The idea that humans only use 10% of their brain is a widely-circulated myth that has been debunked by neurological science many times over.
During Lucy's final transformation sequence, her consciousness starts traveling through time by millions of years. At one point, Lucy pauses in time to observe and interact with an ape-like creature, reaching out to touch its fingertip. Although not expressly stated or explained in the film, this ape-like creature was what scientists today classify as Australopithecus afarensis, the earliest identifiable ancestor of humankind. The first fossils of Australopithecus afarensis were determined to be from a female who was christened "Lucy" by scientists.