Horses can walk shortly after birth, kittens of a few weeks old can be left alone while their mothers search for food, while babies need care for many years.
The fact that birth occurs after 9 months and that the newborn human brain is relatively less developed compared to other primate species has been attributed to some evolutionary processes so far. However, a study presented results that suggest the opposite. Do you think the years-old hypothesis will be refuted overnight?
Our evolution process from four legs to two legs
The skeletons of our ancestors evolved over millions of years from a creature that walked on four legs and had a smaller head. Getting into an upright bipedal position also means that the body supports a wide head. In fact, this is one of the disadvantages of walking on two legs. Other negative effects may include back and neck pain as the viewing angle expands.
Women experienced the biggest disadvantage of the skeleton supporting a large head. Upright posture meant narrower hips. This was an important physical feature that narrowed the birth canal. Women who gave birth prematurely, when their skull and brain were smaller, survived. This actually created natural selection. People whose vital systems were not yet fully developed became people born prematurely.
If there was no evolution and humans were born neurologically like chimpanzees, pregnancy would be at least twice as long. This would be a process that would extend the pregnancy period to 18 to 21 months.
Babies’ brains are developed enough to enter the birth canal.
The larger size of women’s pelvises could prevent them from walking. According to the prevailing belief, babies’ brains only grow large enough to fit into the birth canal. That’s why birth occurs in 9 months, not 18 months.
When babies are born, their brains are less than 30% of an adult brain. This way, they can pass through the narrow channel. This situation, which is one of the reasons for needing care, begins to return to normal after leaving the womb. Brain development continues after birth, and at the end of one year it doubles in size on average.
A study may challenge this process that has come to this day.
Holly M. Dunsworth from the University of Rhode Island and her colleagues tested the “obstetric dilemma” hypothesis and brought a completely different perspective to the event. The hypothesis is that the female pelvis is wider than the male, requiring more energy in activities such as walking and running compared to men. In their research article, they stated that a wider pelvis did not cause such a situation.
Dunsworth also noted that for the human brain to reach chimpanzee levels (40% of the human brain’s adult size), the pelvic size would need to expand by only 3 cm. This is actually considered within the normal pelvic range of today’s women. As mentioned, it also refutes the hypothesis that women should spend more energy.
Babies are born when their mothers reach their metabolic capacity.
Dunsworth and his colleagues also answered why babies are born at nine months from another angle. Dunsworth said that women have metabolic capacity and when this capacity is full, babies are born.
Women spend twice as much energy to maintain their normal processes in the first six months of pregnancy, and this energy increases as the baby grows. The fact that the maximum metabolic rate is between 2 and 2.5 times means that women will not be able to convert energy after nine months.
Although this research, which claims the opposite of the obstetric dilemma that has been around for many years, cannot eliminate the entire hypothesis, researchers still think that these arguments will be valid after a while.
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