There are many reasons why the theory that the rh negative blood factor comes from the Neanderthals holds water.

First off, the reddish hair and the freckles are obviously physical attributes which are frequent within the rh negative population as well as the Neanderthal population of the days of their existence. But who exactly were the Neanderthals? We mainly know them by their name given after the location in Germany where their remains were first found. But what sort of language if any did they have and what did they consider themselves?

"At least two of the extinct, ancient humans had type O blood, making them the "universal donor", according to a new genetic analysis of remains of 45,000 year old individuals."

Source: Neanderthals might have made good blood donors .

The other issue that I have with most of the discussions regarding the Neanderthals is the constant claim that they have simply vanished from the face of the earth when so many reports indicate that Europeans have a small percentage of their DNA within them. What would make the most sense would be that they were simply outnumbered by the other humanoid species on earth and over 10s of 1,000s of years simply mixed into what are now homo sapiens.

"Neandertals, the closest evolutionary relatives of present-day humans, lived in large parts of Europe and western Asia before disappearing 30,000 years ago. We present a draft sequence of the Neandertal genome composed of more than 4 billion nucleotides from three individuals.

Comparisons of the Neandertal genome to the genomes of five present-day humans from different parts of the world identify a number of genomic regions that may have been affected by positive selection in ancestral modern humans, including genes involved in metabolism and in cognitive and skeletal development. We show that Neandertals shared more genetic variants with present-day humans in Eurasia than with present-day humans in sub-Saharan Africa, suggesting that gene flow from Neandertals into the ancestors of non-Africans occurred before the divergence of Eurasian groups from each other."

Source: A Draft Sequence of the Neandertal Genome.

I have also learned that the cranial capacity of the Neanderthals was a lot higher than of the other species on earth including ours, the Homo Sapiens'.

Does that mean Neanderthals were more intelligent than us as a whole?

The truth is we know very little about the Neanderthals, but out of all the theories in circulation about the origin of the rhesus negative blood factor, it makes the most sense to me that it came from them, especially considering the two examined remains tested as O negatives which is most likely their original blood types while A and B and ultimately AB came from mixing with the other existing homo species.