by Steve Connor
12 November 2012
Is the human species doomed to intellectual decline? Will our
intelligence ebb away in centuries to come leaving our descendants
incapable of using the technology their ancestors invented?
In short: will Homo be left
without his sapiens?
This is the controversial hypothesis of a leading geneticist who
believes that the immense capacity of the human brain to learn new
tricks is under attack from an array of genetic mutations that have
accumulated since people started living in cities a few thousand
Gerald Crabtree, who heads a
genetics laboratory at Stanford University in California, has put
forward the iconoclastic idea (see
Our Fragile Intellect)
that rather than getting cleverer, human intelligence peaked several
thousand years ago and from then on there has been a slow decline in
our intellectual and emotional abilities.
Although we are now surrounded by the technological and medical
benefits of a scientific revolution, these have masked an underlying
decline in brain power which is set to continue into the future
leading to the ultimate dumbing-down of the human species, Professor
His argument is based on the fact that for more than 99 per cent of
human evolutionary history, we have lived as hunter-gatherer
communities surviving on our wits, leading to big-brained humans.
Since the invention of agriculture and
cities, however, natural selection on our intellect has effective
stopped and mutations have accumulated in the critical
“I would wager that if an average
citizen from Athens of 1000BC were to appear suddenly among us,
he or she would be among the brightest and most intellectually
alive of our colleagues and companions, with a good memory, a
broad range of ideas and a clear-sighted view of important
issues,” Professor Crabtree says in a provocative paper
published in the journal Trends in Genetics.
“Furthermore, I would guess that he
or she would be among the most emotionally stable of our friends
and colleagues. I would also make this wager for the ancient
inhabitants of Africa, Asia, India or the Americas, of perhaps
2,000 to 6,000 years ago,” Professor Crabtree says.
“The basis for my wager comes from new developments in genetics,
anthropology, and neurobiology that make a clear prediction that
our intellectual and emotional abilities are genetically
surprisingly fragile,” he says.
A comparison of the genomes of parents
and children has revealed that on average there are between 25 and
65 new mutations occurring in the DNA of each generation.
Professor Crabtree says that this
analysis predicts about 5,000 new mutations in the past 120
generations, which covers a span of about 3,000 years.
Some of these mutations, he suggests, will occur within the 2,000 to
5,000 genes that are involved in human intellectual ability, for
instance by building and mapping the billions of nerve cells of the
brain or producing the dozens of chemical neurotransmitters that
control the junctions between these brain cells.
Life as a hunter-gatherer was probably more intellectually demanding
than widely supposed, he says.
“A hunter-gatherer who did not
correctly conceive a solution to providing food or shelter
probably died, along with his or her progeny, whereas a modern
Wall Street executive that made a similar conceptual mistake
would receive a substantial bonus and be a more attractive
mate,” Professor Crabtree says.
However, other scientists remain
“At first sight this is a classic
case of Arts Faculty science. Never mind the hypothesis, give me
the data, and there aren’t any,” said Professor Steve Jones, a
geneticist at University College London.
“I could just as well argue that mutations have reduced our
aggression, our depression and our penis length but no journal
would publish that. Why do they publish this?” Professor Jones
“I am an advocate of
- facts, facts and more facts; but we need ideas too, and this
is an ideas paper although I have no idea how the idea could be
tested,” he said.
THE DESCENT OF MAN
Hunter-gatherer man - The human
brain and its immense capacity for knowledge evolved during
this long period of prehistory when we battled against the
Athenian man - The invention of
agriculture less than 10,000 years ago and the subsequent
rise of cities such as Athens relaxed the intensive natural
selection of our “intelligence genes”.
Couch-potato man - As genetic
mutations increase over future generations, are we doomed to
watching soap-opera repeats without knowing how to use the
TV remote control?
iPad man - The fruits of science
and technology enabled humans to rise above the constraints
of nature and cushioned our fragile intellect from genetic
Research Suggests That Humans Are Slowly But
Losing Intellectual and Emotional
by Lisa Lyons
12 November 2012
Human intelligence and behavior require optimal functioning of a
large number of genes, which requires enormous evolutionary
pressures to maintain.
A provocative hypothesis published in a
recent set of Science and Society pieces published in the Cell Press
journal Trends in Genetics (see
Our Fragile Intellect)
suggests that we are losing our intellectual and emotional
capabilities because the intricate web of genes endowing us with our
brain power is particularly susceptible to mutations and that these
mutations are not being selected against in our modern society.
"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 the papers' author, Dr. Gerald Crabtree, of Stanford
In this environment, intelligence was
critical for survival, and there was likely to be immense selective
pressure acting on the genes required for intellectual development,
leading to a peak in human intelligence.
From that point, it's likely that we began to slowly lose ground.
With the development of agriculture,
came urbanization, which may have weakened the power of selection to
weed out mutations leading to intellectual disabilities.
Based on calculations of the frequency
with which deleterious mutations appear in the human genome and the
assumption that 2000 to 5000 genes are required for intellectual
ability, Dr. Crabtree estimates that within 3000 years (about
120 generations) we have all sustained two or more mutations harmful
to our intellectual or emotional stability.
Moreover, recent findings from
neuroscience suggest that genes involved in brain function are
uniquely susceptible to mutations.
Dr. Crabtree argues that the combination
of less selective pressure and the large number of easily affected
genes is eroding our intellectual and emotional capabilities.
But not to worry...
The loss is quite slow, and judging by
society's rapid pace of discovery and advancement, future
technologies are bound to reveal solutions to the 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."