The course followed by humans on the path to language was a progression through natural signs to human symbols. Signs and symbols are explained in reference to a theory of "semiotics," the study of signs, in the writings of Charles Sanders Peirce (1839-1914, usually known simply as C.S. Peirce). Peirce was perhaps the most brilliant American philosopher who ever lived. Bertrand Russell said of him, "Beyond doubt ... he was one of the most original minds of the later nineteenth century, and certainly the greatest American thinker ever."
He contributed to mathematics, to science, to the study of language, and to philosophy. He is the founder of multiple fields of study, including semiotics, the study of signs, and pragmatism, the only uniquely American school of philosophy, further developed by William James and others.
Peirce's theory of semiotics outlines a conceptual progression of signs from indexes, to icons, to human-created symbols. This progression moves to an increasing complexity of types of signs and the evolutionary progression of Homo species' language abilities. A sign is any pairing of a form (such as a word, a smell, a sound, a street sign, or Morse code) with a meaning (what the sign refers to). An index, according to Peirce, as the most primitive part of the progression, is a form that has a physical link to what it refers to. The footprint of a cat refers us, makes us think of, a cat. The smell of a grilling steak brings to mind the steak and the grill. Smoke indicates fire. An icon is something that is physically somehow like what it refers to. A sculpture represents the real-life object it is about. A portrait likewise is an icon of whatever it was painted of. An onomatopoeic word like "bam" or "clang" bears an iconic sound resemblance to another sound.
It turns out that Peirce's theory also predicts the order of language evolution we discover in the fossil record. First, we discover indexes being used by all creatures, far predating the emergence of the genus Homo. Second, we discover the use of icons by Australopithecines in South Africa some 3 million years ago. And finally, through recent archaeology on their sea voyages, settlement, and burial patterns, we discover that 1.9 million years ago the first Homo, erectus, had and used symbols, almost certainly indicating that human language—the ability to communicate most anything that we can communicate today in principle—began far before our species appeared. What is most fascinating is that Peirce's semiotics is a theory of philosophy that inadvertently makes startlingly accurate predictions about the fossil record.
The influence of Peirce's semiotics throughout world philosophy, influencing figures such as Ferdinand de Saussure, among others, extends to industry, science, linguistics, anthropology, philosophy, and beyond. Peirce introduced the concept of "infinite semiosis" long before Chomsky raised the issue of recursion as central to language.
Perhaps only Peirce, in the history of inquiry into human language, has come up with a theory that at once predicts the order of language evolution from the earliest hominins to Homo sapiens, while enlightening researchers from across the intellectual spectrum about the nature of truth, of meaning, and the conduct of scientific inquiry.
Peirce himself was a cantankerous curmudgeon. For that reason he never enjoyed stable employment, living in part off of the donations of friends, such as William James. But his semiotics has brought intellectual delight and employment to hundreds of academics since the late 19th century, when he first proposed his theory. His work on semiotics is worthy of being much more widely-known as relevant to current debates. It is far more than a quaint relic of 19th century reflection.