Not only did Charles Darwin document a compelling explanation of how the human species originated from earlier primates, he also offered an unequivocal opinion on the difference between humans and other animals. "The difference in mind between man and the higher animals," he wrote, "great as it is, certainly is one of degree and not of kind." And so the "degree versus kind" debate was launched.
There are two approaches to settle this argument: the scientific and the philosophical. Both point to the same conclusion, which is why it is mystifying that we are still debating this question.
Research in comparative psychology reveals that behavioral traits once considered unique to humans are, in fact, shared with other animals, including cognitively sophisticated ones such as deception and self control. A low-rank male baboon will, for instance, threaten a top-rank male for the sole purpose of distracting him so that another low-rank male can have sex with one of the top rank's mates. Chimps will even distract themselves to enforce their willpower: in experiments where they are rewarded with more candy if they accumulate it rather than eat it right away, they will play with toys to resist temptation. Chimps will also stockpile and hide rocks for future use as weapons, indicating an ability to think outside the present moment to a hypothetical future.
But it is not just great apes that demonstrate cognitive sophistication. "Uncertainty monitoring experiments" reveal that dolphins and rhesus macaque monkeys will monitor their thinking when they are faced with uncertain outcomes; they will, for example, opt for easier tasks that offer lower rewards, rather than difficult tasks that offer higher rewards. The burgeoning research that reveals the incredible ingenuity of other animals does not detract from how much more advanced human cognition is, but it does suggest that there are precursors in other animals for our cognitive skills. This makes a strong case favoring the argument of degree, rather than kind.
Beyond the research, there are two conceptual arguments that challenge the notion of a discontinuity between human and animal thinking. First, just as there is an enormous cognitive gap between chimps and humans, so too is there an enormous gap between ants and chimps, which invites this question: if the former represents a gap in kind, why would the latter not be a gap as well? If so, just how many discreet gaps are there between ants and humans? Put another way: on the tree of great apes, are we really that much farther out on our own branch than, say, self-aware dolphins are on the tree of marine mammals or tool-using crows are on the tree of birds? The degree side of the argument does not have to contend with this problem: all animals, including humans, have cognitive powers that lie on a spectrum of varying degrees of complexity.
Nor does the degree side have to contend with a second conceptual problem: if the difference is one of kind, when exactly did ancient, apelike humans "jump the gap" to become modern humans, different in kind from their immediate ancestors? In the gradual evolutionary process of ape becoming human, the transition from non-human to human is only intelligible in the context of a protracted period of time in which degrees of cognitive complexity accumulated.
Do science and philosophy conclude the argument in favor of the degree side? Yes, but the "win" is an empty victory, and this is the key point: outside of a religious context, the degree-kind argument is more of a semantic game than an ontological dichotomy. The issue collapses into how we define "kind." Specifically, are we making a meaningful distinction when we contrast "difference in kind" and "huge difference in degree"? No: in a non-theistic context, they mean the same thing. We can stop debating because there is no substantive debate: this is a case of what philosophers call "a distinction without a difference." We are extraordinary animals -- the most complex genus of the primate order in the mammalian class of animals.
So does it matter which words we use to describe the difference? Absolutely. The language we use to describe our reality influences how we think about it. The problem with viewing ourselves as being "fundamentally different" reinforces a view of being separate from, rather than part of, the rest of the world. Perceiving ourselves as "further along the complexity spectrum" is a more productive way of describing our relationship with the other species with whom we share the planet. Acknowledging a continuum of cognitive complexity reinforces much-needed humility upon us, reminding us of our obligation to use our cognitive complexity in thoughtful, responsible and useful ways.