Body design of the organisms change because of the error in DNA copying or genetic mutation.
If the error in DNA copying occurs the offspring might grow some new characteristics. For example in the case of beetle if genetic mutation takes place then the offspring might grow with the heat resistance characteristic which might help it to survive in harsh conditions even if the others cannot.
Similarly in the case of ape which because of sexual reproduction the genetic mutation took place leading the evolution of the ape to human. Changes like loss of tail, difference (like legs and arms), loss of hair etc.