There have been different views on which aspects of terrestrial life would have required the morphological changes that the human lineage has experienced, so a large number of different explanations have been put forward for each trait. Bipedalism and hairlessness are disadvantageous traits only those who could survive trials and tribulations in cooperation with family members must have been able to evolve as humans. A single mutation in animals with scalp hair is known to induce hairless phenotype (ectodermal dysplasia). Molecular and paleontological dating indicates that CLCA existed 6 Mya, and early hominin fossils show that they were bipeds, indicating that humanization from CLCA occurred rapidly. Sexual relations irrespective of estrus cycles might have strengthened the pair bond. The mother would have sexually accepted her partner at any time as a reward for food. The male partner would have to collect food and carry it to her by hand to keep her and their baby from starving irresponsible and selfish males could not have left their offspring. Her activities would be markedly limited. A hairless mother would be forced to stand and walk upright. All primates except humans can carry their babies without using their hands. A hairless mutation introduced into the chimpanzee/human last common ancestor (CLCA) 6 million years ago (Mya) diverged hairless human and hairy chimpanzee lineages. Three major characteristics distinguish humans from other primates: bipedality, practical nakedness, and the family as a social unit.
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