A FLAT face, big eyes, small brows, little chin and patchy hair. Adult people look a lot like baby chimps.
These similarities underpin an idea first proposed more than 170 years ago, that our species underwent a developmental delay during its evolution which resulted in mature humans retaining some of the features of juvenile apes.
Not only do we look babyish, this process, called neoteny, could also help explain why we are so much smarter than our chimp cousins, despite having almost identical DNA to them.
It meant our brains gained extra time during a long childhood in which to master complex feats like language, said Cyndi Shannon-Weickert, a researcher in the school of psychiatry at the University of NSW.
"We have a protracted period of development that gives us a great capacity to learn and be influenced by our environment," she said. "It's essentially what makes us so successful."
Until now, skull comparisons between humans and chimps, and the fact that we sexually mature about five years later than them, have been the main evidence for neoteny in humans.
But a landmark study comparing the activity of thousands of genes in the brains of 39 people, 14 chimps and nine rhesus macaques has provided the first genetic evidence for this process.
An international team including Professor Shannon-Weickert identified a small group of genes in the front parts of the brain that are switched on much later in humans than in other primates, around the start of adolescence.
"These are genes which make us uniquely human," she said, and the timing of their activation is the critical factor.
The delay in switching them on might not only allow the brain to be "plastic" for longer, soaking up new information.
These genes might also have a role in a downside of our unique brain capacity - the susceptibility of adolescents to mental illnesses such as schizophrenia, bipolar disorder and depression."
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