Are we truly human, or just a borrowed collection of cells, molecules, and atoms? This question has intrigued scientists for centuries, and the answer is far from simple. In 1683, Antonie van Leeuwenhoek, a Dutch scientist, made a groundbreaking discovery. He scraped plaque from his teeth and, using a homemade microscope, observed a bustling world of 'animalcules'—what we now know as bacteria. This was the first recorded observation of bacteria, and it sparked a fascination with the microscopic world.
Van Leeuwenhoek's curiosity didn't stop there. He collected samples from various sources, revealing that human bodies are teeming with tiny, invisible life forms. But how much of our body is truly human? Three centuries later, Thomas Luckey estimated the bacterial population in the human colon, suggesting a 10:1 ratio of microbial to human cells. However, a 2016 study revised this estimate to a more realistic 1.3:1, indicating that humans are, cell for cell, marginally outnumbered by microbes.
These microbes, living in and on our bodies, play a crucial role in our health. They train our immune system, aid digestion, and synthesize essential vitamins. But are they part of us? Associate Professor Andrea Stringer poses an intriguing question: if microbes live within us, are they also part of us? The answer is complex. While microbes reside on us, they are not strictly 'in' us, but their functional role in our bodies is undeniable.
Our dependence on microbes extends beyond humans. 'Germ-free' animals raised in sterile environments suffer from various health issues, including poor immune function and digestion. Moreover, studies show that microbes influence brain functions, such as stress response and anxiety-like behavior. In humans, the gut microbiome is linked to psychiatric conditions like anxiety, depression, and PTSD, highlighting a complex communication system between the digestive and central nervous systems.
The discovery of DNA by Francis Crick and James Watson in 1953 was a significant milestone, but it built upon earlier work by Rosalind Franklin and others. DNA revolutionized our understanding of heredity and genetic traits. However, it also introduced genetic essentialism, a belief that genes define who we are, which has had dark implications in history.
But how unique is our DNA? We share 98-99% of our DNA with other primates and around 80-90% with most other mammals. Even with non-animals, we share a substantial portion of our genetic material. For instance, 20-25% of plant genes have equivalents in the human genome, and 30% of yeast genes have human counterparts. Some of our DNA may even have been acquired from microbes through horizontal gene transfer.
The boundaries between 'human' and 'non-human' become even more blurred when we consider our physical composition. The average human cell contains trillions of atoms, each with a tiny nucleus containing most of its mass. The vast majority of an atom is 'empty space,' and electrons, with their unpredictable behavior, further complicate this picture. When we touch solid objects, it's not a simple collision of surfaces but a repulsion of electron clouds due to electrostatic forces.
So, what defines our humanity? Geneticist Dr. Vicki Jackson emphasizes that while genes are important, we are more than just our DNA. Our health, behavior, and personality are shaped by the interplay of genes, environment, lifestyle, and experiences. Consciousness, the essence of 'us,' remains a mystery, arising from complex electrical transmissions in the brain.
In the end, the question of what makes us human is a deeply philosophical and scientific conundrum, one that continues to captivate and challenge researchers. It invites us to explore the intricate interplay between our physical, genetic, and microbial selves, and to ponder the very nature of our existence.
