Introduction

Aristotle thought that an embryo emerged from an amorphous mass, a “less fully concocted seed with a nutritive soul and all bodily parts”. The much later development of the microscope led to more detailed (if inaccurate) descriptions of embryonic development. In 1677, Anton von Leeuwenhoek, looking at a human sperm with his microscope, thought he saw a miniature human inside. The tiny human, or homunculus, became the epitome of preformation theory.

Also in the 17th century, William Harvey described morphological changes in developing chicken embryos and coined the term epigenesis to counter the notion that any tiny adult structures in eggs or sperm simply grew bigger during embryonic gestation. Over time, experiments began to show that the physical and chemical environment, such as temperature, pH, or even the position of incubation in chicken eggs, could significantly influence embryonic development.

In 1924, Hans Speeman conducted a series of elegant experiment demonstrating that cells responsible for differentiation in one region of an embryo could be transplanted to another region, or to another embryo entirely, where it would induce new tissue development. He won the 1935 Nobel Prize in Physiology and Medicine for his discovery of embryonic organisers that induced morphogenesis. Around the same time, other embryologists, including Conrad Waddington, demonstrated that cells killed by freezing or boiling still induced morphogenesis, suggesting that chemicals signals, not just living cells, influenced development.

These findings led many to conclude that environment played the dominant role in shaping an organism’s phenotype, with genes seen as secondary. Waddington, however, proposed a more balanced view, believing that both genes and environment contributed to development. In 1942, he coined the term epigenetics to describe how environmental factors influence embryonic development.

This idea sparked a nature-versus-nurture debate that continues today. While we now understand that individuals with the same genotype can develop different phenotypes due to environmental influences, questions remain — especially regarding complex traits like intelligence, behaviour, and psychological tendencies. Despite ongoing research, the interplay between genes and environment in shaping these traits remains difficult to disentangle.

This idea sparked a nature-versus-nurture debate that continues today. While we now understand that individuals with the same genotype can develop different phenotypes due to environmental influences, questions remain, especially regarding the complex traits like intelligence, behaviour and psychological tendencies. Despite ongoing research, the interplay between genes and environment in shaping these traits remains difficult to disentangle.