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Sexes Over the next 200 million years, the protozoans evolved into cellular colonies, the porifera. Porifera, such as today's sponges, are truly colonies, with each cell essentially the same as every other. No cellular specialization took place.
Eventually, some cells started specializing in locomotion while others specialized in food gathering, and so forth. This lead to the evolution of the coelenterates, with different cells performing different tasks. Today's jellyfish are coelenterates.
With this complexity, there could no longer be a simple random merging. All this specialization required that some cells spend their time reproducing not themselves, but the creature as a whole. These cells must, then, carry the genetic code for the entire creature. Since the new creature produced by a division and merging would start as the merger of two gametes, hence a single cell, it follows then that all cells in a creature must contain the entire genetic code for the creature. This is indeed the case.
Those cells that specialized in reproduction must produce gametes that attract each other. If all were identical, there would be minimal attraction, so the concept of opposites arose. The gametes became divided into two groups: sperm (male), and eggs (female).
If there are opposite gametes, there are opposite reproductive organs to produce them. Voila, male and female creatures. This proved to be so efficient at mixing the gene pool that it became a survival characteristic. Those species had the greatest urge to merge survived, and elaborate and downright peculiar means have evolved to ensure the urge to merge. Sexual reproduction has been the norm for virtually all species more sophisticated than a bacterium ever since.
In the Sea Since the great pollution, everything ate everything. Except the algae, who were (and still are) the bottom of the food chain: everything ate algae, directly or indirectly.
About 570 million years ago, some critters became tired of being eaten, and decided (so to speak) to do something about it. Hard parts evolved, most noticeably shells, and the Paleozoic era began.
The first things to evolve shells were, not surprisingly, mollusks. They shared the oceans of their day with a grand assortment of cephalopods (head-footed creatures, such as squid and octopi), arthropods (jointed-footed creatures, such as lobsters), annelids (worms), and echinoderms (spiny-skinned creatures, such as starfish). All of these forms survive today, though specific creatures don't.
The evolution of the annelids and echinoderms was soon followed by the first primitive chordates (creatures with a central nervous system). The central nervous system allowed co-ordination between the various parts of the body by channeling their neurological signals through a central organ, the brain.
By 500 million years ago, the early chordates had become vertebrates (creatures with skeletons, although of cartilage and not bone) had evolved. Primitive jawless fish swam the seas. Current examples of jawless fish include the lamprey.
Cartilage evolved into bone, and led to the evolution of osteichthyes, the first bony fish. Most of today's fish are bony, though there are still some cartilaginous fish around, such as sharks.
Some 405 million years ago, two significant events occurred. The obvious event was a sudden proliferation in the number of fish--fish became the dominant lifeform in the sea. A more significant but quieter revolution was also taking place: the plants were invading land, rapidly changing rock and sand into topsoil, and laying the paths the animals would later follow.
Ferns evolved shortly thereafter, and were present to greet the animals as they left the sea. These animals were arthropods: scorpions, spiders, and bugs. Arthropods still outnumber all other species of land animal life except the microscopic.
Of concern to us at this time is the evolution 370 million years ago of rhipidistan, the first lungfish, which were the direct ancestors of all higher forms of life: amphibians, reptiles, birds, and mammals. These early lungfish lived in the coastal bogs and estuaries, occasionally venturing onto land for brief periods.
On the Land By 345 million years ago, rhipidistan had evolved into eogyrinus, the first amphibian and a true land animal. The vertebrates had invaded the land. Amphibians were still tied to the water, however. Their eggs had no shells, and had to be laid underwater. The young were (and still are) born with gills, which they lost as they reached adulthood.
About 290 million years ago, a creature called eosuchian learned the trick of enclosing its eggs in a calcium shell: the first reptile had evolved. Unlike amphibians, young reptiles did not have gills and did not require standing water. They soon developed scales to preserve body moisture as well.
The Paleozoic era came to an abrupt end some 230 million years ago. Most of the marine invertebrates, fish, amphibians, early reptiles, and everything else vanished. The first Great Dying had occurred.
Great Dyings The history of the Earth is punctuated with many Dyings and two (maybe three) Great Dyings. In a Dying, vast numbers of species vanish suddenly (geologically speaking) over a wide area. In a Great Dying, this area is world wide. Such an occurrence leaves uncounted ecological niches empty: those species that do survive the Dying are then presented with an opportunity to undergo rapid radial evolution, a phenomenon wherein each surviving species quickly evolves to fill as many ecological niches as possible.
The reasons behind the Dyings are not clearly understood. Possibilities include asteroid impact, climatological change, volcanic activity, and disease. Whatever the causes, their occurrence is clearly established.
Two (three) Great Dyings occurred in Earth's history. The Permian Great Dying, 230 million years ago, terminated the Permian period and the Paleozoic era. The Cretacious Great Dying, 65 million years ago, terminated the Cretacious period and the Mesozoic era, and brought about the demise of the dinosaurs. Both these Great Dyings are generally believed to be the result of asteroid impact, though other explanations are possible. The argumentative Quaternary Great Dying is currently underway, and promises to destroy the greatest number of species of any Great Dying. Its cause is man.
Reptiles The Mesozoic era had begun. The surviving eosucians evolved into the anapsids.
The early anapsids had an interesting problem to face: body heat. Coincident with the Permian Great Dying (possibly caused by the same event) the climate became cooler. Being cold blooded, the anapsids would assume a body temperature about the same as that of the surrounding air. This meant that they simply couldn't get their motors turning over on a cold morning. They solved this problem through solar power.
By evolving huge fins on their backs, they could position themselves broadside to the sun on a cold day and absorb large quantities of solar energy. Once they were warm enough, they could then face towards or away from the sun. One can see several drawbacks to this scheme: cloudy days, strong winds, etc. These sail-backed reptiles are often depicted in grade-B monster movies by gluing a fan to the back of an iguana.
As a dominant group, the anapsids were short-lived, surviving today only as the turtles and tortoises. They evolved into four other reptile groups: the diapsids, which became the dinosaurs, pterosaurs, lizards, snakes, tuatara, crocodiles, /alligators, and birds; the euryapsids, which became the plesiosaurs; the parapsids, which became the ichthyosaurs; and the synapsids. The dinosaurs, pterosaurs, plesiosaurs, and ichthyosaurs are all extinct (except for Nessie, the Loch-Ness Monster, a lone surviving plesiosaur [if you are a believer, that is]). The lizards, snakes, tuatara, crocodiles, alligators, and birds are still with us.
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