(Cross-posted on 3 Quarks Daily, where it has received many comments.)
It is often said that humans are the only animals to use symbols. So many other claims of human uniqueness have fallen away—thoughts, emotions, intelligence, tool use, sense of fairness—what's so special about symbols, you ask? I share your skepticism, dear reader, and in the next few paragraphs I'll tell you why.
Let's begin by clarifying what "symbol" means here. One way to do this is to contrast symbols with signs. A sign, such as a red light, a grimace, a growl, or a thunderstorm, signifies something direct and tangible, making us think or act in response to the thing signified. Issuing and responding to signs is commonplace in Animalia. A symbol, on the other hand, is "something that represents something else by association, resemblance, or convention". A symbol allows us to think about the thing or idea symbolized outside its immediate context, such as the word "water" for the liquid, "7" for a certain quantity, and "flag" for a community. What is symbolized doesn't even have to be real, such as God, and herein lies the power of symbols—they are the building blocks of abstract and reflective thought. Evidence of material symbols used by humans dates back at least 60-100K years, when burial objects and decorated beads start to appear in archaeological finds. Linguistic symbols were almost certainly in use long before then.
According to Susanne Langer, symbols serve "to liberate thought from the immediate stimuli of a physically present world; and that liberation marks the essential difference between human and nonhuman mentality ... Words, pictures, and memory images are symbols that may be combined and varied in a thousand ways." It is only through symbolic thought that we imagine the past or the future—mental time-travel, including episodic memory, requires the use of symbols. Indeed, language is really a system of symbolic communication, combining words (which are symbols) and syntax. If non-human animals lack symbols, what and how do they really think?