The Neandertal EnigmaFrayer's own reading of the record reveals a number of overlooked traits that clearly and specifically link the Neandertals to the Cro-Magnons. One such trait is the shape of the opening of the nerve canal in the lower jaw, a spot where dentists often give a pain-blocking injection. In many Neandertal, the upper portion of the opening is covered by a broad bony ridge, a curious feature also carried by a significant number of Cro-Magnons. But none of the alleged 'ancestors of us all' fossils from Africa have it, and it is extremely rare in modern people outside Europe." [pp 126-127]
by James Shreeve
in local libraries
Neanderthals really are an eigma. They were stronger than us. Their brain was of statistically equal size.
I've often wondered if they had a must faster rate of development, essentially skipping the prolonged adolesence, which is a key feature of modern humans. Going from infant to adult in 2-5 years would be very advantageous to survival in many ways (its hard to imagine any mammal surviving 5-7 years of near helplessnes, coupled with not reaching reproductive maturity until the 13th or 14th year). Fast (normal) growth would developmentally retard intellect for sure.
We may also be assuming improperly that they were less intelligent based on false qualifiers of intelligence. We presume intelligence leads to cooperation and cooperation is evidenced by living in groups, but are wildebeast more intelligent than jaguars? A lack of grouping could explain a lack of tool development (specialization). Of course there are all of the standard theories as well.