Cognitive Science: An Introduction/Brain Development

Sexual Reproduction
Animals that reproduce sexually, like human beings, generate offspring by combining the genetic material from both parents. The mother's DNA is in her eggs, and the male's DNA is encoded in the sperm. When a sperm fertilizes an egg, these genes combine to form the genetic blueprint of the new individual. The vast majority of things we think of as inherited traits of a person are not the result of single genes that code for one protein. Rather, they are the result of many, many genes. Some of these genes come from the mother, some from the father, so the genetic mix an individual gets, and the trait that manifests, is unpredictable in its specifics.

Humans have 23 chromosomes, which contain the 20,000 or so informational genes that describe human beings. DNA encodes how to make proteins and active RNA, when to make them, where to make them, and how much of them to make. See this sidebar for what is meant by informational gene.

What a Protein is: You probably know the term protein in the context of nutrition. In biology, though, proteins are the drivers of the vast majority of subcellular activity. A stretch of physical DNA encodes the base pairs of a protein, which starts as a long string of amino acids, which folds into a complex shape. This shape determines its function. Different mechanisms detect proteins to know when to turn on and off, and how to behave in general. The biological world runs on proteins. See this video: protein folding for a visualization of a protein being created and folding into its final shape.

But not all genetic effects are inherited. An individual egg or sperm might have a mutation or a copying error, which gets passed on the offspring, resulting in a mutation given to the child that neither parent has in their other cells. Each zygote has an average of 70 of these new mutations, but because only about 3% of our genes are used, most of these have no effect. These might contribute to a trait in the child that neither parent has. Down syndrome is a good example of this: a particular egg or sperm gets an extra chromosome, causing the syndrome that neither parent has. It is almost never inherited, but it is genetic.

Development Starts with a Single Cell: The Fertilized Egg
DNA contains instructions for building a body, including brains (for animals that have them.) But the DNA does not encode a full representation of the body that it will help generate. For one thing, DNA does not have enough information to fully describe the location of every cell and every neuron's connections to other neurons. What it has can be thought of as a program that gets carried out by thousands of mindless biochemical machines, and when this happens, a body is formed.

Human beings start off as a single cell that replicates, and then those cells replicate. As they do, the cells start to differentiate. Some cells are neurons in the brain, other cells are skin cells on the foot.

Randomness in Development
But this process is fraught with randomness and noise. The brain has thousands of different neuron types, and not all are created exactly where they are supposed to be. During early development, they move toward where they are supposed to go, crawling like an amoebae. How to these cells know where to go?

As the initial cell divides into two, then four, then eight cells, and so on, the cells start to keep track of three axes in the body: head to tail (called Anterior to Posterior; even humans start out with a tail), back to belly (Dorsal to Ventral), and left to right. Cells release proteins according to where they are on these three axes, and each individual cell "knows" what level of each protein is optimal for its placement in the body. The concentration of this or that protein for this or that axis determine the three-dimensional coordinates in the body, and each cell knows it's supposed to go. When it finds itself in the right place (because the concentration of proteins is correct), then it stops moving.

Once neurons are in the right place, then they need to connect to the correct other neurons. The information receivers of the neurons (the dendrites) and the information senders (the axons) are like arms that grow out of the neuron. Using the same protein gradients that were used to place the cell in its correct location, dendrites and axons snake out into the brain, looking for the right connections. Some of these axons stretch all the way across the brain!

The particular chemical environment surrounding the developing cells in the mother's womb will have subtle effects on early development that have larger effects later on. Small differences in the developmental environment start the brain (and body) in a particular direction, and further development builds on what came before (it it's own environment). As such, two beings with identical DNA will develop into individuals who are not identical.

We can see this with identical twins. They look and act a lot alike, but they are not perfectly similar, even though they were built from the same DNA. We can even see it in our own bodies. The genetic coding for each side of your face is the same, but our faces are not perfectly symmetrical. So when we talk about differences being innate, it refers not only to genetic differences, but developmental differences that have nothing to do with the environment as it is typically conceived (for example, how you were raised by your parents).

The Nature/Nurture Debate
What are you born with, and what is the result of your experiences? Although this question makes intuitive sense, when we look at how development happens, the difference becomes very nuanced. Even if a child has genes that would make them very tall, if raised in an environment with low nutrition, they might still be short. Genes are instructions, and how and when those instructions are carried out depends on the environment.

A good example is puberty. There is no doubt that puberty changes are based on genetic instructions, but these instructions are not used (the genes are not expressed) until a certain point in a child's life, and these triggers are environmental, broadly speaking.

Another example is tanning. When you are out in the sun, it triggers certain genetic codes to express proteins that make you get a tan. When you are out of the sun, the tan fades. Everyone has genes for tanning, but they are on or off depending on environmental conditions.

See the chapter on the Nature-Nurture Debate for more information.