While closely related, Neanderthals and modern humans split from our last common ancestor between 650,000 and 500,000 years ago and evolved into distinct species. Among other contrasts, Neanderthal fossils show that our faces developed strikingly different shapes: modern human faces are generally smaller and have more delicate features. The reason for this difference has stumped paleoanthropologists for years, but new research has identified a tantalizing clue.
In a study published Monday in the Journal of Human Evolution, a research team shows that modern human faces reach their final adult size much earlier than Neanderthal faces, and that Neanderthals had more bone formation around their cheekbones and noses. While this doesn’t resolve the matter of why our faces were shaped differently, it does answer the question of how, and highlights a unique process that shaped human faces into what they are today.
“Our findings reveal that a change in development—particularly during late growth stages—led to smaller faces,” Alexandra Schuh, a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology’s Department of Human Origins, said in a Max Planck statement. “Compared to Neanderthals and chimpanzees who continue growing longer, human facial growth stops earlier, around adolescence, resulting in a smaller adult face.” Schuh is the study’s first author.
To investigate this, Schuh and her colleagues tracked the changes in face shape and bone cell activity throughout the lifetime of modern humans, Neanderthals, and chimpanzees by comparing the skulls of 128 humans, 13 Neanderthals, and 33 chimpanzees. The comparison revealed that “the midfaces [the region between our eyes and lips] of Neanderthals are on average already larger at birth than those of modern humans and continue growing for a longer period, contributing to their distinctive facial projection,” Schuh explained. Neanderthal faces also experience a dramatic expansion during childhood and adolescence.
In contrast, modern human faces usually reach their final size around adolescence, giving us a smaller and more gracile midface. While chimpanzee faces had different growth patterns compared to both modern humans and Neanderthals, they were most similar to Neanderthals.
“Earlier growth cessation is a distinctive feature of our species,” Schuh told Live Science. “We have identified a unique developmental pattern seen exclusively in Homo sapiens.”
“Microscopically, this is reflected in reduced amounts of bone resorption [in modern humans], indicative of decreased cellular activities linked to bone development,” the researchers wrote in the study. Our bones continuously change throughout our life in a process called bone remodeling, which breaks down old bone tissue (bone resorption) and replaces it with new tissue. “Greater amounts of bone formation in the infraorbital and nasal regions and faster growth rates are responsible for the large Neanderthal midface,” they added.
The researchers noted that chimpanzees, on the other hand, had different bone growth compared to both Neanderthals and humans, especially in their protruding canine region.
The new study sheds light on how humans uniquely developed smaller and more gracile faces, but the question of why is still up for debate. Some scientists have suggested a change in diet or a link to facilitating speech, while others have theorized a new social preference for less aggressive features—an idea called the human self-domestication hypothesis.
“Facial gracilization in modern humans may be linked to behavioral changes, such as increased social cooperation and reduced aggression,” Sarah Freidline, a biological anthropologist at the University of Central Florida and co-author of the study, told Anthropology.net.
Ultimately, the researchers have revealed yet another trait that makes modern humans unlike any other creature on Earth—even our closest relatives.
