Gill Slits: Ontogeny Recapitulates ...? Haeckel's Dishonesty.

Answered by Diane Eager, lecturer in medical biology.

The original question was:
I read in one of my school textbooks that human babies have gills slits before birth and this is proof for evolution. How do you answer this? 

It is very common belief that as the human baby grows in the womb, it goes through an evolutionary process with a fish-like stage, a frog-like stage, and so on until ends up mammal-like and finally human.  This concept goes back to a man called Ernst Haeckel (1834-1919) who was Darwin’s main proponent in continental Europe.  His theory is called “recapitulation,” also known by the longer phrase; “Ontogeny recapitulates phylogeny,” meaning a creature shows its evolutionary history in the various stages it passes through as a developing embryo.  “Ontogeny” means development; “phylogeny” means evolutionary history.

One of the “best evidences” presented for this theory has been that humans (and all mammals) went through a stage where they had gill slits, like a fish, in their neck region.  The study of embryonic development from the early 20th century onwards has long shown this to be completely false.  In spite of this, many school biology textbooks clung to the idea long after it was known to be erroneous, simply for the sake of maintaining the evolutionary story,  e.g. the 1981 edition of the Web of Life biology textbook for Australian High Schools stated: “Chordates also developed slits in the sides of the throat usually early in life. Fishes have gills on these slits. In some of these animals the slits never form a complete connection from the throat to the outside.” (p51)  (Chordates are all animals that have central rod named a “notochord” during embryonic development. This includes all vertebrates.) Notice the admission that the “slits” do not form a connection from the inside to the outside.  If that is true, they are not slits.  Slits by definition are a type of hole.What are the “slits”?

As an embryo begins to develop its three dimensional structure and its body organs start to form the tissue between the head and torso starts to grow around and as it does this it forms a series of ridges and hollows.  For a brief time (28 days to 44 days) the grooves between the ridges have a superficial resemblance to slits when viewed from the outside, as seen in this illustration from an early edition of Gray’s Anatomy.  However, as the Web of Lifetextbook admitted, they do not form holes that penetrate from the inside to the outside as gills in a fish do.  Neither do they ever develop any of the structure of a fish gill. 

The correct name for the series of ridges and grooves is “pharyngeal arches and clefts” and they develop into parts of the face and the throat region, including the jaws, chewing muscles and larynx (voicebox).  The only groove that remains as an indentation is the one closest to the head end of the embryo.  It forms the external earhole.  The others are incorporated into the structures of the face and neck. 

Heads and Tails

As well as the claim about gill slits, other embryonic structures are claimed to the evidence of evolution.  For example human embryos are claimed to have a tail.  They do not.  It simply appears that way because the backbone develops before the hips and legs, and therefore projects beyond the place where the pelvis and legs grow from.  As the pelvis and legs grow the lower end of the spine is completely enclosed.In fact, embryos of different kinds of animals are distinct from conception onwards, and modern research has confirmed this.  In 2007 a group of researchers from University College London, UK and University of Virginia, USA, published a study of the process where a single layer of cells forms into three layers during the early stages of embryonic development. The three layers give rise to different organs and structures in the body. The top layer forms the nervous system and skin; the middle layer the musculo-skeletal and blood circulation systems; the bottom layer the inner organs. The researchers found that in birds and mammals the middle layer, called mesoderm, is formed from cell movements in the centre of the embryo under the influence of a growth factor from a cell layer called extra-embryonic endoderm – a cell layer found in birds and mammals but not fish and amphibians. This process is initiated very early in embryonic development, before the formation of a structure named the primitive streak, which defines the centre axis of the body. This means the development of the body is different in birds and mammals compared with fish and amphibians even before any basic body tissues and structures are formed.  (Reference: ature vol. 449, pp1049-1052 doi:10.1038/nature06211)

Ten years prior to that Michael Richardson and colleagues photographed embryos of many animals and compared them with Haeckel’s theory.  He found great variation between the embryos, and most looked nothing like Haeckel’s drawings of embryos that were supposedly at the same stage of development.  (Reference: Anatomy and Embryology vol. 196, No. 2 (1997), pp91-106, doi: 10.1007/s004290050082)This research fits well with the Biblical account of creation recorded in Genesis 1, which tells us living things were created as separate kinds and have multiplied after their kinds.  Any similarities between embryos are no more significant than similarities in organs and tissues in different kinds of adult animals.  The fact that an organ or tissue is found in different animals at any stage of life does not prove that one is derived from the other.  It simply means all these animals share a similar need for it, and so a similar solution has been built in by the Creator.

For further evidence of how biology reveals Biblical creation rather than evolution see the Creation Research leaflet Evidence from Biology.  Download PDF from here. Creation Research.