Monday, September 12, 2005

Quadrupeds have no birth-pangs

Dr. K.Rohiniprasad

One characteristic that sets humans apart from their evolutionary ancestors is their erect, two-legged locomotion. The other is the process of human pregnancy. The human female is physically incapable of assisting herself or the baby during birth. Assisted labour resulted in surviving and healthier offspring compared to the ancient mammalian pattern of delivering alone. Natural selection probably transformed birth from an individual to a social enterprise. The role and importance of a midwife is perhaps almost as old as bipedal walk. These women were an invaluable and established part of human society.

As humans evolved from quadrupeds to bipeds, the process of giving birth evolved from a private to a social process. This could have been a link between the biological and sociological evolution. Bipedal walk gradually made the pelvis and the birth canal of human females much narrower than that of other primates. Therefore childbirth became physically more painful.

The growth of brain and cranial size among hominids added to the difficulty of labour and delivery. The human brain triples in size between birth and adulthood, whereas the brain of other primates only doubles. The human brain keeps growing at the embryonic rate for one year after birth. It can be said that human babies are born 'early' before the head grows too large and results in the death of the mother. Compared to, say, chimpanzees, young humans are born in an earlier stage of development relative to their adult selves although the length of pregnancy is nearly the same.

Two-legged walking brought about revolutionary changes in the evolution of hands. Freed from occasional help in walking and running, hands and fingers gradually developed specialized functions like tool making. Apart from helping early humans to survive against hunger and physical danger, the process of hand manipulation resulted in unprecedented expansion in certain areas of the cortex. As a result children were born with much larger heads.

As social evolution progressed, man began to control food sources through farming. Human societies became less mobile and new types of staple food became part of their diet. Increased consumption of food rich in carbohydrate, decreased mobility, and nursing at infrequent intervals enabled women to conceive within 10-15 months of the last birth. Food in the form of cereal grains and milk from domesticated animals helped to wean infants from breast-feeding earlier than ever before. This ultimately reduced the birth interval to about 2 years resulting in population growth and more social evolution. It is said that the birth and care of the young accelerated community living and social organization among early human settlements.

As the success of human birth and the ability to conceive more frequently in a lifetime became greater, the obstacles to two-legged walk were surmounted more easily. Increased birth rates meant increased variation, providing a larger pool of genetic traits to be selected for or against. Early humans used their intelligence to compensate for deficiencies in speed and agility. Birth evolved from a private to a social process in order to increase the rates of survival for both mother and child. With time, this socialization led to the development of various techniques and technologies capable of compensating for the physical limitations on birth in bipeds.

With changing times, the growth in the size of the human brain will be inevitable and the process of birth is bound to get more difficult. The development of more effective medicines for pain, use of tools such as forceps and other methods during delivery tend to reduce this trauma. It is therefore not surprising that caesarean section has become a viable option and is more common today.


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