Alden is beginning to learn how to write. He can hold a pencil in one hand and steady the paper with his other hand. This new ability to coordinate the duties of his right and left hands is likely due to .

the growth of his corpus callosum

Explanation:

Alden is beginning of learning, and her coordination from right to left right is due to the growth of his corpus callosum. As the growth of his corpus callosum is responsible and made from fibers which play role to communicate two different sides of mind i-e right hemisphere and left hemisphere. corpus callosum is like message transmitter from one to another hemisphere.

there is an explationation of this on a site called crash   course world history 1