New light has been shed on one of the secrets of human success - the relatively delayed development of human children - in a new study published in the Proceedings of the National Academy of Sciences.
By comparing the timing of thousands of genes being activated in the developing brains of humans, chimpanzees and macaque monkeys, an international research team has shown that our brains do indeed seem to be "programmed" to slow down cognitive and sexual maturity.
The findings support the idea that humans are subject to a process of neoteny - which delays physical development so that juvenile features are retained in mature animals. Axolotls, for example, are a type of salamander that has retained the juvenile gills that allow it to breath underwater.
The idea that humans are neotenous was first put forward in the 1830s, with observations that - compared with apes - humans have a relatively flat face, lack of body hair, large brain relative to body size, prolonged infantile dependency and a long lifespan.
It has been suggested, for example, that a human foetus would have to spend about 21 months in the womb - instead of just nine - to be as physically developed at birth as a newborn chimpanzee.
By taking longer to mature, it is argued, humans have more time to develop complex brains and absorb more information - such as complex language - and skills to bolster their chances of survival and success.
"In human evolution, developmental retardation, or neoteny, has been
proposed as a possible mechanism that contributed to the rise of many human-specific features, including an increase in brain size and the emergence of human-specific cognitive traits," the authors say.
Until now, however, such comparisons between human and chimpanzee have been made largely on the basis of skeletal forms and developmental landmarks. Female chimps, for example, reach sexual maturity between 8 and 9 years of age but in humans not until between 13 and 14 years.
This is the first study to look at the process at molecular level. One of the authors is Professor Cyndi Shannon-Weickert, Macquarie Group Foundation Chair of Schizophrenia Research at UNSW.
"For instance, it is unknown whether all genes expressed in the human brain show a consistent delay in expression timing relative to the chimpanzee or, alternatively, whether different structures or molecular networks are affected to different extents," the authors say.
They found that the delay is not uniform across the human transcriptome (all the gene transcripts present in a given cell) but affects a specific subset of genes that play a potential role in neural development.
They note that humans differ from their closest living relatives, chimpanzees, in brain size and many cognitive traits , as well as in the timing of particular developmental events.
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