Does the human's "secret weapon" also have a "bug"? Monkeys, please don't blindly worship "walking upright"!

Primates, including humans, apes, monkeys, etc., obviously have well-developed brains and flexible limbs. Humans are the only primates that can walk upright. The advantages of walking upright are not mentioned, but what are the disadvantages of walking upright? Does this disadvantage have a big impact on humans? Let's talk about it today.

01 What are the disadvantages of walking upright?

That’s right, some netizens mentioned the problem of difficult labor caused by upright walking, which was further caused by caesarean section, leading to more difficult labor.

It cannot be denied that walking upright is a huge leap in human evolution, which not only allows us to have a better field of vision to improve our survival rate, but also frees our hands to engage in more complex activities. However, walking upright also brings a problem, that is, the weight of the whole body is pressed on the pelvis, which limits the opening and closing of our pelvis.

The birth of a baby needs to pass through the pelvis, thus forming a restraint mechanism of the pelvis on the baby's head.

Simply put: the pelvis limits the overall size of the fetus, including constraints on the fetal head.

02 Why are humans born prematurely?

Although in humans, the female pelvis is wider and shallower than the male pelvis, with a greater angle at the pubic symphysis, this structural feature helps the fetal head pass smoothly.

But it is still limited overall, so the size of a human baby's head is generally limited. There is no way around it. If the head is too big, it will easily lead to dystocia. To this day, head presentation is still an important cause of dystocia.

This also results in the fetus's brain not being fully developed, and it will undergo a period of rapid growth after birth.

So people often say that humans are premature babies, which does make sense.

03 The birth of cesarean section

Of course, some people say that the brain capacity is not important, but the sulci and even the brain connections established later are more important. This also ignores the fact that brain capacity itself is a basic existence. If the brain capacity is too small, the cerebral cortex, the foundation for the development of sulci and brain connections, will undoubtedly be affected.

In this way, humans have been undergoing long-term selection, and the size of the head circumference has basically maintained a relatively constant range. If the head is too large, it will be difficult to give birth, and this is a strong natural selection. However, in the 20th century, a technology was developed, that is: Caesarean section.

After all, saving lives is more important, and the abdominal cavity is much larger than the pelvis.

Therefore, after entering the 20th century, a large number of caesarean sections began to be performed, which also saved many lives.

04 A new problem has emerged: the rate of fetal pelvic disproportion is increasing

But what's interesting is that this has led to a phenomenon, that is, after a baby with a big head is born, according to the relationship between genes and traits, the frequency of the gene behind the trait of large head circumference will increase accordingly, so their offspring will have a certain probability of having a large head circumference, which will further lead to difficult labor and necessitate caesarean section. This phenomenon can be called the fetopelvic disproportion rate. In other words, caesarean section will lead to an increase in the fetopelvic disproportion rate.

Scientists have crunched birth data from the past 50 years and found that the global rate of fetal pelvic disproportion has increased from 3 percent of newborns in the 1960s to 3.3 percent of newborns today.

the regular use of Caesarean sections throughout the last decades has led to an evolutionary increase of fetopelvic disproportion rates by 10 to 20%.

In other words, with the advent of caesarean sections, babies with large heads can be born, but as they grow up, the birth canal does not change significantly between generations. As a result, the proportion of babies with large heads born by the next generation of mothers increases.

What the author did is a mathematical model.

Model of fertility selection, ratio of infant size to maternal pelvic birth canal size

The higher the maternal and infant discomfort, the worse the population adaptation rate (nonsense)

Simple conclusion: The authors calculated through two models and found that the head-pelvis ratio imbalance between mothers and babies due to caesarean section increased by 20% and 9% respectively.

In other words, due to the widespread use of caesarean sections, babies' heads are getting bigger and bigger. If it weren't for the blessing of modern technology, it would have been a strong evolutionary selection in the past, but with modern technology, it has been solved. However, it also means that from the perspective of humans as natural creatures, it is more difficult to get rid of other external factors such as the blessing of technological caesarean sections, which makes it increasingly difficult for humans to give birth naturally.

This is better when technology is developing. If technology cannot meet the needs, it will be difficult to solve, just like the situation in the movie 3 Idiots. But this trend is unstoppable.

In fact, when humans entered the era of tool manufacturing, they had to a certain extent freed themselves from the natural constraints of primitive creatures and began to move towards an era of tool dependence. This is actually, to some extent, comparable to a small-scale "creatures landing from the water" change.

Mitteroecker P, Huttegger SM, Fischer B, et al. Cliff-edge model of obstetric selection in humans[J]. Proceedings of the National Academy of Sciences, 2016: 201612410

Note: The cover image is from Xinhua News Agency