If we were simple single-celled animal, we would not need a complicated body system to stay alive. Single-celled animals absorb oxygen directly from the water around them, and they get rid of waste in the same way. Many feed by simply crawling over food material and absorbing it, or by letting food pass through the flexible membrane around it.
Human cells operate in a similar way. Our organ systems simply provide the basic conditions that the billions of individual cells in our bodies need to survive. In general, the bigger an animal is, the more complicated its body needs to be in order to survive and reproduce.
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