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.
The survivors of the 12 colonies were definitely a superior intellect, since they had space travel 200,000 years before we did and came here fleeing the Cylon genocide of the human race. We have developed greatly from those 38,000 colonials and those Cylons who allied themselves with the survivors of Caprica and the 11 other colonies. That is the true source of this DNA quandry.
Thanks SM. There are a couple of topics regarding earlier physical development inferred from the remains/fossils of Neandertal juveniles.