Many hypotheses have been put forth to explain the unique traits of Homo sapiens, especially in comparison to its closest genetic relatives, Pan troglodytes and Pan paniscus. Conventional thinking in evolutionary biology and biological anthropology has it such that the ancestors of the genus Homo transitioned from a largely arboreal lifestyle in the forest to a terrestrial lifestyle in the savannah and scrub, and that in the case of Homo sapiens, unique selective pressures caused our species to lose much of our furry covering, reduce the robusticity of our skeletons and dentition and drastically boost our encephalization.
Elaine Morgan posited that the aforementioned traits were in part the result of a stage in our evolutionary lineage that took place in an aquatic or semi-aquatic habitat. This hypothesis is known as the 'Aquatic Ape Hypothesis', and has ignited more than its own share of controversy. While I do not outright dismiss the Aquatic Ape Hypothesis, I believe the evidence points to an even more intriguing set of selective pressures that explain the unique traits of Homo sapiens --- flight.
Homo sapiens has greatly reduced dentition and skeletal robusticity than most other apes, a clear adaptation to flight, where any extra weight exerts significant metabolic penalties in flight. While our bones are not hollow and braced like those of avians, they are very gracile in comparison to earlier hominids, possibly pointing to a relatively recent evolutionary phase when we were able to take to the skies.
Critics have objected to this line of reasoning, claiming that no matter how much weight is saved by a gracile skeleton and reduced dentition, Homo sapiens simply cannot furnish sufficient muscular effort to become airborne, pointing to a number of early attempts at powered flight involving avian-style flapping. Even the most physically fit human using the best-engineered machine cannot take to the air by the muscular effort of the arms alone --- for that, we would need an enlarged pectoral keel to anchor the necessary flight muscles.
Instead, I propose that our flight was more akin to the sugar glider or flying squirrel, involving stretched membranes of skin extending from the arms to the legs, and that furthermore the primary muscular effort was furnished not by the arms, but the legs. After all, Homo sapiens has proportionately much more heavily-muscled legs than any other primate. Kinesthetic models suggest gliding flight with bursts of intense kicking to remain aloft, perhaps suggesting large webbed feet as well to more efficiently provide propulsive force.
While muscular and skeletal adaptations are quite important to flight, other adaptations are equally crucial. The avian brain is proportionately much larger than other animals of equal body mass, and it would likely be an explaining factor in why Homo sapiens has a vastly greater degree of encephalization than any other creature extant or extinct. The cerebral hardware necessary to make the fine motor adjustments in flight likely exerted selective pressure for larger brains. Current anthropological consensus is that the large human brain is the result of an increasingly complex social environment, and I have no objection to this assertion, since humans exhibit what might be considered 'flocking' behavior very much akin to that of passerine birds. A combination of social pressure and the increasing need for rapid fine motor control could very well have kick-started the upward trend in brain size evident in the hominid fossil record.
Other minor physical adaptations are evident in modern humans. We likely shed our body hair to reduce aerodynamic drag penalties, while retaining thick hair coverings on the scalp, face, armpits and groin for sexual display purposes. The human nose took on a pointed, slender profile to function as a sort of rudder and reduce the frontal area of the human face.
The flocking behavior of Homo sapiens also necessitated an increasingly complex means of communication and social organization, likely the beginnings of language and culture. Echoes of flight still resound in our daily lives, and one in particular: witness the behavior of individuals and groups at airports. There is a palpable tension as each individual recalls the exhilaration and inherent danger of flight embedded deep within our collective cultural memory. Passengers remove shoes, belts and baggage, reminiscent of a ritual meant to shed excess weight before taking to the air. Many consume high-calorie foods rich in fats and simple carbohydrates, and caffeinated beverages, something that our ancestors likely did to quickly meet the vast metabolic demands of flight. This may also explain the greater role of animal proteins in the human diet, as compared to other primates: meat is much more nutritionally-dense than plant foods, providing a much more concentrated energy source for the intense demands of flight.
The evidence for an avian stage in human evolution is mounting, and textbooks of the future will likely require a redrawing of the universally familiar 'ape to man' sequence to include a winged ape at some point before the upright modern Homo sapiens. Further research must be conducted to determine when this hypothetical Homo icarus (as myself and my colleagues have named it) split off from the hominid line, as well as when and why we lost our powers of flight.
wertperch suggests that avian flu likely contributed to their extinction, a hypothesis eminently worth considering. It would certainly not be the first time that disease has been transfered between humans and animals.
Much credit is due to the various members of the Richard Dawkins forum for sparking this hypothesis.
Much credit is due to Glowing Fish for suggesting that the retention of head hair served to reduce the risk of windburn for Homo icarus, in addition to its sexual display purposes.