Re: What is the purpose of the fibula?

The bones of the lower (hind) leg are called tibia and fibula, which in 
latin mean 'flute' and 'needle'.  Archaeological examples of sheep tibias 
made into flutes have been found, and the resemblance of sheep and human 
fibulas to needles is obvious.

The paired bones of the second segment of the terrestrial vertebrate limb 
(shin and forearm in humans) are part of the basic body plan laid down 
when tetrapods evolved from fish way back in the Devonian period, around 
300 million years ago.  Bones become adapted for various purposes through 
evolution, and may in some circumstances become reduced in size or even 
disappear - like your fibula.

The question to ask, therefore, is not so much 'Why do we have fibulas ?' 
but 'How do fibulas fit into the latest human body plan ?' Paired bones, 
such as our radius and ulna, give manouverability to the lower limb, and 
of course two bones are likely to be stronger than one.  Two bones in a 
limb which is only required for running in a straight line, could, 
however, be seen as unnecessarily complex.  Clues can be obtained from 
other land vertebrates.  

In flying animals, such as birds and bats, the fibula has more or less 
completely disappeared, presumably to save weight.  Cursorial (fast-
running) creatures such as horses have also dispensed with fibulas almost 
entirely.  Massive animals such as elephants and rhinos (and dinosaurs)
have substantial fibulas, probably to help support their weight, 
and 'primitive' land vertebrates such as amphibians have undifferentiated 
fibulas, very similar to the tibia, harking back to the days when it was 
cool just to have a pentadactyl limb.

Our own half-hearted fibula, especially when compared with our intricately 
shaped and muscled ulna, emphasises the division of labour between our 
front and hind limbs.  All of a bat's limbs are adapted for flight, and 
all of a horse's limbs are adapted for running in straight lines, but we 
have 'hands' at one end and 'feet' at the other, and, except in the case 
of one famous US president, are therefore able to do two things at once.  
We  could be described as cursorial, in that we are able to walk and run 
for long periods, and do not use our legs for any other purpose.  It is 
therefore to be expected that our leg bones will evolve away from the 
condition of our slow-moving, tree-climbing ancestors, and become more 
simplified, like those of horses.

For further reading, try basic high school/undergraduate zoology and 
anatomy texts such as J Z Young's 'Life of the Vertebrates', or get down 
to the museum and look at some skellies !