White Americans’ heads and faces have been changing in shape on average, and no one knows quite why, according to new research.
In a trend that can be identified going back to the mid-1800s, U.S. skulls have gotten bigger, taller and narrower as seen from the front, said Richard and Lee Jantz, a husband-and-wife team of forensic anthropologists at the University of Tennessee, Knoxville. They also found that faces have become significantly narrower and higher, though this shift is less pronounced than those affecting the whole cranium.
The changes continue into the generation born in the 1980s, from which come the latest skulls available for the research, according to the Jantzes, who presented their findings April 14 at the annual meeting in Portland, Ore. of American Association of Physical Anthropologists.
“I don’t have any reason to believe” the changes have stopped, said Richard Jantz in an interview.
He cited dramatic increases in the availability of nutrition, better medical care and lower infant mortality as possible factors behind the changes, but expressed pessimism that a definitive reason can be identified. The sheer number of changes that have swept American life make that an “endlessly complicated” proposition, he said.
“We are living in an environment that’s totally different from what’s ever existed in the past. It’s like putting experimental animals in an extreme environment.”
A larger head could allow for greater intelligence, but it’s unclear whether the increases are related to improvements in intelligence scores, Jantz said. Some aspects of the shifts in skull shape aren’t necessarily healthy. Earlier puberty, which has led to reports of girls getting pregnant before their teens, may be reflected in the earlier closing in youth of a separation in the bone structure of the skull called the spheno-occipital synchondrosis, he observed. America’s obesity epidemic is the latest development that could affect skeletal shape, Jantz said, but its precise effects are as yet unclear.
Although the changes in skull structure may be likely to go on, “they don’t necessarily have to continue in the same direction,” he added.
The research only assessed Americans of European ancestry because these provided the largest sample sizes to work with, said Jantz. Over 1,500 skulls were included in the research, many of them coming from the donated collection at the University of Tennessee.
The average height from the base to the top of the skull in males has increased by 8 millimeters (0.3 inches), the Jantzes found; skull size has grown by 200 cubic millimeters, a space equivalent to a couple of small peas. In females, the corresponding increases are 7 millimeters and 180 cubic millimeters.
Changes in skeletal structure are taking place in many parts of the globe, not just the United States, Jantz said. But they tend to be less well studied elsewhere, with the exception of a well-documented increase in human height across the industrialized world in recent centuries. “From what we know, in Europe there are some” shifts in skull shape, Jantz said, but “not as dramatic as seen in the U.S.”
Jantz tends to focus on lifestyle as a principle reason for the changes, not human evolution, although he said he doesn’t rule out the latter. The trend in skull shape “tracks calories available pretty strongly” in the industrialized world, he noted.
The observed growth in skull height is to some extent part of an overall documented increase in whole-body height. But Jantz has found that the skull-height increases are considerably out of proportion to those elsewhere the body, and also have continued whereas the overall heightening has slowed or stopped in recent years.