The evolution of cognition is the process by which life on Earth has gone from organisms with little to no cognitive function to a greatly varying display of cognitive function that we see in organisms today. Animal cognition is largely studied by observing behavior, which makes studying extinct species difficult. The definition of cognition varies by discipline; psychologists tend define cognition by human behaviors, while ethologists have widely varying definitions. Ethological definitions of cognition range from only considering cognition in animals to be behaviors exhibited in humans, while others consider anything action involving a nervous system to be cognitive.