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Activity: Can You Identify the Age?

Background

Skeletons are good age markers because teeth and bones mature at fairly predictable rates.

How Teeth Reveal Age

Drawing of half the lower jaw showing the four types of teeth.
Figure 1. Human teeth.
(Source: Gray's Anatomy)
For toddler to age 21, teeth are the most accurate age indicators. Like many other mammals, humans have two sets of teeth - "baby" teeth and permanent teeth.

Baby teeth, also called milk or deciduous teeth, start erupting at about six months, beginning with the central incisors in the lower jaw (mandible). Each tooth type - incisors, canines, premolars, molars - erupts on a predictable schedule.

Permanent teeth begin to replace deciduous teeth at about six years of age and finish erupting by about 21 years of age.

X-rays provide views of the unerupted permanent teeth and tooth roots still within the bone. The crown of a tooth forms first, followed by the root. Scientists estimate age by comparing the stage of tooth formation in the X-rays and bone with known dental growth standards.

Below are three X-rays showing dental development:

Two x-rays of a child's jaw, side and frontal views. Some of the teeth have not yet erupted.

Figure 2. Dental development in the lower jaw of a four- to five-year old child.
(Source: Smithsonian Institution )

Left and right side x-ray views of an adolescent's jaw show that at least one toot at the back of each lower jaw has not yet erupted (the 'wisdom tooth').

Figure 3. Dental development in the lower jaw of an 11- to 13-year old adolescent.
(Source: Smithsonian Institution)

Left and right side x-ray views show that this adult has a full set of teeth.

Figure 4. Dental development in the lower jaw of an adult.
(Source: Smithsonian Institution)

Growth Standards for Permanent Teeth

Tooth Development in the Skeleton in the Cellar


Notes from the skeleton's forensic file:
The left 3rd maxillary molar has incomplete root formation but is partially erupted. The right 3rd mandibular molar has three-fourths of its root formed and remains unerupted. All other represented teeth are fully erupted with complete crown and root development.

X-rays of the upper and lower jaws of the skeleton in the cellar.

Figure 5. Upper (maxillae) and lower (mandible) jaws of the skeleton in the cellar.
(Source: Smithsonian Institution)

How Bones Reveal Age

Like teeth, bones mature at fairly standard rates. A baby's bones begin to grow in the womb. At birth, the skeleton is partially formed and many bones are still in parts. Throughout childhood, the bone parts grow and unite. The process is complete between 17 and 25 years, when normal growth stops. At this point, you and your skeleton are as tall as you are going to get - with many fewer bone parts than you started with!

Drawing of an adult and an adolescent femur. The ends of the adult's femur are clearly joined to the shaft. The epihyses in the adolescent femur still have areas at the edges that are not attached to the neighboring shaft.
Figure 6. Proximal (near to the torso) and distal (far) ends of adult and subadult (adolescent) femora (thigh bones). (Source: Smithsonian Institution, illustrated by Diana Marques)
Some of the separate bone parts in immature skeletons are called epiphyses, which are the ends of the bony shafts (diaphyses) of the arm and legs (the long bones). Epiphyses are growth centers, often evident on the ends of long bones. At birth, the ends of the long bones are mainly cartilage (a firm, elastic type of connective tissue), with centers of bone beginning to form inside. As a child grows, the shafts get longer, and bone gradually replaces the cartilage epiphyses. Through the growing years, a layer of cartilage called the growth plate, separates each epiphysis from the bone shaft.

In the teenage years, the epiphyses unite to the main bone shafts, as the cartilage growth plate is completely replaced by bone. This process occurs slightly earlier in females than in males. The age of a person can be estimated by looking at the stages of union for the epiphyses in the different bones and comparing it to the standard growth tables.

Age of Union for Epiphyses of Some of the Long Bones

Age Markers in the Skeleton in the Cellar



 

Notes from the skeleton's forensic file: The top (proximal) epiphysis of the humerus is unfused, as are the end (distal) epiphyses of the ulna (outer forearm bone) and radius.


 

The proximal humerus from the skeleton in the cellar has a distinct gap between the end of the bone (the epiphysis) and the shaft.
Figure 7. Proximal humerus of the skeleton in the cellar.
(Source: Smithsonian Institution)


 

Notes from the skeleton's forensic file: On the bones of the legs, the right lesser trochanter (node where muscle attaches to bone) has united, but the line of union is still clearly visible. The femoral heads have begun to unite, but the distal epiphyses of the femora are unfused. The epiphyses of the tibiae are united with the shafts, but only recently, as clear lines of union are still visible.


 

Two bones from the lower leg of the skeleton in the cellar are displayed.  The top of the femur is similar to the humerus in the previous image - the proximal epiphysis has not yet fused, although the epiphysis at the distal end is farther along in fustion. The tibia shows only a small indentation at the points of fusion.
Figure 8. The femur and tibia of the skeleton in the cellar.
(Source: Smithsonian Institution)

All of the bones recovered from the cellar are laid out as if the subject is on his back with his arms at his side and his palms up.

Figure 9. Skeleton in the cellar arranged in anatomical position. (Source: Smithsonian Institution)