TAG | evolution
Our story begins long ago with an ancient jawless fish from whom came every tetrapod, from an axolotl to a zebra-finch, and almost every fish in the seas, lakes and rivers. This fish’s parents were ostracoderms (hard shelled dermis), extinct fishes covered in an armour made from scales of dermal bone. They had cartilaginous endoskeletons to provide some structural support. But this fish was on its way to gaining a feature which would separate it from the ostracoderms, developing a jaw. The first stage was to develop jointed branchial arches rather than the one-piece branchial basket we now see in lampreys, this in itself allowed better ventilation which provided the oxygen needed for a fish that was becoming an active pelagic predator. Over generations these joints would have become more pronounced, until eventually they were such that they allowed the next step in the journey towards jaws.
The two front gill arches developed into the mandibular and hyoid arches. The former the jaw itself, and the latter the support from which it hung. This allowed a buccal pumping mechanism to better ventilate the gills. The gill slit between them became the spiracle or in our case the ear cavity. This jaw was adapted for co-option into the predatory life style as a weapon.
Some of these gnathostome fish (jawed mouth fish) were living in freshwater, they made their bodies more dilute in an attempt to become more isotonic with the water. These fish, still heavy with their dermal bone used their fins as wings to generate lift and wafted a heterocercal tail to force water downwards generating lift. This required a great deal of energy and so posed a problem, it was solved differently in two different groups.
In one which would give rise to the chondrichthyans (cartilaginous fish) the dermal bone was lost and replaced with dermal denticles, a skin of tooth-like projections. This decreased the density of the fish and allowed them a role as important pelagic predators, still beating their tails to generate lift and using an oily liver to increase their buoyancy. Their only structural support came from cartilage, which they made stronger in their vertebrae by using calcium phosphate, creating a form known as prismatic cartilage. The chondrichthyans would go on to form the sharks, skates and rays and the chimaeras.
Another group of gnathostomes would give rise to the osteichthyans (bony fish). They would have lived in anoxic conditions, perhaps caused by vegetation decomposing in warm water. Such fish would have stayed near the surface of the water, where oxygen would be most concentrated and here they evolved the strategy of swallowing air from above the surface, so that it passed through their gut. It could be inefficiently absorbed as it passed through the gut. But there was selective pressure for an invagination of the gut wall to allow a greater surface area for oxygen absorption. Gradually this pocket would have become a specialised lung. This lung provided a store of air in the body, decreasing its density. This allowed the potential for the evolution of endochondral bone, created by the ossification of cartilage by osteoblasts laid down during development, the feature that gives the osteichthyans their name.
The buoyancy these fish now enjoyed freed their fins from generating lift as wings, they could now be adapted as flexible appendages for steering. This was achieved in two different ways by different groups.
The actinopterygians (ray finned) withdrew their endoskeleton and left their fins supported by rays connected by webs of skin. This gave them greater flexibility and manoeuvrability but did not provide very much strength. However when surrounded by water strength is not a very important characteristic and the actinopts have gone on to dominate the sea, comprising 95% of all fish species. Many moved out of anoxic environments and separated their lungs from the gut, so that it was used simply for buoyancy and known as a swim bladder. Only the basal Polypterus has lungs that look very similar to those of other osteichthyans. Many moved into marine environments but they betray their freshwater past with hypotonic bodies. A major group, the Teleosts, use their original lungs as swim bladders but have re-evolved secondary lungs. The electric eel is one which has a secondary lung in its mouth. The actinopts also no longer needed their heterocercal tail for upthrust given their newfound buoyancy and so withdrew the notochord which had run along the top of their ventral tail. This formed a symmetrical, homocercal tail with which to generate simply thrust.
The sarcopterygians are another group of osteichthyans which took a different path when their fins were freed up. Again they achieved greater fin flexibility, but they did so without withdrawing their endoskeleton. Instead they reduced the number of basal elements to allow flexibility while maintaining strength. In the water column this was probably not of great significance. But such strong limbs were ready to be co-opted firstly for strong underwater movement along the bottom of water, as in Acanthostega, and later after further skeletal modifications, to form the terrestrial tetrapods to which we belong. Sarcopts also adopted a different approach to modifying their freed tail to provide forward thrust instead of upthrust. They did not withdraw their notochord but placed it down the middle of a symmetrical, diphycercal tail.