Little human embryos don’t look too human to the average lay person. Images like these lead to a lot of misconceptions usually centered on abortion debates, but they also lead to evolution debates.
Charles Darwin theorized that animal embryos went through developmental stages that mirrored our evolutionary history, called embryogenesis, or biogenetic law. Each embryo goes through a fish stage, amphibian stage, reptile stage and so on. German Professor of Zoology Ernst Haeckel studied embryos from various different animals, documented and drew their features in a series of 24 drawings. These drawings revealed that animal embryos are almost identical in early stages of development and thus embryogenesis was true. One of the vindicating evidences being what appeared to be gills on all vertebrate embryos. But there were two small problems. One; those gills weren’t really gills at all, and two; Haeckel doctored the drawings!
Other German scientists began to call Haeckel out stating that his drawings were falsified, and eventually Haeckel admitted to the deed. Yet, his drawings still appear in textbooks to this day as accurate, despite being proved false over 100 years ago!!! Stephen Jay Gould wrote about this atrocity as, “[W]e do, I think, have the right to be both astonished and ashamed by the century of mindless recycling that has led to the persistence of these drawings in a large number, if not a majority, of modern textbooks!”
To this day you can find pictures of human embryos in science textbooks with captions and footnotes pointing out supposed gills that are evidence of our evolutionary fish ancestry. Despite evolutionary scientists today pointing out the that Haeckel’s drawings were, “highly inaccurate, exaggerating the similarities among embryos, while failing to show the differences.” Even the journal of Science reported, “[g]enerations of biology students may have been misled by a famous set of drawings of embryos published 123 years ago by the German biologist Ernst Haeckel… the impression they give, that the embryos are exactly alike, is wrong.” Renowned anatomist Erich Bleschschmidt wrote, “…the so-called law of biogenetics is wrong. No buts or ifs can mitigate this fact. It is not even a tiny bit correct or correct in a different form. It is totally wrong”
So what are these little pouches that look like gills?
Not gills: Fish absorb oxygen through the water using their gills. Not once during development does a human embryo ever absorb oxygen through the water. Human embryos receive oxygen from the umbilical cord.
Not slits: Those slits aren’t slits at all. They’re folds in the embryo neck.
The folds are caused by the over growing of the embryo brain and spinal column starting on the 28th day. The growth is so rapid that it causes the embryo to curl forward and bend the neck forward causing the tissue on the neck to fold. Just as bending your own neck forward can give you a temporary double chin… or a third chin if you already have a double chin…
So calling these folds gill slits is incredibly misleading since they are neither gills nor slits. They’re often called bronchial arches, but this too is misleading since bronchial comes from the Greek word for “gills.” The proper scientifically sound terminology is pharyngeal arches, pharyngeal clefts or pharyngeal pouches.
From these folds and pouches come our thymus, thyroid, and parathyroid glands. These are a far cry from fish gills.
Biologist Jonathan Wells writes, “[H]uman embryos do not really have gills or gill slits: like all vertebrate embryos at one stage in their development, they possess a series of ‘pharyngeal pouches,’ or tiny ridges in the neck region. In fish embryos these actually go on to form gills, but in other vertebrates they develop into unrelated structures such as the inner ear and parathyroid gland. The embryos of reptiles, birds and mammals never possess gills.”
It is only at a superficial level that some past scientists concluded the folds to be gill slits, but sadly it has remained in textbooks to this day…
 Gould, S. J. (March 2000) “Abscheulich!(Atrocious!),” Natural History
 Richardson, M.K. (1997) “There is No Highly Conserved Embryonic Stage in the Vertebrates: Implications for Current Theories of Evolution and Development,” Anatomy and Embryology, Vol. 196:91
 Pennisi, E., (1997) “Haeckel’s Embryos: Fraud Rediscovered,” Science, Vol. 277:1435
 Wells, J., (May 1999) “Haeckel’s Embryos & Evolution: Setting the Record Straight,” The American Biology Teacher, Vol. 61(5):345-349