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Do Human Embryos Have Gills?

Casey Luskin
Photo credit: John Alan Elson, CC BY-SA 4.0 , via Wikimedia Commons.

During the premiere of The Paradigm Project last night we had over 1,400 viewers, which led to a lively online chat during the initial stream. The documentary features pro-intelligent design scientists such as Stephen Meyer, Jonathan Wells, and Douglas Axe arguing that the best explanation for much of the complexity we see in nature is intelligent design. We’re excited to see where this project goes in the future!

As soon as Dr. Wells appeared on camera one naysayer in the chat attacked him personally, claiming that in the book Icons of Evolution, he “lied his head off about the evidence for gills in human embryos” because he purportedly “left out all the evidence and then said there wasn’t any.” Let’s review what Wells says in the book and see if it’s backed by the evidence:

Midway through development, all vertebrate embryos possess a series of folds in the neck region, or pharynx. The convex parts of the folds are called pharyngeal “arches” or “ridges,” and the concave parts are called pharyngeal “clefts” or “pouches.” But pharyngeal folds are not gills. They’re not even gills in pharyngula-stage fish embryos. (p. 105)

So Wells fully acknowledges the evidence that humans possess structures in the neck region midway through development. As we’ll see below, he notes that these structures even have a “superficial” similarity to structures found in fish embryos. But he also points out that in humans and other mammals they aren’t gills at all.

Defining “Gills”

How do we know they aren’t gills? Well, let’s define a gill (from Wikipedia):

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist. [Emphasis added.]

Thus, a gill is a respiratory organ — i.e., something that you breathe through. Do humans ever use their pharyngeal ridges to breathe? No, they don’t. This is noted by Britannica.com:

The embryos of humans and other nonaquatic vertebrates exhibit gill slits even though they never breathe through gills. [Emphasis added.]

Imposing an Evolutionary Interpretation

Of course this latter quote imposes an evolutionary “gills slits” interpretation and is therefore somewhat self-contradictory: if we “never breathe” through the structures, then in what sense is it appropriate to call the structures “gill slits,” since something can only be a “gill” if it’s being used as a respiratory organ? It isn’t appropriate because humans don’t breathe through these structures and therefore they can’t be gills.

We could go round and round on this, but the question is what does the evidence say? Here, Dr. Wells provide us with quotes from leading embryologists discussing how these pharyngeal folds in humans only have an “illusory” similarity to fish gills — which “never exist” in humans:

In a fish, pharyngeal folds later develop into gills, but in a reptile, mammal, or bird they develop into other structures entirely (such as the inner ear and parathyroid gland). In reptiles, mammals, and birds, pharyngeal folds are never even rudimentary gills; they are never “gill-like” except in the superficial sense that they form a series of parallel lines in the neck region. According to British embryologist Lewis Wolpert: “A higher animal, like the mammal, passes through an embryonic stage when there are structures that resemble the gill clefts of fish. But this resemblance is illusory and the structures in mammalian embryos only resemble the structures in the embryonic fish that will give rise to gills.”

In other words, there is no embryological reason to call pharyngeal pouches “gill-like.” The only justification for that term is the theoretical claim that mammals evolved from fish-like ancestors. Swiss embryologist Günter Rager explains: “The concept ‘pharyngeal arches’ is purely descriptive and ideologically neutral. It describes folds which appear [in the neck] region … In man, however, gills never exist.” (pp. 105-106)

Facts About Gills

So according to the evidence, yes, human embryos have structures that are superficially similar to what we find in fish embryos. But there’s a major difference: in fish it’s appropriate to call them gills because they develop into a respiratory organ. In humans these structures are never used for respiration and they develop into something entirely different from gills. To call them gills is to impose an evolutionary interpretation that is in no way required by the data. Wells thus concludes:

The only way to see “gill-like” structures in human embryos is to read evolution into development. But once this is done, development cannot be used as evidence for evolution without plunging into circular reasoning — like that used to infer common ancestry from the neo-Darwinian concept of homology. To put it bluntly: There is no way “gill-slits” in human embryos can logically serve as evidence for evolution. (p. 106)

So no, Wells didn’t lie his head off. But he did teach us some facts about gills.