MadSci Network: Anatomy |
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 !
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