They say too much of anything is back you. believe it or not, that's true of the c oxygen that powers our bodies. if we brec cedar that was more than 75% cryger, we could die within days. our luring couldn't cope. our bodes are used used to air that's 21% keygen - and even that's harmful over a lifetime. the body makes use of around 90% of the cxygen it takes in and the rest transforms into troe radicals - molecules that attack and damage our cells over time. oxygen, then, she ally touch in large doses. but any 3-4 billion years of evolution of on earth should tour bodes be better at processing crugen? to try to answer that question it's important to understand that oxygen wasn't always present in the atmosphere. because of this, scientists believe that the first life forms on the planet were anaerobic; they were able to survive without crygen. it was only water plants established themselves that oxygen appeared in the atmosphere. this occurrence of oxygen in the earth's atmosphere some 22 billion ago is known as the great oxidation event. plants, animals, don't need anything, but produce through 15 photosy: - the process by which they convert suright, water, and carbon energy. because is a by-product of photosynthesis, it took mamemon a billion years tor enough of to build up in the atmosphere and give foss#evidence suggests animals first appeared about 700 million years ago, in 20 oceans. the first life forms had extremely thin bodes in relation to size. scientists believe this allowed them to make maximum use of low amounts of c oxygen present in the ocean. around 500 milion years ago. atmospheric oxygen levels were high enough to enable animalis to venture out of the ocean and onto land. 25 the amount oxygen in our atmoschere has not increased steadly, in fact, 300 milion years ago, oxygen wiss actually more abundant in the ar than it is now, making up 50% of it. as aresit, anmas were much bigger than they are today. there were insects, for example, which measured more than 2.5 feet from the to of one wing to another. scientists recently bred dragonflies this size in 50% oxygen environments. they say the dragonfies were acte to grow so big because in oxygen-rich atmospheres, insects need the large breathing systems which normally limit the size of their bodies. by 240 milion years ago, though, oxygen levels had fallen to just 1256 of the ar. it's surprising that the amount of cargen in the atmosphere has varied during 35 animal evolution, it's perhaps even more surprising to learn that the planet is more dependent on oxygen produced in the oceans than on land. scientists estimate that sea algae replace around 90% of the oxygen in the biosphere. so, today's most complex forms not only onginated in an environment in which they now cannot breathe - the ocean they rely on it for their survival это текст

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Excellent show complicated challenge to prove yo anyone discover strangeness explain the lesson

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