Describe the Evolutionary History of Vertebrates, Essay Example

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Essay

Describe the evolutionary history of vertebrates from fishes leaving water to their later diversification to amphibians, reptiles, dinosaurs and birds, and mammals. Include in your answer the significant adaptations that supported each transition and how these advances allowed the diversification of each group.

It is theorized that terrestrial vertebrates evolved from lobe-finned fishes.  Only five genera of lobe-finned fish survive today.  Four genera of lungfishes in oxygen depleted environments and the coelacanth.  Lungfish have lungs, which supplement gill breathing.  Lungs and swim bladders both develop as outgrowths of the embryonic gut.  The fossil record shows that animals much like modern lungfish evolved in the Carboniferous era, about 300 million years ago.   These creatures had fish shaped bodies, short stubby legs and no gills.  During the Cambrian period when the vertebrates evolved, there was no plant life except probably a film of bacteria and algae.  Bu the Devonian period, about 200 million years later, the fish evolved.  In addition, plants began spreading on land and providing food and shelter for terrestrial invertebrates.  By the end of the Devonian period, insects evolved and the first land vertebrates had evolved from lobe-finned fishes.  There are selective pressures that probably contributed to evolution of land vertebrates.  The seas had carnivorous fish and fish that could move themselves to land decreased their mortality.  In addition, vertebrates that ate plants or insects and could survive on land would have little competition for food. However, any fish that adapted to land must have had adaptations for living in air.

Air is less dense than water and also provides less support.  In addition, a fish cannot support the body on land without bones in the pectoral and pelvic fins and in its backbone.  Furthermore, gills can’t be used on land as well as dehydration that would occur on land.   The amphibians, reptiles, birds and mammals are known as tetrapods.  These are the land vertebrates.  Even though the adult stages of most amphibians are terrestrial, they still need to live in damp areas.  This is due to the fact that the gas exchange occurs through the skin which needs to be kept moist.

The earliest full fossil of a tetrapod is from the Devonian genus Ichthyostega.  These animals had no gills but were fish-like in having scales and a tail supported by fin rays.  They had adaptations to land that included well-developed limbs, ribs, and limb girdles.  In tetrapod’s, the function of the pectoral and pelvic girdles is to form a strut between the limbs and the spine.

The two groups of amphibians are the urodeles (newts and salamanders) and the anurans (frogs and toads).  The Urodeles show the transition from fish to tetrapod.  Their limbs contain small bones and muscles, like those found at the base of the lung fish.  The Anurans show specialized skeletons that give them the ability to jump.  The Caecilians are another ground that are legless, worm-like and have adapted to burrowing in leaf litter on the forest floor. Amphibians also have the soft glandular skin that allows for gas exchange; however, most have small lungs as well.  Only the aquatic larvae have gills. The sense organs of amphibians are also adapted for land, such as their simple ears that respond to sound. The amphibians were also the first group to develop true tongues.

During the late Carboniferous period, about 300 million years ago, there was a huge increase in the tetrapod evolution.  Insects and plants evolved symbiotic relationships and it was the first time there was enough animal life to support terrestrial vertebrates predators.  The theory is supported by fossils showing adult amphibians and early reptiles with jaws and teeth.  The early reptiles were very small, about 20 cm long. They had strong limbs, which enabled them to move faster. In addition, their jaws were more firmly attached to the skull, allowing them to eat larger prey.  Furthermore, their skin was waterproofed and scaly.  Therefore, reptiles had to breathe entirely through lungs.  One of the major characteristics in the evolution for reptiles is the amniotic egg.  Here there is an egg in a shell or some developed in the mother’s body. The animals evolved adaptations in order to keep the eggs warm.  For instance, snakes coiled around the eggs and some other reptiles buried their eggs in the sand.

The amniotes can be divided into three different groups; the anapsids are the early amniotes, including the turtles that have no holes in the area behind the eye.  Later amniotes evolved holes in this region and are called the diapsids, which includes the dinosaurs, birds, snakes, and crocodiles. The third group is the syanpsids that have a single temporal arch.  These are the mammals and their extinct reptile relatives.  The Diapsids are the only group with Jacobson’s organs, a chemical sensing organ in the roof of the mouth. The Mesozoic era is sometimes referred to as the “age of reptiles”.  Reptiles moved into all types of terrestrial habitats.  There were species that fly, swam and walked.  Within the adaptive radiation there are several evolutionary trends that can be seen.  For instance, low-slung animals with long snouts adapted to eating fish evolved in several diapsid lines. An example is the crocodiles.  Flight developed independently in pterodactyls and birds.  Many of the species became bipedal, walking on their hind legs and using their tail for balance.

The mammals, birds, lizards and snakes all evolved in the Mesozoic age.  In fact the lizards are the most widespread of the modern reptiles today. Snakes are the legless squamates that most likely their anatomy by parallel evolution from several groups of burrowing lizards.  The crocodiles are the closest living relatives of the dinosaurs and of their descendants, the birds.  There are only three groups of crocodiles alive today and include the crocodiles in Africa, Asia and America.  Birds are defined as the only group with feathers.  They are diapsids that evolved in the Mesozoic from theropods, the bipedal carnivorous dinosaurs.  Birds were able to fly and the evolution of feathers made this possible.  The earliest known bird is Archaeopteryx, which was a fossil found from the Jurassic period, 150 million years ago.  It was classified as a bird because it had feathers.  Without the feathers, the bird would have been classified as a dinosaur.

Mammal characteristics evolved from different lines of synapsid reptiles during the Permian period.  Mammals and dinosaurs both appeared during the Triassic, before the birds appeared. By the late Mesozoic age, all three modern groups of mammals had evolved the monotremes, marsupials and placentals.

A mammal is defined as a vertebrate with only one bone on either side in the lower jaw.  This is considered an adaptation that helped them hold the prey more tightly.  The early mammal evolution is the evolving of the quadrupedal locomotion and the new adaptions of carnivory.  The quadrupedal contained the narrow foot track, with the legs placed further under the body compared to the reptiles.  In addition, muscles that moved the limbs became more prominent.  Furthermore, the evolution of specialized teeth is another advancement seen in the mammals.  The fish and reptiles have teeth that are rough and the same size, but early mammals had different kinds of teeth used for cutting, like incisors and canines for gripping. Most of the mammals are viviparous, where the embryo is dependent on the mother.  In addition, all female mammals have parental care for their young and feed the young with milk from mammary glands. The mammals also evolved integument, consisting of skin in order to control temperature. The success of the mammalian group has led to the evolution of mammals exhibiting herbivory, bipedalism, and flight and other adaptations.

All of the people on Earth today belong to the species Homo sapiens, in the mammalian order Primates.  The main evolutionary trend that occurred with primates is the expansion of the cerebral hemispheres of the brain, particularly in the frontal lobes.  Also, the evolution of the well-developed eyes and sensory system were found. There is molecular evidence that shows that African apes and hominids diverged around 5-10 million years ago.  The first hominid fossils appear in Africa and are called Australopithecus africanus.  This fossil is believed to be the ancestor of the species Homo.  The fossil had human teeth with small incisors and canines, but was ape-like in having large, heavy jaws and their brains were small, like those of modern apes.  It is said that Homo sapiens evolved from Homo erectus about three-five thousand years ago.

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