Shelled Eggs Crack Evolution
Explaining shelled eggs by
natural selection is a
major problem for evolution
by Jerry Bergman, PhD
Introduction
Evolutionists believe that all terrestrial vertebrates evolved from marine ancestors that laid shell-less eggs in the water. Scientists have long taught that the first terrestrial animals must have been laying eggs which eventually evolved into eggshell-enclosed eggs in order to conquer living on land. How this occurred is a major problem for evolution.[1] Evolving eggs without shells that were fertilized in water into eggs that are fertilized when in the mother’s body requires hundreds of structural changes. A revolution in egg evolution theory by Jiang et al., discussed below, attempts to move toward bridging his gap.
Amniotic Eggs Enclosed by an Eggshell
Diagram of the parts of an egg. From Wikimedia Commons.
The amniotic egg enclosed by an eggshell design is employed for reproduction by most terrestrial animals, including many turtles and other reptiles, birds, and prototherian (egg-laying) mammals, (amniotes), in which the embryo develops inside of an amnion. This design is very different from the anamniotic egg (an-amniotic or non-amniotic) which is not surrounded by an eggshell and contains the embryo of a fish, an amphibian, or a mammal. This egg design is typically much smaller than the amniotic egg and is more dependent on environmental conditions than are shelled eggs.
Anamniotic eggs are enclosed in chorion membranes and lack both an eggshell and extraembryonic membranes. In contrast, the amniotic egg consists of a set of fetal membranes including the amnion, chorion, and allantois surrounded by an external shell that is either strongly mineralized (as in rigid-shelled eggs) or weakly mineralized (as in parchment-shelled eggs).
The extra-embryonic membranes enclose specific egg elements, store nutrients, collect wastes, and regulate gas and fluid exchange between the egg and the external environment.[2] To this over simplification must be added at least five main classes of egg reproduction that greatly complicate egg evolution: ovuliparity (external fertilization), oviparity (embryos in eggs), ovoviviparity (internal retention of the egg until it hatches), histotrophic viviparity (zygote development within the mother’s oviducts and nutrients are provided by intra-uterine cannibalism), and hemotrophic viviparity (nutrients are provided by the mother via a placenta).[3]
The Importance of the Evolution of the Amniotic Egg
The evolutionary importance of the amniotic egg was because the “amniotic egg with its complex fetal membranes was a key innovation in vertebrate evolution that enabled the great diversification of reptiles, birds and mammals”[4]. Evolution of this design was first required for life to evolve from the marine life of fish to the terrestrial life of reptiles.
Evolutionists have never been able to document even the most fundamental steps of this required evolution, even admitting that “It is debated whether these fetal membranes evolved in eggs on land as an adaptation to the terrestrial environment or to control antagonistic fetal–maternal interaction in association with extended embryo retention (EER).”[5] EER means that the embryo evolved to stay in the womb longer for further development. The Jiang study “suggests that EER was the primitive reproductive mode in basal archosauromorphs. Phylogenetic comparative analyses on extant and extinct amniotes suggest that the first amniote displayed EER (including viviparity).”[6] Viviparity is the development of the embryo inside the body of the mother. The mother provides the maternal circulation to meet the metabolic needs of the embryo’s development until its birth as a fully, or partially, developed infant that can survive on its own outside the womb. In contrast, oviparity involves mothers depositing fertilized zygotes outside her body, (laying or spawning eggs), in metabolically independent incubation organs called eggs, which nurture embryonic development outside of the mother.
In short, evolution of oviparity required the innovation of the many fetal membranes in the egg including the yolk sac, amnion, the chorion, and allantois.[7]
Structure of a Shelled Egg
The Germinal Disc is a small white spot on the yolk where the egg can be fertilized. If the germinal disc is fertilized, an embryo will start to grow. It will then be called a blastoderm. The yolk sac supplies food material to the embryo including minerals, vitamins, fat, and protein. The amnion is a transparent sac filled with a colorless fluid that serves as a protective cushion during embryonic development.
Lying between the eggshell and egg white, are two transparent protein membranes that provide efficient defense against bacterial invasion. These surprisingly strong layers are constructed partly of keratin, a protein that’s also in human hair. The chorion membrane that surrounds a developing fetus provides nourishment and protection for the developing embryo. The allantois is a hollow sac-like structure that is critical to help the embryo exchange gases and handle liquid wastes. The chalazae are ropey strands of egg white that hold the yolk in the egg center. Like little anchors, they attach the yolk’s casing to the membrane lining of the eggshell. The vitelline membrane is a clear casing that encloses the yolk. All of these parts must exist and be functionally integrated in order for oviparity to exist.[8] In other words, shelled eggs are irreducibly complex.
An evolutionary consensus on this issue has not been reached: “Where and how the fetal membranes of the amniotic egg evolved has been debated, and two competing hypotheses have been proposed.”[9] The conventional, ‘terrestrial model’ postulates
that the precursor to amniotes laid eggs on land, similar in many respects to the directly developing eggs of a variety of extant amphibians, and the fetal membranes were gradually acquired so that the egg could adapt to terrestrial environments by retaining water inside and allowing oxygen and carbon dioxide to pass through the eggshell. This widely accepted model has been challenged by the ‘extended embryo retention model that the extraembryonic membranes appeared in the oviducts of the amniotic ancestor as specializations to control fetal–maternal interaction in association with extended embryo retention (EER)’.[10]
Even the most basic question about egg evolution has not been answered by this model, namely:
“Evolutionary studies based on extant amniotes give equivocal results about whether oviparity or viviparity arose first. Circumstantial evidence for the EER model is the near absence of fossils of amniotic eggs before the Late Triassic Period and the discovery of viviparity in many extinct amniotes as old as the Early Permian Period.”[11]
Some of the variety of bird eggs (Science Magazine).
Eggshell Evolution Models
Two models of eggshell evolution exist: “In the terrestrial model, non-EER oviparity was the primitive condition; oviparity with EER and viviparity evolved multiple times in amniotes. In the EER model, the evolutionarily labile reproductive mode of EER across oviparity to viviparity was primitive, while non-EER oviparity evolved multiple times in amniotes.”[12]
The first detailed evolutionary theory was developed in 1957 by
Alfred Sherwood Romer (1894–1973), the leading vertebrate paleontologist of the 20th century, [who] wrote, “One of the most important steps in the evolution of vertebrates was the “invention” of the amniote egg… Its appearance marks the beginnings of the history of the reptiles and the potentialities of evolution of the great groups that are dominant today, the birds and mammals. The evolution of the amniote type of development was a necessary antecedent to the true conquest of the land.”[13]
Romer’s conclusion “has been the textbook view ever since, despite dissenting voices”. This orthodox theory changed due to an analysis of “51 fossil species and 29 living species to explore the likely ancestral states in each major clade,” has been completed by Jiang et al. The result of this analysis was that “the classic ‘reptile egg’ model of Romer and the textbooks” has been
consigned to the wastebasket. The first amniotes had evolved EER as a means to protect the developing embryo for a lesser or greater amount of time inside the mother, so birth could be delayed until environments became favorable. Whether the first amniote babies were born in parchment eggs or as live, snapping little insect-eaters is unknown, but this adaptive parental protection gave them the advantage over spawning earlier tetrapods.[14]
Although Jiang et al., concluded that the EER theory has overturned the old theory, the new theory does not answer even the basic issues involving irreducible complexity. Extended embryo retention theory is largely a tweaking of the existing theory and does not explain any of the major problems with the evolution of hard-shelled eggs.
Furthermore, if the new research concluding EER is correct, it raised several basic questions, such as if the first amniote babies that evolved were born in parchment eggs or as live, snapping, little insect-eaters.
Summary
Although the researchers have concluded that the EER model has overturned the once widely accepted Romer model, I fail to see how allowing the embryo to stay in the womb longer to develop further solves the many problems of the evolution-of-the-egg problem. Evolutionists believe that the amniotic egg evolved but do not know where and how, and both existing theories have major problems. What is totally ignored in this, and most, reviews of egg evolution, is the fact that the yolk sac, amnion, the chorion, and allantois are all required as a functional set because the system is irreducibly complex.[15] The amniotic egg requires all these functional structures to survive.
See also our 27 June 2017 article, “Egg-Shape Evolution Theory Cracks Under Pressure,” where a writer in Science Magazine quoted Thomas Wentworth Higginson who called an egg “the most perfect thing in the universe.”
References
[1] Bergman, J. “The Egg: Creation’s Perfect Package.” Creation Science Dialogue 39(1):4-5, January 12.
[2] Jiang, B. et al. “Extended embryo retention and viviparity in the first amniotes.” Nature Ecology & Evolution 7:1131-1140; https://doi.org/10.1038/s41559-023-02074-0, July 2023.
[3] Lode, T. “Oviparity or viviparity? That is the question…” Reproductive Biology 12(3):259-264.
[4] Jiang et al., 2023, p. 1131. Emphasis added.
[5] Jiang et al., 2023, p. 1131. Emphasis added.
[6] Jiang et al., 2023. p. 1131.
[7] Bergman, Jerry. “The Egg—Irreducible Complexity of Creation’s Perfect Package.” Journal of Creation 33(1):119-124, 2019.
[8] Hedin, E. “Evolution’s Chicken and Egg Problem — Explained.” Evolution News, 9 January 2024.
[9] Jiang et al., 2023, p. 1131.
[10] Jiang et al., 2023, p. 1131.
[11] Jiang et al 2023, p. 1131. Emphasis added.
[12] Jiang et al., 2023, p. 1131.
[13] Benton, M. “Which came first: The reptile or the egg?” Nature Research Communities., 12 June 2023.
[14] Benton, 2023. Boldface added.
[15] Hedin, E. “Evolution’s Chicken and Egg Problem — Explained.” Evolution News, 9 January 9, 2024.
Humpty Darwin sits on a wall of foam bricks held together by decayed mortar. Cartoon by Brett Miller commissioned for CEH. All rights reserved.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,800 college libraries in 27 countries. So far over 80,000 copies of the 60 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.