What tweeters say

72% prefer the spelling ‘Neanderthal’ (survey responded by 828 tweeters on April 10-11 2018).

What Google Books says

The presence of the spelling ‘Neanderthal’ is 4x times more frequent in Google Books than that of ‘Neandertal’ (1864-2008, Engligh language, source: Google Ngram).

What TalkOrigins says

– The first such fossil was discovered in 1856 in the Neander Thal, or «Neander Valley» in German, and became known as «Neanderthal Man». In 1904, German spelling was regularized to be more consistent with pronunciation, and «thal» became «tal». In 1952 Henri Vallois proposed that it should be spelt as the Germans spell it, and the «-tal» spelling has become widely used since then. The «-thal» spelling persists most strongly in England (source).

And yes! Look what happened in the early 1900’s in the books written in German…

What John Hawks says

– I have a couple of reasons why I prefer my «Neandertal» with a «T»:

1) William King was the first to make a taxonomic name for the group we call Neandertals. He named it Homo neanderthalensis — that’s «neanderthalensis»with a «TH». By the almighty rules of taxonomic nomenclature, that’s the name our poor heroes are stuck with (instead of Schwalbe’s vastly better, but slightly later, Homo primigenius, for instance). So using «Neandertal» with a «T» is an act of taxonomic subversion. Let’s call it a pique.

2) «Neanderthal» with a «TH» has an ordinary English meaning that is well understood by everybody — it means «stupid,» «clumsy» and «brutish» all in one!Since that’s not ordinarily what I mean when I’m describing Neandertals, I take advantage of the unfamiliarity of the alternate spelling to get people to think about them in a different way. (source).

Back to Twitter: some individual opinions

• Neanderthal, but pronounced like Neandertal (with a hard ‘t’), for historical reasons: that’s how the word was spelled when it first entered the scientific language. @mik3caprio @marg_leaf
• «Thal» is the antiquated spelling of modern German «Tal» and is cognate to English «Dale». The name itself has become a sort of orthographic fossil or relic in its own right. @malfeasible
• Some languages do not have a ‘th’ sound at all. Listen to French or Dutch speaker trying to say this, that, hither & thither. So probably could not get tongue around Neanderthal even if it was made compulsory. @airpotgardener
• My thesis advisor, who works on Neandertal fossils, insisted we drop the h (…) Insists it is the proper German pronunciation based on the Neander (Neandertal)Valley where the first was found @Bananaaquamelon
• The Neanderthal voters are a bunch of Neandertals. @Boomersaurus

¿Y en español?

Neandertal en el Diccionario panhispánico de dudas: Forma adaptada a la ortografía española del nombre de este valle de Alemania: «La capacidad craneal del cráneo de Neandertal no difería […] de la humana moderna» (Arsuaga Enigma [Esp. 2001]). Aunque también se utiliza la grafía alemana Neanderthal, se recomienda el uso de la forma hispanizada. Cuando se emplea como nombre común para designar a los homínidos en él encontrados, debe escribirse con inicial minúscula: «Hubo, pues, un período de coexistencia en Europa de neandertales y cromañones» (Arsuaga Enigma [Esp. 2001]).

Aun así, no se observa gran diferencia entre escribirlo en español con ‘t’ y con ‘th’ en Google Books…

Artículo relacionado: Neandertales y cromañones [acceso].