Bacteria outnumber the cells in our own body by a factor of 10 to 1 — as many as 100 trillion microbes per person — and yet we know almost nothing about how bacteria have changed as humans evolved.
Although they're much more numerous than our own cells, bacteria are so tiny that they represent only a small fraction of our overall mass. Admittedly, it isn't that small a fraction: about 5 pounds of each person is actually all the bacteria living inside us. It's not the most pleasant thought, but it's been a basic, vital part of our biology going back to the very origins of multicellular life.
But that raises an interesting point — whenever we talk about human evolution, we're only talking about the adaptations of Homo sapiens, not the trillions of microbial passengers living inside it. It's like we're always omitting five pounds of every human body — given the estimated 108 billion people who have ever lived, that's a total of 270 million tons over the past 50,000 years that our current evolutionary story just ignores.
And for the most part, that isn't likely to change, as human remains generally don't preserve ancient bacteria. The one major exception is found in dental plaque. Researchers at the Centre for Ancient DNA at Australia's University of Adelaide examined the preserved tartar from 34 prehistoric skeletons found in northern Europe. They used those skeletons to construct a bacterial timeline spanning from the age of hunter-gatherers to the Middle Ages, with modern dental records bringing us up to the present day.
In a statement, study leader Professor Alan Cooper explains the find:
"This is the first record of how our evolution over the last 7500 years has impacted the bacteria we carry with us, and the important health consequences. Oral bacteria in modern man are markedly less diverse than historic populations and this is thought to contribute to chronic oral and other disease in post-industrial lifestyles. The composition of oral bacteria changed markedly with the introduction of farming, and again around 150 years ago. With the introduction of processed sugar and flour in the Industrial Revolution, we can see a dramatically decreased diversity in our oral bacteria, allowing domination by caries-causing strains. The modern mouth basically exists in a permanent disease state."