Research at the Shanghai Institutes for Biological Sciences in China has discovered a link between intelligence and Alzheimer’s disease. Kung Tang, lead author, has proposed the idea that Alzheimer’s arises due to the difficulties of meeting the high metabolic demands of the ageing brain as intelligence decreases.
Humans are the only species that develop Alzheimer’s disease, the most common form of dementia. More that 520, 000 people in the UK are living with Alzheimer’s. As the disease progresses, proteins accumulate in the brain, forming plaques, and disrupt the connections between nerve cells. The disease also causes a shortage of neurotransmitters (chemical messengers that are integral in the signalling of the brain), death of nerve cells and loss of brain tissue.
The researchers examined the DNA of 90 people of African, Asian and European descent, searching for patterns of variation that are the result of natural selection or changes in population size. They found out that six genes, key in brain development, evolved fifty thousand years ago. These six genes might have contributed to the maximisation of neuron connectivity which made modern humans more intelligent than their ancestors. They are also involved in Alzheimer’s.
The analytical approach used by the Shanghai Institute for Biological Sciences to investigate evolutionary changes in DNA is new. The impact of population size and natural selection on the genome is very similar. The research team estimated how the population size varied over time and then looked for DNA segments that did not correspond with the population history. These segments are the result of natural selection.
The method allowed the research team to study natural selection that took place up to 50 000 years ago, while most current research has succeeded in tracking genome alteration to only 30 000 years ago, giving a more detailed insight into the evolution of this disease.