The reason why animals need continuous supply of oxygen is to survive. Almost all of the animals rely on the oxygen in order to be able to survive and function properly.
The oxygen is what enables the respiration, and the respiration is what allows for the cells to be alive and be able to function. Without constant oxygen supply, the cells will not be able to function, thus they will die, which will cause the whole organism to die.
Also, the oxygen is a gas that is used for the burning of glucose into fuel. This basically means that the oxygen is the final detail that enables the organism to obtain nutrients. If the oxygen is not present, than the organism will do everything to create glucose and it will manage to, but it will not be able to turn into a fuel for itself, thus it will not be usable.
Answer:
If humans were still hunting and gathering food instead of farming - taken into account that the entire population hunts and gathers food in the context of this question - the majority of people would have dark-skin complexions.
Explanation:
This is easily explained by a hunter-gatherer's way of living. Before humans learned to raise animals and grow crops, the majority of us were hunters and gatherers. Hunters and gatherers lived, and still continues to live, their lives very differently from how we are living at present. They'd spend hours hunting - as their name suggests - and catching food out in the wild, which exposes them to a higher amount of sunlight than our everyday way of living. To cope with this, evolution also played an important part in determining the survival of these people in the scorching sun - they evolved to producing high amounts of melanin. Melanin is a dark pigment that helps protect our skin cells against the harmful UV rays from the sun. Lighter skinned people have less melanin than darker skinned people, and when that extra melanin is produced, you'd see it as a tan. Hunter-gatherers produced high amounts of melanin, primarily because they had adapted to spending a large fraction of their time out in the open. The melanin acts like a built-in sunscreen for them, protecting them from getting skin cancers like melanoma.
Tundra biome has a very harsh winter and a cool summer. It has a very low level of precipitation. Because the ground is permanently frozen, only algae, mosses, and lichens are found as vegetation. Animals that live in this biome must be well equipped to combat the very cold weather like the polar bear, artic fox, and snow owl.
Amino acids are the building blocks of proteins. Amino acids polymerize together into protein strands.
