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Updated: May 19, 2025
Because the embryos of some four-footed animals have gill-slits, this is pointed out as evidence that land animals are evolved from fishes. In geological time, natural barriers have sprung up which separated the species which have since developed. The so-called Tree of Life. All living forms can be arranged in a diagram called the Tree of Life.
This ancestor, we may recall, is also reflected for a time in the gill-slits and arches, with their corresponding fish-like heart and blood-vessels, during man's embryonic development, as we saw in a former chapter. These are only a few of the more conspicuous instances of vestigial structures in man. Metchnikoff describes about a hundred of them.
Indeed, as we trace back the still earlier history, more and more characters are found which are the common properties of wider and wider arrays of organisms, for at one time the embryo exhibits gill-slits in the sides of its throat which in all essential respects are just like those of the embryos of birds and reptiles and amphibia, as well as of other embryo mammals and these gill-slits are furthermore like those of the fishes which use them throughout life.
The suggestion has been made that these structures do not recall the gill-slits and gill-arches of the fish, but are folds due to the packing of the embryo in the womb. In point of fact, they appear just at the time when the human embryo is only a fifth of an inch long, and there is no such compression.
To begin with less characteristic organs the digestive system is much like that of the annelid or schematic worm, but with greatly increased glandular and absorptive surfaces. The present mouth of nearly all vertebrates is probably not primitive. It is almost certainly one of the gill-slits of some old ancestor of fish, such as now are used to discharge the water which is used for respiration.
We have already seen that the embryonic bird has the long tail of his reptilian ancestor. In early embryonic life it has gill-slits leading from the pharynx to the outside of the neck like those through which the water passes in the respiration of fish. The Eustachian tube and the canal of the external ear of man, separated only by the "drum," are nothing but such an old persistent gill-slit.
After three or four days the egg changes from its globular form into an oval or elliptical mass, and from one end of this a small knob projects to become a flattened waving tail a few days later. On the sides of the larger anterior portion shallow grooves make their appearance and soon break through from the throat or pharynx to the exterior as gill-slits.
In brief, the young chick possesses a series of gill-slits, for these structures are the same in essential plan and relations as the clefts of tadpoles and fishes. Does this mean that even birds have descended from gill-breathing ancestors? Science answers in the affirmative, because evolution gives the only reasonable explanation of such facts as these.
This trickles down on both sides of the throat, and drains right into the open mouth of the Eustachian tube. Not only so, but these Eustachian tubes are the remains of the first gill-slits of embryonic life, and, like all other gill-slits, have a little mass of this same lymphoid or tonsilar tissue surrounding them.
The case seems different from that of the frog, because gills are used by the tadpole, but gill-slits and gill-bars can have no conceivable value for the chick as organs concerned with the purification of the blood.
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