Subphylum Vertebrata includes organisms with several basic features, in addition to the typical chordate characteristics. Vertebrates exhibit the most pronounced cephalization, in which the anterior portion of the nervous system (the brain) is enclosed inside a protective case (the skull). These animals all have a vertebral column (backbone), an axial endoskeletal system that forms dorsally to the notochord. This system serves to protect the dorsal nerve cord, support the head, anchor muscles, and protect internal organs. The anterior portion of this nerve cord is protected by a cranium in all of the vertebrates.
The figure above illustrates the evolutionary relationships among members of Phylum Chordata. As you look at the evolution of the group, note the acquisition of the more derived features: jaws, teeth, lungs, legs, amniotic eggs, and hair. We will explore these features (and their origins) in more detail in this tutorial.
During vertebrate evolution, many modifications have occurred in the pharynx. If you have an opportunity to take a class in mammalian embryology, you will learn that many structures are derived from different regions of the embryonic pharynx (e.g., human nostrils originate from pharyngeal slits, which are present in the early human embryo). We will examine the evolution of the jaw so that you can appreciate how evolutionary biologists think these more derived structures evolved.
These jaw images are arranged left to right, with the more ancestral state on the left and the more derived state on the right. In the ancestral state, endoskeletal elements are seen in the pharyngeal arches between the pharyngeal slits. It is likely that in this configuration the endoskeleton evolved to support the pharyngeal slits. However, in an event that probably involved a change in the growth of the more anterior arches, the endoskeletal elements became thicker and surrounded the mouth; primitive jaws appeared. These likely allowed the fish to capture food more effectively. In the most derived state (depicted on the right), further changes took place and these anterior bones became thicker and new bony protuberances appeared in the form of teeth. Teeth made predation more efficient, and are found in many vertebrates; most vertebrates that lack teeth are very specialized feeders, consuming food that does not need to be “handled”, including nectar, blood and termites/ants.
You should recognize this gradual adaptation of existing structures for new purposes as a reoccurring theme in evolution. The evolution of other pharyngeal structures, which gave rise to lungs and their derivatives, has also contributed significantly to vertebrate evolution.
Legs and amniotic eggs are two additional derived characters that have played a significant role in the evolution of vertebrates. The significance of legs in the terrestrial environment is obvious. How do evolutionary biologists think they evolved?
The endoskeletal system of vertebrates consists of the axial elements that are associated with the vertebral column. In addition, more lateral elements, termed the appendicular skeletal system, are found in some groups. The fossil record indicates that the appendicular skeletal system first appeared in a group of fish that have more robust fins with a main boney element, the "lobefin" fishes. (as opposed to the light, ray-like boney elements in the rayfin fishes) A progressive thickening of these elements occurred. The fossil record (and other data) indicates that the tetrapods, terrestrial vertebrates, arose from lobefin fish, and that legs evolved from these highly modified lobe fins.
Early vertebrates arose in the oceans, where desiccation was not a problem. However, with the invasion of land about 365 million years ago, ancient vertebrates had another problem to face, water loss. This was similar to the problem faced by land plants. The vertebrates evolved a toughened egg that was resistant to drying, the amniotic egg, which would have been selectively advantageous in a drier environment.
In reptiles, birds and mammals, the amniotic egg has several unique features, including a specialized membrane system that has many functions, including gas exchange and the transfer of vital nutrients to the developing embryo, and a protective and porous shell. The amniotic egg is name for the amnion, a membrane that surrounds the embryo and contains amniotic fluid, which bathes and protects the embryo.