In 1683, Antonie van Leeuwenhoek, a Dutch scientist, observed "animalcules" (bacteria) in plaque scraped from his teeth using a homemade microscope. This observation is credited as humanity's first documented viewing of bacteria. Van Leeuwenhoek continued to collect samples and observe a miniature world, revealing that human bodies contain an invisible populace of microorganisms.
Revisiting the Microbial Self
Three centuries after van Leeuwenhoek, scientist Thomas Luckey estimated in 1972 that microbial cells outnumbered human cells at a 10:1 ratio. This figure was widely cited for decades. However, a 2016 study provided a revised estimate of 1.3:1, suggesting approximately 30 trillion human cells and 39 trillion microbial cells in an average adult, indicating humans are marginally outnumbered cell for cell by microbes.
Microbes residing in and on human bodies are not merely passengers; they perform crucial functions. Associate Professor Andrea Stringer notes that while gut bacteria technically live on us (within the intestinal lumen), their roles in training the immune system, digesting complex carbohydrates, and synthesizing vitamins (biotin, vitamin K, folates) suggest they are functionally integral to human life.
The Role of Microbes in Health
Dependence on microbes extends across animal life. "Germ-free" animals raised in sterile environments develop significant health issues, including impaired immune function, poor digestion, and increased susceptibility to infection.
Research also indicates a connection between microbes and brain function, known as the gut-brain axis. Studies on germ-free mice demonstrate microbial influence on stress response, anxiety-like behaviors, and cognition. In humans, the gut microbiome is increasingly linked to psychiatric conditions such as anxiety, depression, and PTSD, with observed differences in the microbiome composition of individuals with these disorders.
Human DNA and Shared Genetics
The discovery of DNA by Friedrich Miescher in 1869, followed by Crick and Watson's structure description in 1953, revolutionized understanding of heredity. While genetic essentialism has been a prominent concept, the uniqueness of human DNA is less pronounced than often assumed.
Dr. Vicki Jackson, a statistical geneticist, states that humans share approximately 98–99% of their DNA with other primates, and 80–90% with most other mammals. Even with non-animals, humans share about 20–25% of genes with plants and roughly 30% with yeast, indicating common ancestry spanning over a billion years.
A small fraction of DNA in the human genome may originate from microbes through horizontal gene transfer (HGT), where genes are exchanged between microbes or taken up from the environment. While rare and often debated in human genomes, some candidate genes may trace their origins to HGT. Jackson notes that the unique aspects of the human genome derive more from