Does Gill Embryology Show Fish Evolved from a Common Ancestor?

Keywords: gills, evolution, creation, embryonic research, fish, embryo, endoderm, ectoderm

Can a landmark discovery about how fish embryos grow their gills connect us firmly to roots under the sea? Cambridge University zoologists J. Andrew Gillis and Olivia R.A. Tidswell think so.

Fish use gills to extract oxygen from water. Evolutionists maintain that vertebrates without gills—like us—have gills “present as vestiges in our own embryology.”1 (More on that below.) But where did gills come from in the first place? Enquiring evolutionists want to know! To find out, they look for similarities in the gills of different sorts of fish embryos. They hope to thereby unveil the gills of the common evolutionary ancestor of all fish and to gain a clue about how very different groups of fish—jawless, bony, and cartilaginous—diverged.

A Fishy Controversy

The skate is a jawed fish with a cartilaginous skeleton. Like all fish, it has gills. Gillis and Tidswell have used modern methods to study the skate’s embryonic gill development. Their surprising discovery has resolved a long-standing controversy and overturned information accepted since the 19th century. The controversy has hinged on the cellular origin of gills within a fish embryo.

Evolutionists believe observable embryology offers clues to the unobservable evolution of fish and ultimately of us. There are three main categories of fish: jawless (hagfish and lampreys), jawed cartilaginous (sharks and skates), and jawed bony. Bony fish can be ray-finned (trout) or lobe-finned (Tiktaalik), and evolutionists believe land vertebrates evolved from the lobe-finned ones. They believe that the jawed placoderm evolved from jawless fish and eventually gave rise to all sorts of jawed fish. (To learn more, see “Fish Brains Grew Till We Have Faces, Evolutionists Say.”) Because different groups cells seemed to differentiate to form gills in the embryos of jawless and jawed fish, evolutionists have long thought that gills had to evolve separately in the jawless and jawed lines of fish.

Early on, all vertebrate embryos form three layers of cells—endoderm, ectoderm, and mesoderm. As an embryo develops, cells from these three categories differentiate and multiply to form all the structures in the mature organism. In jawless lamprey and hagfish embryos, gills form from endoderm. But by the dawn of the 20th century, scientists were convinced they saw the embryonic gills of bony, jawed fish like sturgeon and lungfish forming from ectoderm. The observably different gill embryology in disparate fish groups seemed to paint a history in which jawless and jawed fish diverged before evolving gills. From an evolutionary point of view, then, gills would have had to evolve twice—in jawless fish from endodermal cells, and in jawed fish from ectodermal cells. Evolution of something independently in different lineages is called convergent evolution.

A century later, this evolutionary applecart was upset by the discovery that gills in the bony zebrafish develop not from ectoderm but from endoderm, like those in jawless fish. From an evolutionary point of view, this discovery created confusion and controversy. Did gills really evolve twice? If not, did that tighten the family connection between jawless and jawed fish? And how could cartilaginous fish fit into this confusing story?

The Gill Story

As gills form in any type of embryonic fish, endodermal pouches fuse with overlying ectoderm. Gill slits in this fused layer become the familiar gill openings on a fish. Arches between the slits support them and differentiate into various gill-related structures. Because both endoderm and ectoderm are involved in gill development, it is easy to see how 19th-century technology might not have been able to tell just what kind of cells eventually differentiate into the actual gills.

Advancing technology makes more accurate observations possible.

Advancing technology makes more accurate observations possible. Gillis and Tidswell labeled some of the skate embryo’s endodermal cell membranes with fluorescent tags2 and observed where the descendants of those tagged cells ended up. Their results show that gills in a cartilaginous fish embryo really do develop from endoderm—like those in the bony zebrafish and in jawless fish—not from ectoderm. That’s a fine piece of detective work, and their observations correct misinformation attained through older, less sophisticated means.

Extrapolation of their observations to the larger world of fish indicates that the embryos of all jawed fish (from sharks to salmon) have more in common with embryos of jawless fish (like lampreys and hagfish) than previously believed. Endoderm, not ectoderm, is the embryonic source of the cells that form gills in all sorts of fish. However, the extrapolation of these observations to an evolutionary path deep in unobservable time is a worldview-based interpretation of their findings.

Recapitulating Evolutionary History

Evolutionists assume that embryological development within one kind of organism reveals past evolutionary transitions to organs or organisms of increasing complexity. They have not observed such transitions.

Similarly, evolutionists assume that the layers of dead creatures in the fossil record represent evolutionary milestones. Characteristics of organisms preserved in ascending fossil layers are believed to demonstrate when various anatomical features evolved over millions of years. Like the evolutionary interpretation of embryology, this interpretation of the fossil record is based upon unverifiable evolutionary assumptions and worldview-based interpretations of observations, not upon the observations themselves. The catastrophic burial and subsequent fossilization of billions of organisms in layers of sediment associated with the global Flood of Noah’s day is a different interpretation of the observable fossil record, one that is consistent with the history God has provided in His Word. (Read more about this in “Doesn’t the Order of Fossils in the Rock Record Favor Long Ages?”)

Gillis says, “Our embryological research helps us understand exactly how the gill structures in early vertebrates such as Metaspriggina relate to the gills of living forms. . . . Embryology can tell us about the evolutionary relationship between anatomical features in living animals, while paleontology can pinpoint precisely when these features first appear in deep time. I think that this work nicely illustrates how these two areas of research can inform one another.”3 (The Metaspriggina to which Gillis refers is a tiny fish preserved in the Cambrian Burgess Shale. Because it is found deep in the fossil record, evolutionists consider it to be a vertebrate that evolved very early. And because Metaspriggina lacks gills on the first of its seven branchial arches, evolutionists think it was giving up a gill to make room for a jaw to evolve. Read about it in “Jaw-Dropping Discovery in the Burgess Shale.”)

Gillis extrapolates an evolutionary interpretation of his skate research not only to the depth of the fossil record but also to its breadth. He believes that his clarification of the fish family tree shows how active swimming vertebrates evolved in the first place. He says, “These findings demonstrate a single origin of gills that likely corresponds with a key stage in vertebrate evolution: when some of our earliest relatives transitioned from filtering particles out of water pumped through static bodies to actively swimming through the oceans.”4

Gillis explains how he connects the advantage that gills confer on their owners with the evolutionary origin of those owners, saying,

Gills provided vertebrates with specialist breathing organs in their head, rather than having to respire exclusively through skin all over the body. We can't say whether these early animals became more active and needed to evolve a new respiratory mechanism, or if it was gill evolution that allowed them to move faster. However, whether by demand or opportunity, our work suggests that the physiological innovation of gills occurred at the same time as the lifestyle transition from passive to active in some of our earliest ancestors.5

Evaluating the Embryologic-Evolutionary Connection

Are these valid conclusions? It is easy enough to understand why evolutionists would look for the unobservable path evolution supposedly took in the observable development of living embryos. After all, on its journey from a single fertilized egg to a mature organism, an embryo’s cells must differentiate and organize themselves into many complex structures. An embryo’s increase in anatomical complexity throughout its development would seem then to be a viable path along which various complex organs could have evolved if it were possible for such evolution to occur.

Gills are complex and very common anatomical organs among active animals too big to absorb sufficient oxygen through their skin. They appear deep in the fossil record and form from the same types of cells in all fish embryos. An evolutionist would then logically assume that early evolved gills were the key stepping stone to an active underwater lifestyle. If a person accepts the fish story6 of molecules-to-man evolution hook, line, and sinker, he would readily see this skate-gill discovery as evidence of our inner fish. Presuming an evolutionary history, this would look like a unifying link connecting us to distant cousins like hagfish, lampreys, sharks, and salmon. Gillis sums up the belief of his fellow evolutionists when he proclaims that “evolutionarily speaking, we are all bony fish.”7 (Mr. Henry Limpet of The Incredible Mr. Limpet would be so pleased.8)

Experimental biology provides no mechanism by which such evolving complexity could occur.

Embryology and evolution are not the same thing, however. Embryologic development in a particular organism—though technically difficult to see in some cases—is observable. And evolutionary development—the changing of one sort of organism into a different and more complex sort of organism over time—is unobservable. In fact, experimental biology provides no mechanism by which such evolving complexity could occur. Nevertheless, evolutionary scientists believe that new and more complex organisms can evolve through the alteration and repurposing of structures in an embryo. They believe that the embryologic development of structures like gills “recapitulates” their evolutionary history, repeating and picturing the various stages of their supposed evolution while their embryonic form takes shape and matures.

This notion—called embryonic recapitulation—is commonly accepted in one form or another by those who already believe molecules-to-man evolution happened, but it is a belief based in a worldview—a worldview that rejects the history in God’s Word—rather than the scientific method. The sequence of steps through which an embryo’s organ transitions in its path to maturity is dictated primarily by information in that organism’s DNA.

Something Fishy About Gill Slits

Furthermore, there is a long-standing and popular myth that human embryos have gills in their so-called (and completely nonexistent!) fish stage. However, the mammalian embryonic structures erroneously called “gill slits” have nothing to do with gills. They are pharyngeal arches. These are collections of embryonic cells that in humans become parts of the jaw, face, ear, middle ear bones, and voice box. We can speak, hear, chew, and smile because of the complex array of structures these pharyngeal arches form. God designed the blueprint for our pharyngeal arches and the structures into which they differentiate. He put that information into human DNA when He created Adam. You can read more about this in “Review: Your Inner Fish.” Your Inner Fish was a PBS program in which paleontologist Neil Shubin mingled observable biological wonders with their unobservable evolutionary origins to support his claim that “we’re all adapted from ancient ancestors; we are, every one of us, just a jury-rigged fish.” Despite evolutionary dogma superimposed upon anatomical observations, however, we are not and never have been related to fish.

Scientific observation does not reveal any mechanism by which an organism can attain new information to develop into a different, more complex kind of creature. In accordance with our Creator’s design, as indicated in Genesis chapter one, animals and humans each reproduce and vary only within their created kinds. It is our Creator God’s design for each kind of organism that unfolds during embryologic development. The similarity of the embryonic gills in all sorts of fish is a common design used by our Common Designer. It is not proof that gills evolved so early in evolutionary history that they enabled animals to start swimming around under the sea and eventually crawl out onto land, grow lungs, and ultimately give rise to people. That’s a very colorful story, but scientifically and historically, it just doesn’t hold water.