Final answer:
At birth, the brain is closer to its adult size than any other physical structure, signifying its early and significant development in human growth.
Step-by-step explanation:
The brain is the physical structure that is closest to its adult size at birth. A newborn's head, which houses the brain, is very large in proportion to the body. By the end of the embryonic period, the head is nearly as large as the rest of the embryo's body, and major brain structures are in place.
The body proportions and especially head size of a full-term newborn indicate that while the body has much growing to do, the brain has already undergone significant development. This early growth of the brain indicates its importance to human development and functioning.