The proximity of key sensory organs to the mouth points to the original function of the vertebrate face. The first vertebrates, which lived about 500 million years ago, were tiny fish that moved in search of food. The close conjunction of mouth with organs of vision and smell would have enabled these animals to sense and detect potential food items and readily ingest them. Had those sensory organs evolved to be near the tail end of the animal, finding and taking in food would have been much more difficult.
This basic function of the face has been retained in all vertebrates, including our own species. But just as our visual and olfactory organs perform other, additional functions to that of finding food, the human face, as a whole, does more than just perform as the "sensory headquarters" of the animal. Chief amongst these is a large expressive capacity, which not only aids in manifesting our feelings but in conveying them to others. Although our nearest animal cousins, the great apes, have great facial expressivity, the human repertoire of expressions is far larger than theirs. Furthermore, since every extended conversation between people involves an accompanying play of facial expressions, and since we talk a great deal, we are undoubtedly the most facially expressive animal on earth.
That conclusion however immediately prompts a question: how and why did our species acquire such expressive faces?
The evolution of the distinctive physical features of the human face, which facilitate our ability to express feelings, can be traced to two evolutionary periods: the beginnings of the "anthropoid" (or man-like) apes, about 55-50 million years ago, and the past six million years, in which humans branched off from the lineage that gave rise to modern chimpanzees. In the first, our ancestors lost most of the fur covering their faces, experienced a large reduction of the muzzle, and developed closer-set eyes than their primate ancestors. In the second period, that of "hominin" evolution, in which the human lineage fully developed, we lost more facial fur, eliminated the muzzle completely, acquired foreheads (due in large part to our enlarged brains), chins, bone-buttressed noses, and, overall, the flat, relatively small, vertically disposed faces that distinguish the human face from all others.
Yet, there was also much accompanying evolution of the brain, as evidenced by the increase in brain size, but there is little fossil evidence of the detailed structural changes. Those changes, however, can be inferred from comparative studies of living species. The remodeling of the brain to make a fully human brain would have involved neural "rewiring" that facilitated greater facial expressivity, the instantaneous interpretation or "reading" of those facial expressions, and, a growing integration of oral communication (speech) with facial expressions. That integration is as distinctive a feature of humans as is speech itself and raises some of the deepest questions. A central one concerns when language, and speech, arose in our lineage. Were there "proto-languages" perhaps as much as a million years ago, in one or more of our ancestral species (Homo erectus, Homo heidelbergensis)? Or did language arise as a fully-developed capacity only in our species, Homo sapiens, sometime in the past 200,000 years? The answer to that question, if it can be found, may help reveal how much language-development itself drove the evolution of increasing brain size in the human lineage.
Irrespective of whether the evolution of language was one factor driving brain size increase, it seems likely that the increasingly complex social relationships our species exhibits, were a major contributor. This idea is known as "the social brain" hypothesis and is attracting increasing support. In light, however, of the complexity of facial expression in serving to convey so much information about the individual, it seems likely that an important addition to this concept may be termed "the social face" hypothesis. This is the idea that the increasingly complex social environment of our ancestral species played a major part in the evolutionary shaping of our species' highly distinctive and expressive face.
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