The human brain does not suffer from catastrophic forgetting (or catastrophic interference), and the explanation for its ability to learn continuously is as follows:
In the human brain, a single concept is memorized through multiple brain cells (neurons). When a new concept needs to be learned, the brain does not reconstruct all the synaptic connections nor does it immediately sever the old ones. Instead, it only strengthens the synaptic connections between the brain cells that represent the concepts most relevant to this new concept.
For example, after learning the concept of walking and its corresponding physical action, the brain has one sequence of connected brain cells representing the physical action of walking, and another group of cells representing the abstract concept of walking.
When the brain proceeds to learn the concept of running, it first attempts to activate the muscle cell groups associated with running. The brain then discovers that running is essentially just increasing the activation intensity of the muscle cells already activated for walking, achieving a faster stride.
Thus, to form the concept of running, the brain only needs to add a concept—that of strengthening the activation of the muscle cell groups—to the existing group of brain cells activated by the walking concept.
This creates a puzzle-piece, modular, and localized learning process, eliminating the need to reconstruct all synaptic connections.