Forensic anthropologists at the University of Tennessee in Knoxville have found that heads of Americans are getting larger.
The scientists examined 1,500 skulls dating back to the mid-1800s through the mid-1980s. They noticed US skulls have become larger, taller and narrower as seen from the front and faces have become significantly narrower and higher. These findings were presented at the annual meeting of the American Association of Physical Anthropologists.
The team cannot pinpoint a reason as to why American head shapes are changing and whether it is primarily due to evolution or lifestyle changes.
“The varieties of changes that have swept American life make determining an exact cause an endlessly complicated proposition,” said Dr Lee Jantz, coordinator of the University of Tennessee’s Forensic Anthropology Center (FAC) and lead author of the study. “It likely results from modified growth patterns because of better nutrition, lower infant and maternal mortality, less physical work, and a breakdown of former ethnic barriers to marriage. Which of these is paramount we do not know.”
The scientists found that the average height from the base to the top of the skull in men has increased by eight millimeters (0.3 inches). The skull size has grown by 200 cubic centimeters, a space equivalent to a tennis ball. In women, the corresponding increases are seven millimeters and 180 cubic centimeters.
Skull height has increased 6.8 percent since the late 1800s, while body height has increased 5.6 percent and femur length has only increased about 2 percent. Also, skull-height has continued to change whereas the overall heightening has recently slowed or stopped.
The team also noted changes that illustrate our population is maturing sooner. This is reflected in the earlier closing of a separation in the bone structure of the skull called the spheno-occipital synchondrosis, which in the past was thought to fuse at about age twenty.
America’s obesity epidemic is the latest development that could affect skeletal shape but its precise effects are unclear.
“This might affect skull shape by changing the hormonal environment, which in turn could affect timing of growth and maturation,” said Richard Jantz, professor emeritus and former director of the FAC. “We know it has an effect on the long bones by increasing muscle attachment areas, increasing arthritis at certain joints, especially the knee, and increasing the weight bearing capacity.”
The research only assessed Americans of European ancestry because they provided the largest sample sizes to work with. Prof Richard Jantz said changes in skeletal structure are taking place in many parts of the world, but tend to be less studied. “The research has uncovered shifts in skull shape in Europe though it is not as dramatic as seen in the United States,” he said.