The appearance of a tail in humans is an ATAVISM how does it happen?
Humans & Apes have developed parrallely,but we have developad frm an Ancestor which was never having tail,then how it appears in sme cases?
I think the reason you are having a problem with this idea is that you are not going back far enough on the "family tree" of the human species. You say that the ancestor that we and the apes have in common had no tail and therefore we should not have one either (even if it is only vestigial). But what you are forgetting is that the vestigial tail does not need to have come from our most recent common ancestor.
So instead of thinking about the most recent common ancestor, think way back. If you go far enough back along our family tree you will of course come to animals that had tails. That's where our vestigial tails come from.
The development of human has arised form the apes. The family primates. The atavism is the reappearance of the ancestral characteristics in the present generations. This directly relates to the genetic terms. The dominant and the recessive characters and sometimes the mutations as well. Its is the mutation that developed the new generation from the old one and the dominance of the one character over the other.The genes are very active in these terminologies that the changes are seen in every new individual. Not all individuals are completely identical ot their parents. Some if identical are takne into account. Some breeds of human may have the tail as the recessive character and then after 100s ganerations may arise again.
Theories don't say but different mind different concepts.
Atavism is the reappearance of those ancestral characteristics in an organism or in the organism of a group,which do not occur normally or which represent the reminiscent of normal structures possessed by the individuals of other groups.Such abnormal structures are known as atavistic characters of reversions or atavisms.
In such cases abnormal characters or structures appear in the embryo or in the adult,which were not present either in the parent or grandparents but in some remote ancestors.
Tail is absent in man,but occassionally a human body is born with a short fleshy tail.It is devoid of vertebrae and is removed by surgeon with no trouble.