The long-standing natural selection process that weeds out the weaker aspects of species was shown to have driven specific “changes in six genes involved in brain development” during the span of 50,000 to 200,000 years ago.
According to a recent article in Nature, the report hypothesizes that the natural selection process may have forced an increase in how neurons connect. This connectivity may have caused humans to become smarter and smarter as the species continued to evolve (2).
The researchers hypothesize that the cost of this increase in human intelligence was Alzheimer’s. That’s partially based on the evidence that these “same genes” are “implicated in Alzheimer’s disease.”
The population geneticist Kun Tang from the Shanghai Institutes for Biological Sciences in China, who led the research, proposes that “the memory disorder developed as ageing brains struggled with new metabolic demands imposed by increasing intelligence” (3).
Alzheimer’s Disease Is a Human One
Alzheimer’s is singular to humans with no other known species suffering from the aging disease. This set the team on a path to develop “a new coalescent-based method that collectively assigned human genome regions to modes of neutrality or to positive, negative, or balancing selection” (3).
Scientific American reported that the team selected the time ranging over the last half million years when the anatomically modern human (AMH) began to emerge. The selection time was supported by analyses the team conducted that were based on the genome sequences taken from “three ancient AMHs and the Neanderthals.”
Within these sequences, a series of “brain function-related genes were found to carry signals of ancient selective sweeps.” The team believes that these may have “defined the evolution of cognitive abilities.” This was possibly before the “Neanderthal divergence or during the emergence of AMH” (3).
What makes this significant is that “signals of brain evolution in AMH are strongly related to Alzheimer’s disease pathways.”
During their research, Tang’s team “searched modern human DNA for evidence of this ancient evolution.” This was achieved by examining human genomes that were taken from 90 people in Africa, Asia and Europe. The scientists looked for patterns that showed variations “driven by changes in population size and natural selection.”
Brain Function and Alzheimer’s
According to American Scientific, this analysis is tricky since “the two effects can mimic each other.” As a way to control population changes and their effects of “signatures of natural selection—the researchers estimated how population sizes changed over time.”
By doing this, they were able to identify any genome segments (4) that “did not match up with the population history, revealing the DNA stretches that were most likely shaped by selection.”
Through this elimination process, the team could look back “at selection events that occurred up to 500,000 years ago.” This revealed “the evolutionary forces that shaped the dawn of modern humans” some 200,000 years ago.
Prior to this, the methods used only uncovered changes dating back to 30,000 years. According to computational biologist at the Broad Institute (Cambridge, Massachusetts) Stephen Schaffner this approach of analysis is “promising” since it treats “all kinds of selection in a uniform framework” while also “treating different eras of selection in a more or less uniform way.”
Schaffner cautions that more research will need to be conducted in order to confirm the team’s new method and determine that it is in fact possible for broad application.
Time will tell if the team’s new findings withstand further scrutiny and analysis. If they do, then perhaps science can take another step forward in unlocking the mystery of Alzheimer’s and possibly one day- a cure.