A new study suggests that humans may be (slowly) losing our intellectual and emotional abilities because they're at risk from mutation or loss from the genome.
Human intelligence and behaviour requires the optimal functioning of an estimated 2000-5000 genes. Crabtree’s theory is based on the idea that these genes are particularly vulnerable to mutations, and that our modern environment does not remove these mutations by natural selection. This means that our intelligence and emotional capabilities are expected to decrease over time as more mutations enter the genome.
"It is very likely that within 3000 years (~120 generations) we have all sustained two or more mutations harmful to our intellectual or emotional stability," writes Crabtree in a study published in Trends in Genetics.
By contrast our ancestors would have had highly selective evolutionary pressures on the genome.
"The development of our intellectual abilities and the optimization of thousands of intelligence genes probably occurred in relatively non-verbal, dispersed groups of peoples before our ancestors emerged from Africa," says Crabtree.
Without a well-functioning intellect, our ancestors would not have been able to survive in an environment that required them to constantly to adapt to new dangers. But our modern environment doesn't select against deleterious mutations in the same way, leading to a gradual loss in our emotional and intellectual abilities. To make matters worse, neuroscientists believe that genes involved in brain function are particularly susceptible to mutations.
But not to worry just yet. The loss is likely to be relatively slow, and judging by society's rapid pace of technological development, we should be able to develop ways to counter this problem.
"I think we will know each of the millions of human mutations that can compromise our intellectual function and how each of these mutations interact with each other and other processes as well as environmental influences," says Dr. Crabtree.
"At that time, we may be able to magically correct any mutation that has occurred in all cells of any organism at any developmental stage. Thus, the brutish process of natural selection will be unnecessary."
Crabtree’s theory is based on the estimate that in every generation there are around 60 new mutations per genome and about 100 heterozygous per mutations genome (mutations that only affect one copy of a gene). Some of these mutations are likely to affect genes involved in human intellect.