As it was done

For the origin of life on Earth, it takes a lot of conditions and coincidence them on our planet in ancient times was the scale of the Universe is extremely rare, if not unique.
No matter how probe space powerful radio telescopes, never managed to catch a response to the earthly calls or catch ordered signals from the abyss of the Universe, which would attest to the existence of advanced extraterrestrial civilizations. While we are alone, and nothing is guaranteed from self-destruction, but this is the way.
Simultaneously with life appeared on Earth evolution. It is assumed that in a strange time of free oxygen on Earth was not the first organisms lived through anaerobic (oxygen-free) metabolism. The legacy of this regime, it is possible, in our days are anaerobic microbes, and can be, and the ability of some organs and tissues of higher animals and humans sometimes lead anaerobic lifestyle, accumulating "oxygen debt", which later is always in demand.
But that green plants formed quite a lot of free oxygen. Appeared aerobic-celled animals. The cell membrane (diaphragm) is allowed to retain the constancy of the internal environment of the cell, necessary for normal metabolism, only in a rather narrow limits, and therefore life nested there, and where the external environment is fairly constant in the water space, near the equator. Consuming food and oxygen from the water, the cell throwing waste into the same water; cilia, flagella - all this enabled the cell to move and ventilation of the surrounding space. So supported the constancy of the internal environment of unicellular organism, its homeostasis, without which life would be impossible in a little bit changeable conditions. In multicellular organism to maintain homeostasis harder and evolution on this way repeatedly suffered an artistic failure. Fused cells interfere with Each other to eat "fresh" food, to allocate waste, breathe, the contact surface with water (in relation to the mass of the body) has decreased, and the attempts of life go to multicellular structures were often unsuccessful.
Apparently, the problem of separation of slag was necessitous food and breath - primary excretory device in the form of branching tubules (protonephridia) appeared before all: the intoxication garbage, apparently, was more dangerous than the rest.
In lower worms allocation of wastes make protonephridia; respiratory system and digestive system appeared later, but until the issue was resolved or retaining only two layers of cells, where each of the layers was in contact with water or branch of the primary body cavity for a few dead-end "vessels" - this is called gastrovascular (gastro-vascular system, which digests food and posts it to all of the cells. Finally, a third option: the animal has a mouth, which in turn ends up other cells to get food and oxygen. After eating and breathing, the cells return to the line, where digest eaten and throw the waste at the place of permanent residence. Garbage through protonephridia excreted into the environment.
Later the digestive system becomes flow and vascular system associated with it, gets some independence. This system first (marine worms of nemertean) has no heart, but the very vessels is not blind branches, and closed circuit, with the capillaries that bring food and oxygen to every cell and carry away waste output away through protonephridia, out of the circulatory system. Protonephridia break in the wall of the vessel. Such perfection circulatory system gives nemertina advantage over other invertebrates: these worms have already developed acropolicy brain with a large number of associative neurons. The muscles of nemertean weighs more than half of their body (in mammals - about 50 %). Thanks to the perfection of homeostasis nemertines - predators, none of the parasite (last maintain homeostasis due to the internal environment of the "owner"). Nemertean first time there was a blood - carrier food, oxygen and waste. However, primitive nemerteans have acquired specialized respiratory system and breathe, obviously, the initial section of the intestines.

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