Skeletal Development : Evolutionary Implications
Comparative biology is not exactly a hot bed of social debate and ethical doubt. However, it does help to place all the anatomy and physiology we learn about ourselves and our fellow vertebrates into perspective. No one knows the exact reasons for why things happened, and in some cases it is barely known how they happened. If nothing else, though, it forces you to look at the way an animal works as a living, moving, eating, surviving whole and how the biology of its different tissues contributes to or endangers its existence.
Bone plays such a huge role in evolution: ultimately, it is one of the features that allowed the first tetrapods to hold themselves up on land without the buoyancy of water to support them against gravity. As a structural tissue, its capacity for flexibility of structure has made possible a huge number of shapes, sizes, and adaptations in movement and locomotion between species and allows one tissue to take on a number of functions within a single individual.
In looking at the skeleton, it is easy to examine it as a whole, though both evolution and embryology separate it in development. Each piece of the skeleton was selected for because of a completely unique function it had:
The possibilities of bone tissue itself for flexibility, strength, and most importantly remodeling and growth over other structural tissues gave the structures it made up the advantage of adaptability, which lead to the vast diversity of bone shapes and functions seen across the vertebrates.
The dermal plates in the head which began as a cumbersome body covering of bony plates ultimately came to provide protection to perhaps the most important organ of all, the brain.
The axial encasings of the spinal cord allowed a freedom of movement with a decreased effect on the nerve tube that allows it. Its segmentation gives it a flexibility unlike any other part of the skeleton. Later on, the development of the sternum allowed for the creation of negative pressure in the lungs that allowed animals to abandon buccal pumping and the long, flat head it required for a more freeing and efficient mode of respiration.
The appendicular skeleton started and remains in many animals today as two pairs of fins which are not used nearly as much for locomotion as is the tail in fish and took on weight bearing responsibilities upon the transfer to land. The limbs can now take on different shapes for climbers, diggers, walkers, bounders, swimmers, and fliers.
One of the most amazing things to realize, however, is how well conserved the signalling mechanisms that determine limb shape are. Though the very process by which bones become bone tissue is different in different parts of the skeleton, limb morphogenesis is directed by genes that are similare between humans, rats, and even drosophila. How the system has come to have such variety and yet such conservation of molecular mechanisms is something to wonder at.