“Our brains use 2,000 to 5,000 so-called Intellectual Deficiency (ID) genes, according to Crabtree’s estimate. Geneticists routinely identify specific mutations in ID genes as the causes of various types of mental retardation and similar intellectual syndromes that arise during imperfections in brain development.3 Proper human intelligence requires at least one clean copy of every ID gene.4 Like a missing wooden slat in a very long rope bridge, a mutant ID gene challenges the brain to carefully “step over” the gap on its way to developing the biological infrastructure for healthy intelligence.
Each new generation accrues about 60 new mutations to the gene-coding DNA regions of the human genome. Crabtree applied this rate to calculate that “every 20-50 generations we should sustain a mutation in one copy of one of our many ID genes.” As a result, “in the past 3000 years then (~120 generations), each of us should have accumulated at the very least 2.5-6 mutations in ID genes.” Accordingly, the human intellect “perhaps reached a peak 2000-6000 years ago.”1″
Here’s another article with a similar theme. I would point out two things about this. One, whether or not the human brain is deteriorating genetically, modern humans’ reliance on machines to help with their thinking may have something to do with intellectual capacity. We are no longer required to exercise our memory and thinking skills as rigorously as our forebears who did not have super-fast computational devices. Two, the argument that Genesis was written so that “primative man” could understand it is unpersuasive, based on these findings.