In 2011 the geneticist working on the Starchild Skull discovered that its mtDNA (the part of DNA passed only through the maternal line) was radically different from human DNA.
The maximum number of mtDNA differences between all humans is 120. The Starchild Skull has between 800-1,000. This is a partial result, but it is enough to be definitive: the Skull’s mtDNA is not human.