Note that the modern skull on the left has a protrusion at the bottom of the face, while the Neanderthal skull on the right does not have this feature. The reason for the difference: Only late Homo sapiens had chins.
Nathan from the University of Iowa? Nathan Holden is an expert in craniofacial features and mechanics. He said: "From a certain point of view, the chin is just an insignificant little thing; But interestingly, we humans are the only animals with chins, and no other animals have them. "
In order to explore the origin of chin, holden and his colleagues at the University of Iowa recently conducted a study. The results show that the chin is not shaped by mechanical forces such as chewing, but is produced with the change of facial size and shape in evolutionary adaptation. The evolution of human beings is more and more adapted to social life, and the hormone level in the body has also changed. The appearance of chin may be related to these changes.
Once this hypothesis is established, it may end a debate that has been going on intermittently for more than 100 years: why did late Homo sapiens have chins? How are their chins formed?
Some people (characters) also have strange chins. ...
Using advanced biomechanical methods, the research team of the University of Iowa analyzed the head and facial features of nearly 40 subjects, and their data covered all stages from infants to adults. The results show that the resistance caused by mechanical forces including chewing is not enough to produce new bone in the mandible. Writing in the online journal of anatomy, they pointed out that the reason why late Homo sapiens grew their chins seems to be simple geometry: from early Homo sapiens to today's human beings, our faces became smaller and smaller (our faces were about 65,438+05% smaller than those of Neanderthals), and in the process, the bones at the bottom of our faces popped out, which became our signature feature.
Therefore, human beings don't "have long chins", but stick out their chins because their faces become smaller.
Holden is currently an assistant professor of orthodontics at the School of Dentistry, University of Iowa. He is also an anthropologist. "In short, we haven't found any evidence about the mechanical function of the chin. Some subjects, even the chin's ability to withstand mechanical forces will weaken with age. " He said, "In short, the chin is unlikely to be created to disperse stress and tension, and it is more likely that other explanations hold."
Neither orangutans nor monkeys have protruding chins.
Also at the University of Iowa, Robert? A group of anthropologists, led by Robert Franciscus Francescus, put forward a more interesting explanation: they believe that the chin is a by-product of the change of human lifestyle. This transformation began about 80,000 years ago, and by about 20,000 years ago, the late Homo sapiens began to migrate from Africa, and their lifestyles changed greatly. Specifically, the ancestors of late Homo sapiens were ancient hunter-gatherer tribes. In the process of evolution, isolated small tribes have become large cooperative groups, forming a social network on the earth. With the closer relationship between groups, people are increasingly using art and other symbolic media to express their feelings.
In this process, the changes of men are particularly obvious. Their personalities have become calm, and they are no longer fighting for territory and property as before, but are more willing to form alliances, which can be proved by the exchange of goods and ideas. Communication facilitates communication between the two sides and benefits all mankind.
At the same time, this attitude change is accompanied by the decline of male hormone levels, especially testosterone levels, which in turn causes significant changes in their craniofacial areas. The biggest change is that the face has shrunk and the chin has naturally stood out.
Because testosterone drops, the face becomes smaller and the chin protrudes.
This theory was put forward by Francesca's team in the journal Contemporary Anthropology in August 20 14, and he was one of the authors of this paper. "We think the advantage of late Homo sapiens is that they have a social network extending in all directions, and they can easily exchange information and find spouses. This is a pioneering work. " He said, "But to maintain such a society, men must tolerate each other. Their curiosity and desire to explore will definitely overwhelm their desire to attack, which is also reflected in their facial structure. "
Current research supports this view, because it excludes the possibility of creating chin by mechanical actions such as chewing.
The researchers investigated the general response of mandible to vertical bending and compression of wishbone. In the compression of the wishbone, one side of the mandible is pulled outward, which makes the outer surface of the mandible compressed. When bending vertically, the ascending branches (that is, the backward and nearly vertical parts on both sides of the mandible) open outward, making the chin tight. According to the old theory, in these two cases, the chin is oppressed by mechanical force, which should lead to the appearance of new bones at the micro level, just as lifting heavy objects will cause tiny wounds and lead to the appearance of new muscles. Therefore, the old theory holds that mechanical forces such as chewing create our chin.
However, researchers at the University of Iowa came to a different conclusion. They looked at the head data of subjects from 3 to 20 years old and found that those tiny mechanical forces would lead to the growth of chin bones. In fact, they found the opposite: the object with the greatest mechanical resistance-the chin-is most similar to a child aged 3 or 4-that is, they have no chin at all.
However, they also noticed that the "growth" of chin is related to another change, that is, the adaptive change of our facial features with the increase of head size-it is like assembling a three-dimensional puzzle with pieces of parts.
For example, children's chins are flat and almost illegible, much like Neanderthals. Only when our heads and faces reach adulthood will the bones under our faces stand out.
When we were three or four years old, our (left) chin bones were very flat. It is not until early adulthood (right) that the chin slowly protrudes.
"Our research shows that chin protrusion has nothing to do with its function," holden said. "It is mostly related to the spatial dynamics in the development process."
Maybe that's what I said above.