We know humans live relatively long
lives and we certainly know we spend
a larger proportion of those lives as
children than other species.
The question is: How did we manage to extend this critical period of our growth? When and where did our ancestors start to stretch out limits of physiology and build that long childhood? Where can we find evidence of this evolutionary process?
The very surprising answer is in the mouths of babies — specifically, their teeth. But to understand how the timing of teeth tells us the story of our wellbeing, we need to first put teeth in context as important milestones on the path to growth.
Different species grow at different rates. How fast you grow is determined by a complicated set of interlocking mechanisms that factor in everything from the mass of the animal to the stability of their environment and has led to development of a branch of evolutionary biological theory that attempts to disentangle factors that propel a species from one developmental milestone to the next ‘life history.’
Understanding species’ life history has major implications for biology. Comparing the rate of growth between two species, for instance, gives us insight into different evolutionary strategies.
For Homo sapiens, who have some of the slowest growth on the planet, looking at life history becomes a critical way to address why our species has moved our milestones so far from those of our nearest relatives.
Teeth are one of the foremost tools in understanding how animals grow because they arrive in the mouth and erupt at a very predictable time.
This regular schedule reflects the critical importance of having the right teeth at the right time. Animals need different sizes and numbers of teeth at different ages.
If you think about trying to fit an entire adult set of teeth into the mouth of a baby, you will understand why humans come with two sets.
Of course, having multiple sets is not the only option — some animals, such as sharks have endless sets. Some, such as hamsters, have teeth that grow continuously.
But for primates like us, there are two sets to worry about: Milk or baby teeth and adult teeth – the permanent set. The schedule of when teeth emerge gives us a clear signal of which teeth are needed and when.
From about five months in the uterus, human teeth start to develop. We are born with some of our baby teeth already partially formed, but still inside our jaws.
The process of teething, which parents, in particular, are acutely aware of, is actually a long and drawn-out period over the first few years of life as the baby teeth erupt out of the jaw and into the mouth.
First, the incisors in the front, which have the job of nibbling and biting, erupt around four and a half months; then the lateral incisors to the sides of them around seven and a half months.
The first big bumpy chewing teeth, molars, around 10 and a half months followed by the ripping and tearing canine teeth until the last big baby teeth.
The chunky second molars emerge at about one and a half and two and a half years old. That’s it for teeth until about five years old, when the very first permanent tooth comes.
Molars have been seen as key to explaining the timing of our life histories. Evolutionary biologists looking to explain patterns of growth and development in primates have observed that the timing of the eruption of first permanent molar is linked very well with the end of dependence on mother’s milk or the end of infancy.
The eruption of the second permanent molar has been linked with a stage of childhood usually referred to as the juvenile period and the ability to forage independently in primates like chimpanzees.
Finally, the eruption of the third of our big chewing teeth — the wisdom tooth — has been associated with reaching adulthood, the end of growth, and possibly the start of reproduction.
Careful reconstruction of fossil teeth has shown earlier probable ancestors such as Australopithecus africanus and Homo erectus, erupted their teeth into their mouths much faster than we do today.
The timing of the eruption of molars particularly probably was very similar to that of our common ancestor with today’s chimpanzees and modern-day chimpanzees erupt their first permanent molar at two to four years old.
Second permanent molar erupts at around six to eight years and finish the last molar at 12 years, a few years before they are ready to behave as full adults. Humans, by contrast, have molars that appear around age five, 10 to 11 and about 18.
However, even though we have many similarities with our nearest primate relatives, we have somehow become untethered from the biological milestones that signal different life history events.
The eruption of our teeth is not timed quite right for when we wean and move our babies onto solid foods. Even in societies where pressure to end breastfeeding early does not exist, humans simply do not spend as long as infants on the breast as a primate our size should.
A large-scale study of forager children around the world found that by 10 years when second permanent molar is not quite erupted, children were still only half as competent at getting food as they would be at 20.
Meanwhile, age at eruption at last permanent molar is highly variable and not as clearly linked to reproductive age, with factors such as adolescence confusing the picture.
This suggests that perhaps mile stones such as the timing of teeth are not all that matters in calculating how we got our unique human life histories.
Perhaps our drive to grow long and slow means we have untethered our teeth from the behavioral milestones that our closest relatives still display or perhaps we have just drawn out the time between these milestones in such a way that it is no longer clear how they fit in the primate pattern.
This does not mean the timing of our teeth does not matter. The factors that push our species out to the extreme ends of life-history schedules are not, of course, ours alone.
Our costly investment in big brains has long been theorized to be behind our extensive lifespans, but the same links between long lives and big brains have been seen in many other mammals.
If you map the size of the brain against the timing of teeth, you get a very neat line right across all of the primates and we fit that line perfectly.
Our first molars emerge at exactly the right time for a primate trying to build an enormous brain the size of ours. If you consider the function of teeth, it makes sense that the emergence of molars would coincide with important points for growth in our species — points at which we need to be able to take in and process more calories, using our molar teeth.
New research shows learning complex skills such as the foraging skills human beings need to exploit their ecological niche may also be an important part of what humans do with their long childhoods — getting the right mix of nutrients for an energy-burning brain is a complicated business, requiring group communication, the invention of tools and complex mental processes.
The writer is a biological anthropologist and archaeologist at the University of Central Lancashire
