The human body undergoes remarkable transformations from birth through adulthood. While most physical attributes change significantly, not all body parts follow the same growth trajectory. This highlights how bodies prioritize certain functions from birth.
The Body Part That Doesn’t Grow
The body part that is nearly fully grown at birth is the eyeball. At birth, a baby’s eyeball measures approximately 16.5 millimeters in length, about 75% of its adult size. While it experiences some rapid growth, reaching about 24 millimeters by early adulthood, its overall change is not as dramatic as other body parts. This limited growth contributes to the perception that infants have disproportionately large eyes.
The ossicles, the three tiny bones in the middle ear (malleus, incus, and stapes), are also fully formed and adult-sized at birth. These are the smallest bones in the human body, with the stapes measuring around 3 x 2.5 millimeters. They are fully ossified (turned from cartilage into bone) by about 24 weeks of prenatal life, undergoing minimal changes after birth.
The Science Behind Early Development
The early maturation of the eyeball and ear ossicles links to their important roles in an infant’s immediate survival and learning. Vision and hearing are fundamental senses that allow newborns to interact with their environment, bond with caregivers, and begin language development. The body prioritizes the development of these sensory organs to ensure they are functional from day one.
The rapid prenatal development of the eye involves the formation of its intricate structures between weeks 3 and 10 of pregnancy. Similarly, the middle ear ossicles begin to form around six weeks of development and are nearly complete by birth. This accelerated growth ensures that mechanical components for transmitting sound vibrations and light signals are ready for use. Maintaining their adult size from birth helps guarantee consistent sensory input, important for the developing brain to process information and form connections.
Growth Patterns of Other Body Parts
Most other body parts undergo significant growth after birth, unlike the eyes and ear ossicles. The human brain, for instance, is only about 25% of its adult weight at birth. It experiences a rapid increase in size during the first two years of life, reaching approximately 75% of adult weight by age two and 90% by age six, continuing to grow until around age 18. This postnatal brain growth involves the formation of new connections between neurons and the development of supporting cells.
The skeletal system also matures considerably after birth. Infants have soft spots in their skull bones that fuse later, and their long bones contain growth plates that allow for linear growth throughout childhood and adolescence. These growth plates fuse by the time an individual is 14 to 18 years old. Muscles, too, experience substantial growth post-birth, with skeletal muscle being one of the fastest-growing protein masses in neonates due to high rates of protein synthesis. This continued development ensures the body can adapt and strengthen as a child grows and becomes more active.