Why Do Humans Possess a Twisted Birth Canal? Unraveling an Evolutionary Puzzle

Doctor Holding Newborn Baby

Researchers unveil fresh insights into the evolutionary factors shaping the complex structure of the human birth canal.

Extraordinary shape makes births more difficult, but guarantees stability.

The relatively narrow human birth canal presumably evolved as a “compromise” between its abilities for parturition, support of the inner organs, and upright walking. But not only the size of the birth canal, also its complex, “twisted” shape is an evolutionary puzzle. Katya Stansfield from the University of Vienna and her co-authors have published a study in BMC Biology presenting new insights into why the human birth canal evolved to have this complex shape. They suggest that the longitudinally oval shape of the lower birth canal is beneficial for the stability of the pelvic floor muscles.

In most women, the upper part, or inlet, of the birth canal has a round or transversely (left-to-right) oval shape, which is considered ideal for parturition, but it is unknown why the lower part of the birth canal has a pronounced longitudinally (front-to-back) oval shape. This twisted shape typically requires the Baby to rotate when passing through the narrow birth canal, which further increases the risk of birth complications.

In comparison with humans, apes have a relatively easy birth pattern that does not require rotation of the baby thanks to the longitudinally oval shape of the birth canal both at its inlet and the outlet. “For giving birth, it would be much easier to have a uniformly shaped birth canal also in our species,” says Katya Stansfield, a specialist in biomechanics. Instead, the twisted human shape requires a complex, rotational birth mechanism: The baby needs to rotate to align the longest dimension of its head with the widest dimension of each plane of the birth canal. Misalignment can lead to obstructed labor and result in health risks for both mother and baby.

A research team of evolutionary biologists and engineers from the University of Vienna, the Konrad Lorenz Institute for Evolution and Cognition Research in Klosterneuburg, and the University of Porto hypothesized that the support function of the pelvic floor muscles, which are suspended across the lower pelvis and also play an important role in sexual function and continence, may have influenced the evolution of the shape of the birth canal. The team carried out extensive biomechanical modeling of the pelvic floor and found that the highest deformation, stress, and strain occur in pelvic floors with a circular or transverse-oval shape, whereas a longitudinally oval elongation increases pelvic floor stability. “Our results demonstrate that the longitudinally oval lower birth canal is beneficial in terms of stability,” says Katya Stansfield. “However, this outcome prompted us to ask why the pelvic inlet in humans is not also elongated longitudinally,” elaborates Barbara Fischer, an evolutionary biologist.

Traditionally, it has been assumed that the transverse dimension of the human pelvis is constrained by the efficiency of upright locomotion. “We argue that the transverse elongation of the pelvic inlet has evolved because of the limits on the front-to-back diameter in humans imposed by balancing upright posture, rather than by the efficiency of the bipedal locomotion,” says Philipp Mitteroecker, who was also involved in this study. A longitudinally deeper inlet would require greater pelvic tilt and lumbar lordosis, which would compromise spine health and the stability of upright posture. These different requirements of the pelvic inlet and outlet likely have led to the evolution of a twisted birth canal, requiring human babies to rotate during birth. 

Reference: “The evolution of pelvic canal shape and rotational birth in humans” by Ekaterina Stansfield, Barbara Fischer, Nicole D. S. Grunstra, Maria Villa Pouca and Philipp Mitteroecker, 11 October 2021, BMC Biology.
DOI: 10.1186/s12915-021-01150-w

21 Comments on "Why Do Humans Possess a Twisted Birth Canal? Unraveling an Evolutionary Puzzle"

  1. thirdwavetrisomite | November 16, 2021 at 5:42 am | Reply

    So you have to explain what transversely and longitudinally mean, yet you do not explain what parturition means. Great consistency.

  2. Babu G. Ranganathan | November 16, 2021 at 7:41 am | Reply

    Babu G. Ranganathan*
    (B.A. Bible/Biology)


    ONLY LIMITED EVOLUTION (micro-evolution or evolution within biological “kinds”) is genetically possible (such as the varieties of dogs, cats, horses, cows, etc.), but not macro-evolution, or evolution across biological “kinds,” (such as from sea sponge to human). All real evolution in nature is simply the expression, over time, of already existing genes or variations of already existing genes. For example, we have breeds of dogs today that we didn’t have a few hundred years ago. The genes for these breeds had always existed in the dog population but never had opportunity before to be expressed. Only limited evolution, variations of already existing genes and traits, is possible.

    The genes (chemical instructions or code) for a trait must first exist or otherwise the trait cannot come into existence. Genes instruct the body to build our tissues and organs. Nature is mindless and has no ability to design and program entirely new genes for entirely new traits.

    Evolutionists believe that, if given millions of years, accidents in the genetic code of species caused by the environment will generate entirely new code making evolution possible from one type of life to another. It’s much like believing that by randomly changing the sequence of letters in a romance novel, over millions of years, can turn the novel into a book on astronomy! Not to worry. We’ll address the issue of “Junk DNA” in a moment.

    WHAT ABOUT NATURAL SELECTION? Natural selection doesn’t produce biological traits or variations. It can only “select” from biological variations that are possible and which have survival value.

    HOW COULD SPECIES HAVE SURVIVED if their vital tissues, organs, reproductive systems, etc. were still evolving? A partially evolved trait or organ that is not complete and fully integrated and functioning from the start would be a liability to a species, not a survival asset. Plants and animals in the process of macro-evolution would be unfit for survival. For example, “if a leg of a reptile were to evolve (over supposedly millions of years) into a wing of a bird, it would become a bad leg long before it became a good wing” (Dr. Walt Brown, scientist and creationist). Survival of the fittest actually would have prevented evolution across biological kinds!

    NEW SPECIES BUT NOT NEW DNA: Although it’s been observed that new species have come into existence, they don’t carry any new genes. They’ve become new species only because they can’t be crossed back with the original parent stock for various biological reasons. A biological “kind” allows for new species but not new genes. Nature has no ability to invent new genes for new traits. Only limited variations and adaptations are possible in nature, and all strictly within a biological “kind” (i.e. varieties of dogs, cats, etc.).

    Dr. Randy J. Guliuzza’s extensive research points to a better explanation than natural selection for variation and adaptation in nature. Dr. Guliuzza explains that species have pre-engineered mechanisms that enable organisms to continuously track and respond to environmental changes with system elements that correspond to human-designed tracking systems. This model is called CET (continuous environmental tracking). His research strongly indicates that living things have been pre-engineered to produce the right adaptations and changes required to live in changing environments. It’s much like a car that’s been pre-engineered so that the head lights turn on automatically when day changes to night.

    What about genetic and biological similarities between species? Genetic information, like other forms of information, cannot happen by chance, so it is more logical to believe that genetic and biological similarities between all forms of life are due to a common Designer who designed similar functions for similar purposes. It doesn’t mean all forms of life are biologically related! Only genetic similarities within a natural species proves relationship because it’s only within a natural species that members can interbreed and reproduce.

    Many people have wrong ideas of how evolution is supposed to work. Physical traits and characteristics are determined and passed on by genes – not by what happens to our body parts. For example, if a woman were to lose her finger this wouldn’t affect how many fingers her baby will have. Changing the color and texture of your hair will not affect the color and texture of your children’s hair. So, even if an ape or ape-like creature’s muscles and bones changed so that it could walk upright it still would not be able to pass on this trait to its offspring. Only changes or mutations that occur in the genetic code of reproductive cells (i.e. sperm and egg) can be passed on to offspring.

    What about the new science of epigenetics? Epigenetics involves inheritable factors which can turn already-existing genes on, but epigenetics doesn’t create new genes.

    Most biological variations are from new combinations of already existing genes, not mutations. Mutations are accidents in the genetic code caused by nature (i.e. environmental radiation), are mostly harmful, and have no capability of producing greater complexity in the code. Even if a good accident occurred, for every good one there would be hundreds of harmful ones with the net result, over time, being harmful, even lethal, to the species. Even if a single mutation is not immediately harmful, the accumulation of mutations over time will be harmful to the species resulting in extinction. At very best, mutations only produce further variations within a natural species.

    All species of plants and animals in the fossil record are found complete, fully formed, and fully functional. This is powerful evidence that all species came into existence as complete and fully formed from the beginning. This is only possible by creation.

    God began with a perfect and harmonious creation. Even all the animals were vegetarian (Genesis 1:30) in the beginning and did not struggle for survival nor kill and devour each other. Macro-evolutionary theory does not begin with a perfect and harmonious creation as the Bible states. The Bible and macro-evolutionary theory cannot both be true.

    All the fossils that have been used to support human evolution have been found to be either hoaxes, non-human, or human, but not non-human and human (i.e. Neanderthal Man was discovered later to be fully human).

    There has never been unanimous agreement among evolutionary scientists on ANY fossil evidence that has been used to support human evolution over the many years, Including LUCY.

    The actual similarity between ape and human DNA is between 70-87% not 99.8% as commonly believed. The original research stating 99.8% similarity was based on ignoring contradicting evidence. Only a certain segment of DNA between apes and humans was compared, not the entire DNA genome.

    Also, so-called “Junk DNA” isn’t junk. Although these “non-coding” segments of DNA don’t code for proteins, they have recently been found to be vital in regulating gene expression (i.e. when, where, and how genes are expressed, so they’re not “junk”). Also, there is evidence that, in certain situations, they can code for protein.

    ARE FOSSILS REALLY MILLIONS OF YEARS OLD? (Internet article by author)

    Visit my latest Internet site: THE SCIENCE SUPPORTING CREATION (This site answers many arguments, both old and new, that have been used by evolutionists to support their theory)


    *I have given successful lectures (with question and answer period afterwards) defending creation before evolutionist science faculty and students at various colleges and universities. I’ve been privileged to be recognized in the 24th edition of Marquis “Who’s Who in The East” for my writings on religion and science.

    • Your writing is garbage. You state things without proving them. Eg, when you say ‘macro evolution’ cannot happen, it’s your job to prove that fairly definitive statement.
      Conversely you can find flaws in scientific papers on evolution, and then get that argument published in a referred scientific journal.

  3. Hey Babu, how’s it going?

  4. Paleontologically unsound questions. Foro news, see G. G. Simpsom, Tempo and Mode in Evolution, 1950. Do the authors still ignore that a holobiont is a multivariate stochastic system? Between your article and the long fundamentalist comment above, I today started my day thinking in committing suicide 🦖🦖🦖🦖 🦕🦕🦕🦕

  5. Ever think that we were made by design and not chance? Psalm 139:13 “for you created my inmost being;you knit me together in my mother’s womb. I praise you because I am fearfully and wonderfully made; your works are wonderful, I know that full well. my frame was not hidden from you when I was made in the secret place . When I was woven together in the depths of the earth , your eyes saw my informed body. All the days ordained for me were written in your book before one of them came to be.

    • Your confidence in written words in a book clearly written by man is illogical. Man created God, at least the one you are talking about. That’s why the Bible is wrong about many things that we came to know about later. If a God who supposedly created the universe doesn’t know whether the sun revolves around the earth or vice versa, then that God can’t be trusted with much of anything.

  6. Perhaps these differences in biology could be better addressed by supporting normal, physiological birth – which is generally not what you see going on in the majority of hospital births. While emergencies do arise, so many problems are created just as often with too many interventions. It is clear a midwife did not contribute to this article LOL.

  7. My first baby was a transverse lie
    And I delivered him naturally
    It was the most painful experience of my life both mentally and physically
    I really don’t know how we survived
    And this was 23 years ago

  8. What the f*ck is a birth canal? I’m a labor nurse and I don’t know, anatomically speaking, what the hell you are studying here. Can we please use appropriate terms to identify parts of our bodies.

  9. Mutation within the gene pool. Creation theory is wrong. Period. Carry

  10. WOW. I call bull. When you lay on your back, the birth canal and pelvis shift so the bone are closer together. When you squat or on fours, it widens. Women thru hisstory have always given birth on side, fours or squating, NEVER back. It was regulated by drs in england when the king ordered it cause he had a fetish for watching the birth, and best way was spread eagal on ur back. We learn in school about movement of body and the science of it, yet we still let women lay on back for birth? It would be much easier. Also NO BABY IN THE WORLD IS EVER EVER born with vitmin k. FOR A REASON. Dont DONT let them inject it. If you do, why? Side effects and injury are different. Allergies are an injury. Vit k is NOT NEEDED in newborns. If it were, why did nature make it so the NO ONE IS BORN WITH IT? You think otherwise then i applaud you for your comitment to cognitive dissonance.

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