"`` Six pairs of these aortic arches loop over the pharynx . In primitive fish , these arches persist and enable the gills to oxygenate the blood . In adult birds and mammals , in which lungs oxygenate the blood , such a system makes little sense - but all six pairs of aortic arches are formed in mammalian and avian embryos before the system eventually becomes simplified into a single aortic arch . Thus , even though our physiology does not require such a structure , our embryonic condition reflects our evolutionary history. `` ( Developmental Biology : Seventh Edition , Scott F. Gilbert , Sinauer Associates Inc. , 2003 , p500 )"
"Another characteristic in this same general area that ties us back to fish is the development of our parathyroid glands - which are very similar to gill buds in fish - from a particular pharyngeal arch : the same one that in fish gives rise to the gill buds ! Moving on to the kidneys for a minute . The human embryo forms not one , but 3 successive kidneys : the pronephric , mesonephric , and metanephric kidneys . Only that last one persists and only it is the kidney you and I and all of our fellow humans use . Why do human embryos go through two transient and simpler kidneys before getting it right ? Here 's a hint : the other two are carry overs from simpler organisms ! The evolutionarily simplest vertebrates , such as hagfish and teleost fish , develop only the pronephric kidney : it is their adult functional kidney . Now move up an evolutionary notch to ( other ) fish and amphibians . In their embryos , the first kidney still develops , but soon begins to degenerate and is replaced by the mesonephric kidney , which is the functional adult kidney in these organisms . Now move up another evolutionary notch , to reptiles , birds , and mammals . Their embryos form the first kidney , which then degenerates and is replaced by the second kidney , which then degenerates and is replaced by the final , metanephric kidney : their adult functional kidney . The above is not anywhere near a complete list of embryological evidences for common descent/evolution . PS : Note the serial addition of developmental stages that occurs in kidneys as one moves up to higher evolutionary levels : `` ontogeny recapitulates phylogeny `` . Of course this `` law `` does not hold for all organs/organ systems in all organisms , but it does hold for a few cases . And that 's enough to clearly indicate evolution of those organ systems by their being retained from ancestors and then modified during embryonic development ."
"our embryonic condition reflects our evolutionary"
"And that 's enough to clearly indicate evolution of those organ systems by their being retained from ancestors and then modified during embryonic development ."