have the highest EQ. Here are some others:
Human - 7.4
Bottlenose Dolphin - 5.3
Orca - 2.6
Chimpanzee - 2.5
Rhesus Monkey - 2.1
Elephant - 1.9
Dog - 1.2
Rat - 0.4
EQ is a very crude measure of intelligence. The structure of the brain rather than its size has more to do with intelligence. But little is known about how a dolphin's brain works so EQ is the best we have. It would seem bottlenose dolphins are in a class by themselves, somewhere between humans and other mammal.
Dolphins and humans common ancestor lived about 50 million years ago. Bottlenose dolphins evolved in their current big brain form about 3 million years ago. Humans in big brain form are less than 1 million years old. In evolutionary time scale, the evolution of the human brain and bottlenose dolphin brain is virtually simultaneous.
So the interesting question is what happened on Earth in both the oceans and on land to drive the evolution of big brains. Something must have changed in the environment in the last few million years. And why did humans triple their brain size while bottlenose dolphins merely doubled it.
Update Nov 5, 2009
The current thinking is that humans ancestors became bi-pedal 6 millions years ago and had brains the size of chimps for 3 million years. Then three million years ago, the hominid brain size suddenly doubles. Then within the last 100,000 years the brain inceases another 50%. So there would seem to be two separate brain evolution events. One 3 million years ago simulatneously for both humans and dolphins and one recently for just humans.