In the northern spring, 250,000 years ago, a Neanderthal baby was born in the southeast of modern-day France. At nine months of age, solid foods were added to the child’s breast milk diet, and in the autumn of its third year, after two harsh winters, it weaned from its mother’s milk entirely.

These are some of the extraordinary insights into Neanderthal life gleaned from an analysis of long-buried teeth and published in the journal Science Advances.

The study, by a team of researchers led by Tanya Smith, a biological anthropologist from Griffith University in Australia, provides a peek into the lives of Neanderthals and the environment they inhabited in unprecedented detail.

“It’s the stuff of science fiction,” says Smith.

Tooth enamel is laid down in layers, similar to growth rings in trees, so it forms a record of the climate and chemical exposures faced by its owner.

The analysis is the first time scientists have gained more than a broad brushstrokes impression of the climate at the time our prehistoric cousins lived.{%recommended 5348%}

“We’ve never been able to really relate this kind of developmental history to environmental variation before,” says Smith.

She and her colleagues initially thought they had three Neanderthal juvenile specimens from Payre in Southern France. Instead, it turned out that one of those teeth was a molar from a modern human who lived 5400 years ago, providing what Smith describes as “a perfect scientific control” to compare climatic conditions faced by the Neanderthals with those endured by pre-industrial modern humans living in the same location.

Smith’s team sliced fine sections from the molars, then drilled out tiny samples to measure minerals trapped in the enamel.

Rising and falling levels of the oxygen-18 isotope in the three teeth showed that the Neanderthals lived in a colder climate a quarter of a million years ago than the one that existed 5400 years ago. There was also greater variation between summer and winter temperatures.

Wintertime was particularly tough for the Neanderthals. Both children experienced periods of illness during winter months.

In one tooth, evidence of breastfeeding was also clear. The researchers looked at levels of the trace element barium, which, being chemically similar to calcium, leaches out of the mother’s skeleton as she produces calcium-rich breast milk.

At nine months, the barium level falls off, probably as the infant began relying more on solid foods. Weaning occurred at 2.5 years, similar to what’s seen in non-industrialised human populations, though additional samples are needed to confirm whether this was normal for all Neanderthals, says Smith.

No evidence of breastfeeding was seen in the second tooth, which formed between the ages of three and six years.

One surprise finding was that both Neanderthal children were exposed to lead as their teeth formed, not once, but twice. On each occasion, the brief but intense periods of exposure to the neurotoxic element coincided with cooler weather, wither in the winter or spring. The likely culprit was contaminated food or water or inhalation of lead-contaminated smoke.

“The lead was just a shocker,” says Smith. She had no reason to look for lead contamination in the Neanderthal teeth – “Nobody really looks for it,” she notes – but her public health colleagues routinely test for the element.

Smith hopes that further tooth samples will provide an even clearer picture of the environments and behaviours of Neanderthals, as well as of other early humans.

The research adds to the researcher’s already impressive body of knowledge regarding the relationship between teeth, anthropology and evolution. She is the author of a new book on the subject, recently published by MIT Press.