By the end of the movie, when Lucy is sitting in the chair and her skin turns black, we can see her arms slowly turning black as well. When camera cuts away and shows Lucy from behind, her arms are naturally white again.
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.