The brain undergoes a complicated pattern of development in the embryo. This development is similar in birds, mammals and humans.

An embryo has three basic layers that begin as simple layers of cells programmed by genes to become different parts of the body. The outer layer, the ectoderm, becomes skin, brain and nervous system. The middle layer, the mesoderm, becomes muscle and bone. The inner layer, the endoderm, becomes the gastrointestinal tract and abdominal organs. The retina, the nerve-cell layer of the eye, develops as a part of the brain and later is connected to the tissues which make up the eye. Thus, the eye is a part of the brain that looks out on the world.

There is, however, an important difference between human brains and the brains of all other members of the animal kingdom. No, I am not referring to size. Porpoises and whales have us beat on that point. The difference is that our brains are more immature at birth than any of the others.

The neurons in the human brain are dividing actively at birth and continue to do so until about age 2. By that time, we have all the neurons we will ever have. In fact, we will have fewer neurons as adults because of a pruning process, called apoptosis, which reorganizes our brains under hormonal control at puberty, so that our thoughts will have structure and reasoning ability. Puberty is the age where we begin to ask important questions and begin to doubt that the world is the perfect place we thought it was as a child.

To emphasize this point, the brain of a chimpanzee is as developed at birth as humans are at 2 years of age, and they are ready to start fending for themselves within a few weeks. Obviously, parental care is not necessary after 1 to 2 years. The question: why?

The answer is not complete and awaits ongoing research, but is appears that the cause is the development of upright posture, which resulted in a closed pelvis, thus limiting head size at birth at the same time that the brain was growing larger. As anyone who has dealt with birth problems, animal or human, knows, a large head and small pelvis always is a problem.

We are severely handicapped by a lack of monkey and chimpanzee fossils. Most of them lived and died in forests that were too wet to preserve fossils, and there was no mechanism to bury the fossils, so we don’t have much with which to work.

We do know that about 8 million years ago, when we estimate that chimpanzees and humans separated from their common progenitor, the forests of eastern Africa became drier. What was thick jungle became a grassland savannah with increasing numbers of herbivores to eat the grass and the leaves of the trees but allowing much less fruit to develop, which had been the predominate food for monkeys and apes until that time. Fruit still is the predominate food of monkeys and chimpanzees, both forest dwellers, but those who lived on the plains became more omnivorous, adding much more meat to the diet, similar to the diet of baboons.

Why do we think that humans once subsisted mostly on fruit? Because we cannot synthesize vitamin C in our intestinal tract as most animals do, but rather resemble monkeys and anthropoid apes in receiving our vitamin C entirely from diet. Thus, we hypothesize that the inability to synthesize vitamin C is an indicator of our past.

Further research to confirm this idea would be helpful. Since fruit is an abundant source of vitamin C, (ascorbic acid), those whose diet consists primarily of that food source do not need to make their own.

One of the rules of evolution is that if a trait is not needed, it will disappear. A comparative example is the loss of vision in fish and crayfish that live in caves. Thus fruit-eaters lose their ability to synthesize their own vitamin C.

The reason for dwelling on this point is be sure to establish what we know so that the reasoning can be easily followed. It is not known why humans achieved the vertical position as their primary mode of locomotion, nor is it known for sure when it developed, but our current best estimate is not before 8 million years ago.

There are many unproven theories to guide future research. We have footprints in hardened volcanic lava made about 3.5 million years ago in Africa that show a normal walking gait with a foot resembling the human foot in a creature about 3 to 4 feet tall walking in what appears to be an upright gait. That fact is the best that we can do at present.

We also have the associated finding of several skulls over the next 3 million years, demonstrating that heads are growing larger; culminating in skulls as large as ours since about 200,000 to 150,000 years ago. Associated with all this is the problem of the closing pelvis. We will start there next time.