Sharing common ancestors with Humans, Neanderthals (or Homo neanderthalensis) were a humanoid species on Earth, which became extinct 28,000 years ago. It is assumed that, in contrast to their Human rivals, that the extinction of Neanderthals was in part due to their inability to cope with rapidly changing environmental conditions.
Henry Starling said that the crew of the USS Voyager might think of him as "some backwards 20th century Neanderthal that doesn't know what he's got", with regards of his 29th century technology, but he proved to them otherwise. (VOY: "Future's End, Part II")
In 2151, Dr. Phlox compared the situation of the Valakians being selected by evolution to become extinct in favor of the Menk to that of the Neanderthals. (ENT: "Dear Doctor")
When Phlox attempted to compare the DNA from a Xindi-Reptilian to a Xindi-Primate, in early 2153, he noted that the Primate's genetic profile was nearly identical, but not quite the same, as the Reptilians. In noting this difference, Phlox agreed with Jonathan Archer that the best analogy might be in comparing a Human and a Neanderthal, rather than a Human to a chimpanzee. (ENT: "The Xindi")