Dental tissue is revealing in same way annual tree rings can tell much about environment tree grew in

Telltale signs of stressful life events can be found in our teeth, say researchers who have found that birth, menopause and even imprisonment appear to leave their mark in tissue that is laid down throughout life.

The phenomenon is similar to the way the thickness of annual tree rings can tell us about the climate and environment in which the tree grew – however in teeth it is changes to the way the tissue interacts with light that offers the clues.

“We didn’t know that a portion of our organism served as such faithful biological archive for the entirety of life,” said Paola Cerrito, co-author of the research, from New York University.

Researchers behind the study say they believe the discovery could help shed light on a longstanding conundrum. .

Humans are unusual as they are one of a few mammals that live for many years after they stop being fertile – a trait that has led to a number of theories including the “grandmother hypothesis”, the idea that post-menopausal women survive to help their children to grow up and reproduce.

The researchers say that, in order to test the theory, they needed a way to look at possible evolutionary links between birth patterns and menopause, and changes in the way children are reared.

The study, they say, may do just that.

The team are now looking at the teeth of primates and human relatives such as Neanderthals to see if they show the same telltale clues. Cerrito said the findings could also be useful in other fields.

“The methods we developed can be used by archaeologists of the ancient world to piece together a more complete understanding of the lives of past civilisations by integrating written records of a person’s social and public life to biological data regarding intimate details such as fertility, menopause, or other physiological stressors,” she said.

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Writing in the journal Scientific Reports, Cerrito and colleagues report how they analysed 47 teeth from 15 modern, deceased individuals, looking at multiple teeth for nine of them.

It has long been known that the enamel of teeth forms layers over time, however this process stops once the teeth have finished forming. As a result Cerrito and colleagues looked at a different tissue, cementum, that grows throughout an individual’s life. This tissue covers and protects the tooth’s root, and helps connect teeth to the jaw.

Looking at the teeth using polarised light microscopy, the team found the interaction of the cementum with light was not the same throughout the tissue. Taking into account the age of the individual at death and the thickness of the cementum, the team found that these distinct changes seemed to line up with the timing of known events in the life of the individual in question, including giving birth and menopause.

“Sadly, in a number of individuals that had been imprisoned prior to death, we found evidence of that event – telling us just how physiologically stressful that must have been for them,” said Cerrito.

“In the case of a young 25 -year-old female, we discovered that the transition from a rural to an urban lifestyle was recorded in her teeth – but not a traumatic physical accident,” she added. “This tells us something about the ‘sensitivity’ of cementum as a recording structure: episodic events are not recorded, while prolonged systemic ones are.”

The approach has, at present, a number of limitations, including that the type of change in the cementum is the same for different life events, while even within the same individual, different teeth offer slightly different timings.

Prof Debbie Guatelli-Steinberg of Ohio State University, an expert in using teeth to explore our evolutionary past and who was not involved in the new research, said the study suggested it was possible to detect changes in cementum linked to pregnancy and menopause.

“That finding is of interest to anthropologists who wish to track the evolution of unique attributes of humans – such as short inter-birth intervals and long post-reproductive lifespans – that differentiate them from their great ape relatives,” she said, although she added more research was needed to differentiate between different events.

Prof Tanya Smith of Griffith University was cautious, noting that many fossil individuals are known only from their teeth, meaning it is difficult to know their sex, while it is not clear at what age our ancient relatives may have started to produce cementum. What’s more, at present, the approach relies on the age of the individual at death being known- which is unlikely for fossilised remains – although Cerrito said a technique in development allows the team to see annual rings in the cementum, allowing years to be counted.

“I don’t believe that this approach alone will resolve how frequently ancient women gave birth nor when they experienced menopause,” she said.