By the time a human embryo is five weeks old, it is just the size of an apple seed. But the brain has already begun to grow. By eight weeks, the basic structure of the brain and central nervous system are in place.
The neural networks are spreading out. And even now, the nerve signals are traveling more than 150 miles an hour. At birth, nearly all 100... billion neurons of the human brain are already in place. But the brain only weighs about 25% of what it will later on.
It's about to embark on its fastest growing period, quadrupling in size by the time a child fits into the brain. finishes preschool. By age six, the brain is 90% of its adult size. During that burst of growth, 700 new neural connections are formed every second as we gain the capacity to smile around two months, to talk usually around a year, and to dress ourselves around the age of three. In those early years, in fact throughout our lives, the brain changes through experience, learning to speak, taking those first steps, understanding colors and shapes, forming novel thoughts.
But as certain neurons are used more frequently, other unused neurons go away. It's a process called pruning. And almost anything can shape us in those baby and toddler years. First words, first ice cream, first TV show, first argument, for better or worse.