Creationism and Evolution (II)
During the Devonian Period the seas were full of invertebrates and also contained a number of fish. One of these was a fish that had the capacity of swallowing air when it reached to the surface. It is supposed that this fish was the first vertebrate to venture onto land. Although these fish were only barely adapted to live out of the water, they survived and give origin to the amphibians because the land was largely void of life.
The early amphibians had the means to move on land, but they spent much of their lives in the water. They never became completely adapted to live on land. This feat was accomplished by the reptiles in the Mesozoic Era, which is known as the Age of Reptiles. Reptiles were able to dominate the land due to the development of an egg covered with a shell and filled with an amniotic fluid. This kind of egg could be laid on land, whereas the amphibians had to return to water to deposit their eggs.
The amniotic fluid resembles sea-water, and a similar fluid is present in the fetuses of mammals, including human ones. The fact that several body fluids have a certain amount of salt indicates that life originated in the sea. The membranes found in reptile and bird eggs are also present in the human embryo, although not fully developed. This is a strong evidence that mammals descend from animals that reproduced by laying eggs.
The dominance of reptiles (including dinosaurs) came to an end before the Cenozoic Era. Although the cause of this decline is not known, it allowed the development of birds and mammals. The Cenozoic Era is known as the Age of Mammals. Mammals diversified into marine and land species, but a close analysis of their limbs demonstrates their common origin. The differences between the arm of a person, the leg of a horse, the flipper of a whale, and the wing of a bat are only superficial; their skeletal elements are structurally similar.
An example of evolution is the horse. Its older antecedent can be traced back to a small animal with the size of a dog in the Tertiary Period (at the beginning of the Cenozoic Era). Thousands of fossil remains show how this animal, known as 'eohippus,' evolved to the horse that we know today. One of the changes that occurred was the transformation of the toes of the eohippus in the hoof of the horse.
Primates, the order to which humans belong, appeared also in the Cenozoic Era but earlier than the eohippus. They are primarily tree dwellers and seem to descend from insect eating animals of the Mesozoic Era. The closest living relatives of humans are the apes, which should not be confounded with monkeys. The family of apes includes animals like the gorilla, the chimpanzee, the orangutan, and other smaller ones, like the gibbon.
Humans are classified with the apes because of their striking anatomical resemblance that that not occur with monkeys. The form of their bones and organs is highly similar. Genetically there is no difference between a man and a chimpanzee. There is a difference with an orangutan, and a greater one with a gibbon. From the point of view of genetics, humans and African apes are closer to each other than they are to their Asian cousins. African apes and humans seem to have diverged approximately 10 to 5 million years ago. The differences have to do with posture and brain development. Humans have an upright posture and faster locomotion. Their cranial capacity at least doubles that of an ape.
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