|MadSci Network: Evolution|
How did the human foot evolve? That is a deceptively simple and extremely difficult question to answer. There have been careers dedicated to this question, and so I'm not sure that I can do it justice in a short little essay. Nonetheless, I'll give it a try. Humans are plantigrade animals, which means that we stand on the whole foot, and not just on the ball of the foot or the toes. Plantigrady is not too common among mammals, and reflects the primitive (closer to the ancestral type) condition more similar to that which is seen in reptiles. What that means is that both the foot and the hand were originally propulsive and manipulative organs. The manipulative ability was adapted into a grasping ability when the first primates evolved. Primates differ from other tree dwelling creatures in that they grasp tree limbs with their fingers and toes rather than using claws. So the human foot past through a grasping phase. But then, somewhere along the line, the human ancestor became a full time terrestrial biped. That means that we walk on the ground all the time (strange, but not unheard of for a primate) and that we walk on only two feet (also strange, but also not unheard of for a primate). One of the ways that human locomotion is special is that we can only walk on two feet on the ground (we are basically incompetent at other forms of travel). One of the major changes that makes other forms of locomotion difficult is the changes of our foot. The human foot still shows the tell-tale signs of our tree dwelling ancestry. But, its shape has changed to facilitate walking on the ground. There are many changes that I could talk about, but one of the more obvious is the adducted big toe. In humans the big toe (the hallux) has moved in line with the other toes (it is permanently adducted). This transforms the hallux into a propulsive organ (we push off with our hallux at each step), rather than a grasping organ. In most other primates the hallux is normally in an abducted position (out of line with the other toes), which allows it to grasp objects like the thumb. The loss of this grasping ability for the hallux is a key change in the evolution of human locomotion. Many studies of the Laetoli foot prints (fossilized human-like prints in East Africa) focus on whether they show an abducted hallux. Some people say they do, although most people say they don't. A recent fossil find in South Africa was of an early hominid foot (designated STW 573, or "Little Foot"). This specimen is usually reconstructed in a way that shows a slightly abducted hallux (more than we would see in modern humans, but less than we would see in modern apes). This reconstruction causes the discoverers to argue that this creature still spent part of its time climbing in trees, and that it was not yet a fully human-like biped. I have had a brief glimpse of this fossil, and I have doubts that its hallux was really as abducted as the discoverers claim. But, until I get to examine the fossil more closely I can only have doubts and cannot make firm conclusions. In any case, I disagree with the conclusion that a partially abducted hallux is a sign of arboreal activity. I would argue just the opposite. There is no benefit to losing the grasping ability of the hallux unless the tree climbing lifestyle had been abandoned. So even a partially abducted hallux is not a fully abducted hallux, which argues for a ground dwelling lifestyle. But, again, I have to say that I have not been able to examine the fossils myself. When I have a chance to do so, I may form a different opinion. There is a fossil foot from Olduvai Gorge (OH8) that has a fully adducted hallux in line with the other toes. Although there are still some minor differences from the modern foot, this foot is essentially the same as ours. So, the human foot, in form and function, is at least 2 million years old. Depending upon your interest in this subject, the most valuable answer I can give you is probably the following list of references. References: Tuttle, RH (1987): Kinesiological inferences and evolutionary implications from Laetoli bipedal trails G-1, G-2/3, and A. In: Laetoli, a Pliocene site in Northern Tanzania. (Eds: Leakey, MD; Harris, JM) Clarendon Press, Oxford, 503-523. Day, MH; Wickens, EH (1980): Laetoli Pliocene Hominid footprints and bipedalism. Nature 286, 385-387. Lewis, OJ (1980): The Joints of the Evolving Foot. Part III. The Fossil Evidence. Journal of Anatomy 131, 275-298. Conroy, Glenn C; Rose, MD (1983): The evolution of the primate foot from the earliest primates to the Miocene hominoids. Foot & Ankle 3, 342-364. Charteris, J; Wall, JC; Nottrodt, JW (1982): Pliocene hominid gait: new interpretations based on available footprint data from Laetoli. Am. J. Phys. Anthropol. 58, 133-144. Lamy, Paul (1986): The settlement of the longitudinal plantar arch of some African Plio-Pleistocene hominids: a morphological study. J. Hum. Evo. 15, 31-46. Clarke, Ronald J; Tobias, Phillip V (1995): Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269, 521-524. Szalay, Frederick S; Langdon, John H (1986): The foot of Oreopithecus: an evolutionary assessment. J. Hum. Evo. 15, 585-621. Szalay, Frederick S; Dagosto, Marian (1988): Evolution of hallucial grasping in the primates. J. Hum. Evo. 17, 1-33.
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