The article below makes me remember that some people are not sure whether you should spell Neanderthal as Neandertal, as both spellings are used in writings on the subject.
The answer, put simply, is that both are correct. Neanderthal remains were first found in the vicinity of the River Neander. Neanderthal meant valley of the Neander, thal being then the German for valley. Since then, the German spelling of thal has become tal. The change need not affect how you spell the name of the hominid, but it can if you want it to.