Re: how did human foot evolve

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.