Bipedalism & Human Birth: The Obstetric Dilemma Over hundreds of thousands of years, the human body has evolved in many different ways to help us adapt to our ever changing environment. The obstetric dilemma refers to changes in the female pelvic region as a result of our evolution towards bipedalism, and the resulting difficulties in childbirth. When assessing gestation periods, premature births and maternal & infant health, the question comes up as to why the human body has evolved in such a way as to make childbirth potentially fatal for both mother and child?
The advantages of bipedalism can be seen when we throw a ball, carry the shopping bags from the car or collect the mail from the letterbox. But are the advantages we gain worth risking the lives of both mother and infant during childbirth? Have we found the balance between possessing bodily adaptations which are essential for bipedalism, and the advantages of having babies that are better developed and are born with larger, more complex brains?
Bipedalism is a form of terrestrial locomotion, where humans and a few other bipedal creatures can move around using their two hind limbs. The earliest bipedal adaptation is thought to have begun with Australopithecines, dating back between 3 and 4. 2 million years ago, with evidence of upright locomotion found in fossil form. A number of selective pressures started to arise with the evolution of the human species, and as a result a number of changes had to be made throughout the body, especially females, to accommodate for these pressures.
Natural selection played a role in the development of bipedalism, as our ancestors were moving through environments that required them to have maximum foraging distance and ground coverage with minimal energy expenditure. This in turn meant that individuals possessing the right characteristics for upright striding had a greater chance of survival as their environments grew less and less accommodating. Among other reasons, such as freeing our upper limbs and assisting thermoregulation, bipedalism allowed us to become more energy efficient.
It provided a way for us to get the most out of our sparse, patchy & unpredictable environments while conserving as much energy as possible while still maintaining our foraging efficiency. The energy we conserved in making these changes went towards the development of larger brains, and has been essential in our evolutionary process. For our bodies to adapt to this radical new method of locomotion, a number of structural changes had to be made throughout the body. Organs & bones shifted to allow these adaptations to be made.
The broadening ilial and the narrowing ischial pelvic bones resulted in the human pelvic structure becoming smaller, narrower, and with a reduced birth canal. Differing largely from the male pelvis, which has a much broader pelvic bowl, the female pelvis is much rounder with less bony structures protruding into the cavity. Babies today are born after approximately 9 months in gestation. In 1994, the average gestation period was 39. 2 weeks, which dropped to 38. 8 weeks in 2004 (Australia’s Babies. ABS 2010).
Compared with chimpanzees and gorillas, this is a relatively short incubation period. Even though a longer period is desirable for prenatal development, there are many risks involved in child birth today and therefore there are a number of reasons why our species cannot afford the extra time in incubation, nor can we afford for the incubation period to be anything less. Chimpanzees’ offspring are born with 45-50% brain development, while human infants are born with just 25% of their brains fully developed (Discovery Media 2010).
Foetal brain development is highly energy costly, and so a prolonged incubation would be detrimental to the health of the mother. This essentially means that a human child will spend close to a year helpless and completely dependent on its mother. There are benefits to this prolonged period of nurture and care for human infants after birth. Although they take up a lot of time and effort, in the interest of successful reproduction, the father is more likely to invest more of his time with the infant, providing necessary food and care while the young are growing up.
This in turn creates an extended transition period from childhood to adults, meaning that a young child experiencing the world under careful guidance and support has a greater chance of survival. Another evolutionary adaptation is menopause. This is a natural response to aging in women, a process in which the menstrual cycle ceases, preventing women past childbearing age from having any more children. This adaption was made in the interest of reproductive success, because once the mother reaches this point she will now invest her time into her existing children and/or grandchildren.
So even though more effort is required from both parents early in life and throughout childhood, it is greatly beneficial in the end as it boosts our chances of reproductive success. Reference Darwin, C 1859, The Origin of Species, London Relethford, J 2010 The Human Species: An Introduction to Biological Anthropology, 8th edition. McGraw Hill, New York. Meyer, J 2010. Lecture notes from ANHB1102; Anatomy of Human Locomotion Obstetrical Dilemma. Available from http://www. servinghistory. com/topics/obstetrical_dilemma Australian Bureau of Statistics. Australia’s Babies is available from http://www. abs. gov. au/AUSSTATS/