Review: Your Inner Reptile

by Dr. Elizabeth Mitchell on April 19, 2014

The stars of the program were a group of reptiles that supposedly evolved into mammals, and the supporting cast consisted of baby opossums that gave their lives for the cause.

Your Inner Reptile, the second episode of the PBS miniseries Your Inner Fish, continued this week as Tiktaalik-discoverer Neil Shubin used reptilian fossils and mammalian embryos to support his contention that the human body was shaped by reptilian ancestry.

Chewing up the Evolutionary Ladder

Using fossils collected from South Africa to Nova Scotia Shubin leads viewers to believe that evolutionists can track the evolution of human features such as our teeth by following a series of transitional mammal-like reptiles through the layers of time. Shubin introduces us to the extinct so-called “mammal-like reptiles” (aka synapsids). He shows us their teeth and points out that this subclass of reptiles, unlike modern reptiles, had some features that would have allowed them to chew their food. Their teeth were not all identical little pegs but were differentiated into incisors, canines, and molars. Because modern reptiles do not typically have this arrangement, evolutionists consider this a transition toward mammal-ness.

Turning his attention from dead fossils to living embryos and modern genetics, Shubin claims additional support for his belief that many human features were just reptilian traits re-purposed by millions of years of evolution. The yolk sac of the mammalian embryo and the series of three middle ear bones found only in mammals are the twin pillars on which his assertions rest.

Egg on Our Face?

Vertebrate embryos do have a “yolk sac” is necessary to the early embryo’s survival and a springboard to further development.
Shubin takes us to visit a woman having an ultrasound and points out the yolk sacs on her tiny babies.1 He tells an evolutionary tale to explain their presence and then tries to support it with genetic claims that are common but do not bear close scrutiny. Shubin’s story is that fish ancestors evolving into land reptiles figured out how to keep their eggs alive in a shell but had to supply their embryos with protein-rich egg yolk. Then, once those reptiles evolved into live-bearing mammals, they didn’t need the yolk sac for food so it just stayed around as a vestigial remnant.

Vertebrate embryos do have a “yolk sac.” It is one of several membranes formed outside an early embryo’s body and, like those other “extraembryonic” membranes, it is necessary to the early embryo’s survival and a springboard to further development.

The embryos of egg-laying reptiles and birds have needs very different from those of placental mammals, like humans. (Biologically, humans are mammalian, though we, being made in the image of God (Genesis 1:26-27 (NKJV)), are distinct from animals.) Inside their tough shells the large yolk sac must supply the nutritional needs for the entire gestational period, store waste material like uric acid, and help with gas exchange. The needs of a human embryo differ from those of these egg-layers, but only the assumption that humans evolved from reptilian ancestors would lead a person to think the human embryo’s yolk sac is just an evolutionary leftover. What Shubin fails to mention is that the human embryo’s yolk sac performs many essential functions, being in place before any embryonic organs develop.

First of all, the yolk sac is, until the 6th week of development, the sole manufacturer of blood cells; thus, by the time the embryo is large enough to need these oxygen-carrying cells, they are all ready to go.2 Additionally, the yolk sac forms blood vessels while the embryonic heart is forming; when fused with the heart they form the embryo’s cardiovascular system. Furthermore, in the earliest weeks of development the human’s yolk sac is involved in transferring nutrients to the embryo, absorbing uterine secretions and keeping the embryo “fed” until the placental circulation can be fully established.3 Finally, once its nutritional and circulatory roles are completed, the yolk sac does not go to waste but gets incorporated into several parts of the developing embryo’s body.4

Yolk sacs thus serve the needs of the species in which they are found. This does not demonstrate an evolutionary progression but only a good design that our wise Designer varied to meet the needs of humans and of each kind of animal He created. The extraembryonic membranes seen in all vertebrates are exquisitely tailored to meet the embryo’s developmental and nutritional needs. Evolutionists may point to chicken and reptile eggs full of gooey yolk and declare that mammals evolved away from needing that source of nutrition, but the truth is that each yolk sac is perfectly designed for the embryo it serves.

Derelict Genes?

Hammering home our supposed egg-laying heritage Shubin says “we carry a genetic signature of our egg-y past” in the form of “derelict” genes, “broken genes [for manufacturing yolk protein that] still lie buried in our genome like fossils from the past.” He is referring to vitellogenin (VTG) genes. VTG genes in non-mammals are responsible for the production of yolk protein. In comparing the genomes of humans and several mammalian animals to the chicken genome in the region of the VTG genes, mammals do differ in many ways. Evolutionists committed to the notion that mammals, chickens, and reptiles share a reptilian ancestor consider these differences to be mutations that eventually destroyed the no-longer-needed yolk-making function in the mammalian line. However, VTG genes are associated with functions other than yolk production. VTG genes in honeybees, for instance, are associated with antioxidant functions, social behavior, hormonal dynamics, and the sense of taste.5 In some non-mammalian vertebrates an immunological function has now been discovered.5 Thus, even in egg-layers, VTG genes do not exist just for the purpose of making yolk protein.

Institute for Creation Research geneticist Dr. Jeffrey Tomkins, having examined the currently available data comparing human and chicken VTG genes, reports that the portions of the human genome that align with chicken VTG genes are not “derelicts” as Shubin indicates but have a number of vital functions, which, incidentally, are completely unrelated to making egg yolk.6 It is presumptuous to assume that just because these genes were first found in association with egg yolk that their mammalian counterparts are vestigial remnants or that they, like so much other so-called junk DNA, will not be found to actually have functions unrelated to any ancestral history of making yellow goo.


Mammals have three tiny bones in their middle ear, though they often are not preserved in the fossil record. Reptiles—like the “non-mammalian amniote” in the drawing—have only one ear bone but have jaws made of multiple bones. Evolutionists claim that two jaw bones in reptiles got smaller over millions of years, migrated to the ear, and evolved in the precisely connected trio of bones found in mammalian ears. This belief is based on an evolutionary desire to “connect the dots” on the tree of life, as reptiles with variations in their jaws are still reptiles, not transitional forms. Image: Philcha, Wikipedia

Mammalian Middle Ear Bones

Middle ear bones are the fodder for a particularly popular evolutionary tale, and Shubin tells it. Mammals have three ear bones—the malleus, incus, and stapes. In mammalian embryos a template called Meckel’s cartilage helps shape the lower jawbone and then, except for two tiny parts that ossify to form the malleus and incus, disappears. Reptiles have only one ear bone, but reptile jaws are composed of several bones that vary depending upon the species. In reptile embryos Meckel’s cartilage does not disappear; it ossifies to form part of the jaw. These are observable embryologic developments that, along with countless other characteristics, distinguish mammals from reptiles.

Evolutionists have long claimed that mammals evolved two of their three ear bones from the quadrate and articular jaw bones of reptilian ancestors. Mammalian middle ear bones only function when they are precisely positioned to transfer vibrations properly from the eardrum to the inner ear. Though no one has been able to demonstrate how these bones shrank to become useless for chewing, migrated from the jaw to the ear, and acquired a brand new and perfectly designed hearing function,7 this idea persists.

Shubin asserts that the fossil record reveals a clear trail of evolutionary advancements with the needed transitions in reptilian jaw bones until something close to the mammalian ear bones developed. But the fossil record has failed to reveal the clear-cut trends in reptilian jaw bone changes that evolutionists often claim is present.8 Finally he introduces us to Dr. Luo, who discovered a fossil thought to be the earliest little proto-mammal to have made the transition and separated its ear bones from its jaw. But again, nothing about this animal’s skull shows that it is a transitional form, only that it is a species of, as well as can be determined from just a tiny skull, an extinct mammal that appears deeper in the fossil record than evolutionists expected to find it.9

Taking viewers to a laboratory to observe the development of the ear bones in opossum babies, Shubin resurrects the discredited embryonic recapitulation theory to gain credence for the evolutionary just-so story of how two parts of the sturdy reptilian jaw migrated and evolved into the tiny, precisely linked bones of the mammalian ear. Opossums, being marsupial mammals, are born very soon after conception and complete their development outside their mother’s womb. Their early development is thus analogous to what we would expect during the embryonic and fetal development of a placental mammal but considerably more accessible for study.

Looking at a series of opossum skulls on Your Inner Reptile, we’re shown that two middle ear bones form in the jaw and grow into their proper position in the middle ear, and we’re told this is an “astonishing” replay of “300 million years of evolution.” In fact, while this embryonic development is fascinating to observe it is not astonishing at all, and does nothing to support reptile-to-mammal evolution.

These two mammalian middle ear bones and the lower jaw ordinarily form from Meckel’s cartilage, which is part of the first pharyngeal arch, and as mammals develop the ear bones are pulled into their proper position. Thus, these opossums are not recapitulating 300 million years of evolution, as the program teaches, but only developing in the way mammalian embryos do.10 They are maturing, not evolving. Maturation is observable. Evolutionary transitions, requiring as they would genetic information to become something new and more complex, are not observed in the study of living creatures and cannot be supported by pointing to embryologic development, which is beautifully designed in each kind of creature and not the result of a blind, purposeless, trial-and-error process of mutations and natural selection. This observable embryonic development within a particular mammal is not a fair analogy for evolutionary migration over millions of years in a supposed transition from one kind of animal to another.  

Mammal-Like Reptiles Don’t Tell the Tale

Despite evolutionary claims like those presented on Your Inner Reptile, the problem with lining up fossils and postulating that each transitioned into the next is that such comparisons focus on a limited number of features, such as jaws and ears. Such an approach ignores many other traits that would have to evolve, many of them at the same time, to build a new kind of animal. Also, close examination reveals that evolution of some characteristics seems at times to have reversed, evolving a trait and then losing it and having to evolve it all over again. The sequence of mammal-like reptiles is simply a group of reptilian fossils that have some features not seen in modern reptiles, but that only demonstrates some of our world’s lost reptilian biodiversity. It does not demonstrate that they were in the process of evolving into mammals any more than observable embryonic development does.

Much as we might like to peer back through time to see what animals in the past were like, our interpretations of a genomic or paleontologic time tunnel are governed by our starting assumptions. If we assume that all living things share a common evolutionary ancestor, then we will naturally perceive or imagine every difference as an evolutionary deviation and evidence of acquired complexity. But the evolutionary starting point is not supported by observable experimental biology in which organisms only vary within their created kinds. The biblical worldview is consistent with the pattern of variation and reproduction that we see in the world. If we begin by accepting God’s eyewitness account of creation and the global Flood as historical, then we understand that these extinct creatures are descendants of those original kinds of created animals, many of which were catastrophically buried and fossilized in the global Flood.


  1. You can learn more about yolk sacs and the other amazing developments that take place in a place ordinarily hidden from human eyes in the video Fearfully and Wonderfully Made and in the chapter on embryonic development in the book Creation Facts of Life.
  2. The liver and eventually the bone marrow take over these functions later, but the yolk sac is there to do the job until they are formed. Read more in Dr. Jean Lightner’s article “Egg on our faces?” at
  3. Guojun Sheng & Ann Foley, “Diversification and conservation of the extraembryonic tissues in mediating nutrient uptake during amniote development,” Annals of the New York Academy of Sciences, 1271 (2012) 97-103. doi: 10.1111/j.1749-6632.2012.06726.x. Some mammals also utilize the yolk sac as a significant part of their placenta, but the nutritional role of the yolk sac in humans differs and is no less essential, as the yolk sac is the primary source of nutrition for about a month, until the placental circulation is fully established.
  4. Each part of the developing gut incorporates portions of the yolk sac. The small part not utilized in building the embryo’s gut remains as part of the umbilical cord. A portion of the yolk sac, the allantois, also contributes to the urinary bladder.
  5. S. Zhang et al., “Vitellogenin, a multivalent sensor and an antimicrobial effector,” The International Journal of Biochemistry and Cell Biology, 43 (2011): 303–305, doi:10.1016/j.biocel.2010.11.003.
  6. Personal communication, e-mail.
  7. Some claim that the new function isn’t so new, since reptiles hear through the vibrations of the bones in their jaw. In fact, humans have their hearing augmented by bone conduction via the skull’s bones too, but this cannot compare with the functionality in modifying and transmitting sound that happens in the mammalian trio of middle ear bones.
  8. See C. Sidor, “Evolutionary trends and the origin of the mammalian lower jaw,” Paleobiology 29, no. 4:605–640 and also “Synapsids and the Evolution of Mammals” at Truth in Science to learn more.
  9. Z, Luo et al., “A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics,” Science 292 no. 5521:1535–1540. Also see Bigger brains for better noses.
  10. Incidentally, in human babies, the ear bones remain encased in a connective tissue matrix until a month before birth. Then the matrix dissolves and the little bones are locked into place by curtain-like tissues in which their supporting ligaments develop. Development in an embryo and fetus is observable and occurs as is programmed in the genome of the animal or person being observed. But embryos are just developing, not recapitulating a mythological evolutionary history.

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Answers in Genesis is an apologetics ministry, dedicated to helping Christians defend their faith and proclaim the gospel of Jesus Christ effectively. We focus on providing answers to questions about the Bible—particularly the book of Genesis—regarding key issues such as creation, evolution, science, and the age of the earth.