Genes that Gave Rise to Human Intelligence Linked to Alzheimer's Disease

Humans are the only species known to develop Alzheimer's that is absent in closely related primate species such as chimpanzees...In autism, the Fragile X syndrome prevents the production of a crucial protein known as FMRP that activates the NOS1 gene. This gene is found throughout the developing brain in humans. Crucially, NOS1 helps guide the maturation of the brain's speech and language and decision-making centers.

http://en.yibada.com/articles/34097/20150522/genes-gave-rise-human-intelligence-linked-alzheimers-disease.htm

The pattern of NOS1 activity in these brain centers does not occur in the developing mouse brain — suggesting that it is a more recent evolutionary adaptation possibly involved in the wiring of neural circuits important for higher cognitive abilities.

http://news.yale.edu/2012/05/10/evolution-s-gift-may-also-be-root-form-autism

Neuronal nitric oxide synthase (NOS1) promotes synaptic plasticity but it also plays a role in excitoxicity which leads to the death of neurons via peroxynitrites.

http://www.ncbi.nlm.nih.gov/pubmed/8613773

http://www.jneurosci.org/content/19/14/5910.full.pdf

So what helped make us smarter than other animals also made us almost uniquely vulnerable to Alzheimer's disease.