Babies and Birthing

Human babies enter the birth canal from the womb in the same way a chimp does but just before the actual birth the skull rotates 90 degrees in order to exit the rounded birth canal that humans have evolved. In Homo Sapiens, evolution reached a compromise that favored even bigger brains at a further cost to birthing and efficient walking. The Homo Erectus pelvis was very narrow. Humans are unique among mammals in the extent to which the brain keeps growing well after birth. The scientific terms for this is secondary altriciality. It involves accelerating the birthing process and arresting the development until after birth. Monkeys and apes are born with brains half as heavy as they will ever be. A chimpanzee brain, for example, will weigh perhaps 7 ounces at birth and about 14 ounces as an adult. Human brains are about a third of their final size in newborns; they more than double in size in the first year after birth. On average, human babies are born with a brain that weighs 14 ounces but reaches 35 ounces in one year. It will continue to grow until it reaches about 45 ounces in size (at age 6 or 7). Gestation in humans should be about 21 months rather than the normal 9 we think in terms of. This is the process of accelerating the birthing process to enable the enlarged brain to "escape" the birth canal. Development of the brain then continues external to the womb for well over the first several years. What this intense development means is that a human infant is born relatively helpless. A baby can neither stand up or in any way fend for itself for a long time. Stephen Jay Gould has written "our sexual maturation comes almost absurdly late in a Darwinian world supposedly regulated by a constant struggle to secure reproductive success and pass more genes along to future generations....slower development must provide some power advantage to evolve, in the face of its obvious drawbacks." In fact, must of what makes us human in the end may stem from this unnaturally long period of helplessness in the very early part of our lives.

Research on a nearly complete skeleton of a young Homo Erectus boy who lived nearly 1.6 million years ago showed that his vertebral canal for his spinal cord is only half as wide as it is in modern humans. This is more apish than human. A spinal cord contains two types of tissue: nerve cell fibers or white matter, and nerve cell bodies or gray matter. The spinal cord serves as a messenger for nerve signals traveling between the brain and the rest of the body. If you think of the cord as a telegraph serve, the fibers are the telegraph lines and the cell bodies the relay stations. Researchers suspect that the Homo Erectus boy may have had less gray matter - cell bodies. These are associated with nerve functions close to them in the body. This is interesting in terms of the fact that it was in the thoracic region of the spinal column that the difference in size was maximized. This would mean that Homo Erectus had less muscle control in the vicinity of the ribs. There are two possible reasons why greater muscle control evolved after this time. The first has to do with greater uprightness of the body trunk as long distance travel became more important. The second, is perhaps more crucial, would relate to finer control of breathing as the use of language emerged. It may have been the case that Homo Erectus. lacked the nerves to control the amount of air passing from the lungs up the respiratory track to the mouth.

If, as evidence shows, language capacities really were a late occurrence in human evolution, then it represents only one by-product of a big brain and could not by itself a driving force behind expanding brain size. This is particular true at the time of emergence of Homo Erectus between 2.0 and 1.5 million years ago when brain size expanded initially from 500 cc to over 800 cc. This is the point that one must consider in assessing why larger brains were become an important theme in human evolution at this time and after.

Keep in mind that hominids show signs of being faster moving and more social, developing better ways to process food (meat), and maintaining lower population densities or increasing geographic range as brain size increased. Later Homo Erectus appears to have been faster and better prepared at getting to meat than preceding forms; they would have been expert in processing the meat thanks to better tools (hand axes) and cleverer in finding new food sources using a wide-ranging travel. Keep in mind that accompanying larger brain size was movement out of Africa. This may have occurred as early as 1.8 to 1.6 million years ago or as late as around 900,000 years ago. Yet larger brains did mean greater travel. We do know this much.