January 31, 2018

Dental problems in humans

Late Iron Age/Roman woman showing large dental calculus deposit, from Cambridge area, UK. Photo: Alan Cooper

Science has now provided an excuse for those of us used to being chided by our dentists for not brushing often enough: blame your cavities on the Industrial Revolution. New research suggests that the dietary changes associated with the Industrial Revolution 150 years ago (and with the invention of agriculture 10, 000 years ago) caused an epidemic of tooth decay and gum disease. The culprits are oral bacteria. The human mouth is the native home of a wide variety of microbes, some helpful and some harmful. Over the course of human history, eating more starch and sugar seems to have tipped the balance in favor of the disease-causing bacteria. Even without ultrasonic toothbrushes and mouthwashes, our ancestors may have had healthier teeth than we do!

Where's the evolution?
To study the ancient microbial communities inhabiting human mouths, an international team of researchers capitalized on the fact that oral bacteria are trapped on teeth by tartar buildup. The scientists collected tartar samples from the teeth of human skeletons ranging from a few hundred to more than 6000 years old and extracted the ancient bacterial, providing snapshots of the oral ecosystem at different points in our history. Analysis of the DNA showed two things. First, different bacterial species became more common after dietary shifts. When humans began farming and eating more wheat and barley, the oral ecosystem changed, allowing gum-disease-causing species to flourish. When humans began eating processed flour and sugar, the ecosystem shifted again, this time favoring decay-causing species. Second, as these dietary changes occurred, the of the oral ecosystem plummeted. As with many other habitats on Earth, there just aren't as many species living in our mouths as there used to be. Since greater biodiversity is associated with healthy, resilient ecosystems, a lack of oral biodiversity may help explain the high rates of dental disease among modern humans.

The link between our diet and rates of tooth decay and gum disease is an example of a to modernity a case in which a disease results from a modern lifestyle feature that our has not experienced during the course of our history. The idea is that our bodies are 'matched' to the environment in which we spent most of our evolutionary history not to the environment that we have created for ourselves today. In these instances, has not been able to keep pace with the rate at which human culture and the innovations that it generates (e.g., farming) change. If we humans had eaten lots of carbohydrates throughout our evolutionary history (for many hundreds of generations) and if the resulting dental disease had reduced our ability to survive and reproduce, we might have experienced natural selection favoring that reduced the severity of the problem. However, because these dietary changes happened relatively recently (and in the modern world, occurred in conjunction with advances in dentistry that lessened their impact on health and reproduction), we do not possess adaptations to deal with an influx of sugar and starch and, hence, are likely to develop dental disease.

Evolutionary mismatches to modern lifestyles seem to contribute to many diseases. Some cases are relatively obvious for example, the high rates of lactose intolerance among human without a history of dairying and the prevalence of type 2 diabetes among populations that have recently adopted a high-sugar, high-fat Western diet. Other mismatches are less obvious. For example, throughout human evolutionary history, we've been more or less continually infected by parasites, such as helminth worms. However, clean water and modern hygienic advances have made such infections rare in developed countries. Now, medical researchers have discovered several autoimmune diseases (e.g., multiple sclerosis) that seem to be caused or made worse by the lack of a helminth infection. In this case, modern, worm-free lifestyles seem to be causing problems for human bodies that evolved under the continual influence of parasites.

Mismatches to modernity are studied by researchers interested in an approach to human health and disease that is informed by human evolutionary history, as well as by an understanding of the real-time evolution experienced by our pathogens, symbionts, and the cells of our own bodies. If you want to know whether the appendix has any function, what regime for antibiotic administration is least likely to result in the evolution of antibiotic resistant bacteria, why chemotherapy doesn't always cure cancer, or why some people don't have wisdom teeth, the field of evolutionary medicine is where you'll find the answers.

Viewing health and disease in the light of evolution is important not just from a theoretical perspective (e.g., understanding the origins of tooth decay), but from a practical one as well. Evolutionary medicine often points the way towards new treatment and prevention strategies. Sometimes an evolutionary perspective supports established treatments like reducing one's consumption of processed sugar and flour to reduce tooth decay and sometimes it presents novel ones. In the case of dental hygiene, one of the researchers behind the new research that if bacterial diversity is important to dental health, mouthwash might actually do more harm than good by wiping out the microbial community that normally lives in our mouths. When it comes to human health (or any biological system, for that matter), studying what an organism is like today paints only half the picture. An understanding of how it got to be that way its evolutionary history fills in the missing half.