How do we know an individual’s age at death?

Teeth and bones

Teeth

Teeth can tell us how old certain individuals were when they died – especially if they died young. Their ages can be determined because teeth appear in a certain sequence and at particular ages.

Much of the information about an individual’s age is gathered by looking at the types of teeth visible above the jaw surface (gum line). Additional information comes from examining the degree of root development deep within the jawbone.

Ages are now also being confirmed by microscopic examination of tooth enamel. When tooth enamel grows, it produces tiny growth lines in the enamel. These lines can be counted to give the tooth’s age.

Our first deciduous ‘milk teeth’ or ‘baby teeth’ begin to appear at around six months of age. These gradually fall out during childhood and are replaced by permanent or ‘adult teeth’. All our teeth appear in a certain sequence and each type of tooth appears at a particular age. The ages at which different teeth appear in humans is shown in brackets.

Baby teeth

• central incisor (6-9 months)
• lateral incisor (7-11 months)
• canine (16-20 months)
• first molar (10-16 months)
• second molar (20-26 months)

Permanent ‘adult’ teeth

• central incisor (6-8 years)
• lateral incisor (7-9 years)
• canine (9-12 years)
• first premolar (10-12 years)
• second premolar (11-13 years)
• first molar (6-7 years)
• second molar (11-13 years)
• third molar (17-25 years)

Bones

An individual’s approximate age can be determined from certain bones. Young individuals who are still growing have special growth plates in their bones but by about 20 years of age, the bones have stopped growing and the growth plates disappear.

Some bones begin to fuse together after the body stops growing. In humans, bones begin fusing from about the age of 16. For example, the sacrum is a structure at the base of the spine which is actually made up of five bones, called vertebrae, but these individual bones fuse together when we are between 16 and 23 years of age. The collarbone becomes fused at about 26 years of age.

By 23 years of age, the five vertebrae that make up our sacrum become fused together into a single unit.

Our skulls contain many bones that tightly join along lines called sutures. The sutures begin to fuse from about the age of 17 and some fuse more quickly than others. In very old age, all the sutures are completely hidden by bone tissue. The basilar suture on the base of the skull is particularly useful when aging an individual. It closes between 18 - 24 years of age.

Rates of development

To estimate an individual’s age, it is important to know the rate at which their body develops or grows. Modern humans, for example, are slow to develop from a baby to an adult. We take twice as long to reach maturity as our closest living relatives, the chimpanzees. This is reflected in how quickly our bodies grow and mature. For example, human adolescence begins at about 12 years of age but in chimps it occurs at about 6.5 years of age.

Why did we evolve a delayed development rate?

Delaying development is a risky strategy as young are reliant on a mother for food and will not survive if she dies early. In addition, the delay in reproducing means offspring risk dying before they are able to replace themselves by breeding. However there may be a number of advantageous factors involved including:

• delaying childbirth allows for older mothers that have had time to grow bigger, stronger bodies. This helps reproduce bigger babies that are more likely to survive but also mothers who can give birth more frequently, or even survive the stresses of childbirth
• an extended childhood allows more time for learning and socialising. Human brains are relatively small at birth but grow quickly, achieving 95% of adult size by five years of life (although white matter continues to grow up until about 18 years of age). This period of rapid growth coincides with a childhood dependent on others and in an environment where learning can occur

When did our ancestors evolve delayed developments?

Combining the evidence from teeth and bones with understandings about growth rates allows the ages of some of our early ancestors, particularly children, to be determined.

Australopithecines

Our early ancestors, particularly the australopithecines, developed quite quickly. Microscopic growth lines in their tooth enamel show that they developed at a rate similar to that of modern chimpanzees.

For instance, the Taung Child, a fossil belonging to the species Australopithecus africanus, was once believed to be about six years old when it died. This was based on the position of the child’s first permanent molar tooth, which was just beginning to appear in the jaw. Comparisons with modern human children suggested an age of six. In the 1980s, a technique called computerised tomography confirmed that the teeth were developing quickly, in a very apelike manner. This showed that the rate of growth was similar to that of chimpanzees rather than modern humans. This new information estimated the Taung Child’s age as three and a half years at the time of death. Enamel growth line counts confirmed this age.

Early humans

Early Homo species appeared to have developed at rates that were intermediate between those of modern apes and modern humans.

A 1.6 million-year-old Homo ergaster skeleton from Kenya, known as the Turkana boy, was assumed to be about 12 years old when he died because he was about 163 centimetres tall and weighed about 50 kilograms. Closer examination of the teeth and skeleton revealed he was probably about 8, meaning that although he had a reasonably long slow period of growth, he developed more quickly and attained more of his height and body weight earlier than modern humans do. A female pelvis from this species, discovered in Ethiopia and described in 2008, was shown to be wide enough to have birthed young that had brains as large as 320cc - about 30-50% the size of an adult H.ergaster. This suggests that this species developed a large proportion of the brain before birth, like modern humans, and less of it during childhood, which is more ape-like.

Studies on a later human species, Homo antecessor, that lived about 800,000 years ago in Europe, offer further clues. A study in 1999, based on rough estimates of tooth eruptions in three juvenile specimens, found that this species matured like modern humans. However, more detailed studies are in the pipeline.

There is considerable debate about the growth patterns one of our closest (in time) relatives, the Neanderthals. Some studies have concluded that their teeth developed at the same rate as modern humans do whereas others claimed they didn't. Confirming this either way would help determine whether our distinctive growth patterns evolved before or after the Homo sapiens line split from the Neanderthal line.

Modern humans

Our own species, Homo sapiens had fully extended chidlhoods by the time the first members of our species appeared over 200,000 years ago. In 2007, scientists studied the teeth of an 8 year old child found in Morocco, dated to 160,000 years old. The study revealed the child had grown as slowly as a modern 8 year old.