Where Do Babies Come From? The Historical Misconception of Conception

By Toby Lanser

Rembrandt’s Aristotle with a Bust of Homer, 1653

“Though they of different sexes be

Yet on the whole, they're the same as we

For those that have the strictest searchers been

Find women are just men turned outside in”

This anatomical rhyme comes from a medieval verse that encompassed the general thought around the differences of the sexes employed by medical students for millenia. To understand how people misunderstood the mechanisms behind sex and development for so long, we must return to the time of Pythagoras in ancient Greece around 530 BCE. One might remember Pythagoras as the name of the guy whose theorem your geometry teacher loved to quote, but we will focus on his more abstract theories surrounding the idea of inheritance.

As most of the ancient Greek thinkers were, Pythagoras was half scientist and half mystic, and for the longest time his theory on inheritance prevailed. Pythagoras’s theory attempted to explain the mechanisms between the physical similarity, or “likeness,” of parents and their offspring. At the heart of his theory lay his suggestion that hereditary information was carried predominantly in male sperm. This hereditary information was gathered by the sperm circulating throughout the body and absorbing metaphysical information from its environment (arms, legs, heart, etc.). Since this theory focused mainly around the sperm, it became known as spermism. This information-infused unit matured in the womb whose main objective, according to Pythagoras, was to provide nutrients for this raw data to be transformed into a child. The focus around the male as the primary source of hereditary information had far-reaching effects in society, with civilizations viewing women as nothing more than “human incubators,” and men being considered the forebearer of all children.

As absurd as this theory sounds when taking into account modern science, it took the razor sharp mind of Aristotle, the first natural philosopher, to even challenge spermism. Aristotle rejected the notion that hereditary information was carried nearly exclusively in the sperm. Since children can inherit features from both their parents, he reasoned sperm could not be the sole contributor. Additionally, Aristotle attacked Pythagoras’s theory that sperm collected information by traveling throughout the body in the most intuitive and self-evident way: Pythagoras’s theory could not account anatomical differences between men and women. There is a clear difference in anatomy between the two sexes which would not arise if offspring were modeled after their father’s physical characteristics.

Though not exactly a hero for the equal rights of women, Aristotle presented a radical notion for his time: men and women must contribute equal amounts of information to generate offspring. Aristotle took it further by theorizing that the sperm and egg are not the essential parts of procreation. The matter transmitted is merely a proxy in the form of a message, or code, used to assemble a human from scratch. This was perhaps the first time in the human history, approximately 350 BCE, that the concept of DNA and a coded inherited messages had been proposed.

However, the idea of sex cells carrying a code as the primary source of hereditary information did not take a smooth trajectory towards the discovery of DNA. Thinkers took Aristotle’s theory and attempted to build off it, though they missed a key concept: differentiation. Rather, thinkers focused on proliferation. To explore the idea of proliferation in development, one has to operate under the assumption that a fetal form constantly exists and only grows, as opposed to our current understanding of a single cell first proliferating, then differentiating.

A clever theory was devised during the European Renaissance, suggesting that all sperm cells contained a mini-human, or homunculus, and when inside the womb, expanded like a bouncy house being inflated with air. The idea of a homunculus as the smallest of humans, and the source of all children, is innately recursive, meaning it is endlessly repetitive, allowing it to explain procreation on a generational scale. But the theory of a homunculus suffers from a key flaw: it does not account for variation among humans and their offspring. If a mini-human were present in all sperm cells, then the exact version of that homunculi would result through maturation, having every offspring from a parent be identical.

As the debate over the the creation of a fetus reached an impasse after hundreds of years of arguing, another conceptual debate about biology was taking place: the mechanisms that cause the anatomical differences between the sexes. Starting with another Roman philosopher and scientist, Galen, in 200 AD, with the dissections of the testes and ovaries. Galen hypothesized that the ovaries were simply internalized male testes, relaying the theme of the opening medieval verse. Another bizarre theory on sex determination was that of a Greek philosopher, Anaxagoras, in 400 BC who suggested that sex was determined by that location of the sperm in the testes, either in the left or right testicle.

It took over two millennia for the biological basis of sex to be discovered. In 1903, a graduate student from Vermont, Nettie Stevens, performed a series of groundbreaking studies by investigating the chromosomes of mealworms. At the time, the biological unit of heredity was unknown, though it was theorized that chromosomes were the basis of inheritance, a concept aptly titled “The chromosome theory of inheritance.” In her studies, Stevens analyzed the morphology of all of the chromosomes in a mealworm cell. She found all of the chromosomes remained the same and were paired, between both male and female worms, except for one pair. For this pair, the females had similar looking chromosomes and the males contained a normal looking chromosome and a short, stubby one. Stevens replicated her studies, producing similar results and bringing her to the conclusion that this short chromosome was the basis for the differences between the sexes. Somewhere in this chromosome, she theorized, was the code that lead to the differentiation between the sexes, shattering the theories offered by some of the greatest minds in human history.

Stevens’s early genetic studies spearheaded modern genetics, and in the following decades researchers identified that chromosomes carried DNA, and that DNA itself was the genetic code necessary to enabled inheritance.

As the history of thought and debate progresses in any field, nascent theories are built upon until their popularity becomes ingrained in a society as fact. Equipped with the critical thinking and curiosity, it is a scientist’s duty to not just discover the workings of the unknown, but to question the fabric of what has been thought of as being known for millennia.


Mukherjee, Siddhartha. The Gene: an Intimate History. Large Print Press, a Part of Gale, Cengage Learning, 2017.

Aristotle, and D. M. Balme. History of Animals. Harvard University Press, 1991.

Stevens, Nettie Maria. “Studies in Spermatogenesis.” 1905, doi:10.5962/bhl.title.56517.


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