According to my research on different types of studies, I can say that based on the information provided within the question lifespan develop-mentalist's are researchers and practitioners from a variety of disciplines. They also believe that no particular, single period of life governs all development.
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It is necessary to separate oxygenated and deoxygenated blood separate in mammals and birds to maintain their body temperature. Also, if they both are mixed, the blood will get impure and it would be difficult for the heart to supply impure blood. We also can't live with impure blood so we most probably will die.
