The most important aspect of human evolution was facilitated not by Darwinian-style natural selection but by a crucial technological device invented by early Stone Age women, shows research by a leading British prehistorian.

Timothy Taylor of Bradford University claims that increased brain size was made possible by the invention of the baby sling, a development which enabled slower growing, physically and mentally immature offspring to survive and flourish.

"In effect, kangaroo-style, early female human ancestors became marsupial, carrying their immature youngsters outside their wombs," said Dr Taylor, who has published his research in a book called The Artificial Ape. "The invention of the baby sling, which allowed more babies to successfully mature outside the female body, instantly removed the barrier to increased head and brain size."

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Before the invention of the baby sling, dated by Dr Taylor to at least 2.2 million years ago, when human ancestor head size suddenly began to increase, physically mature infants were more likely to survive, because caring for slower-developing immature ones was difficult, uneconomic and often dangerous. Mothers holding their infants were more vulnerable to attack from predators or other humans than those using baby slings. They were also less able to perform other more economically productive tasks.

Most importantly, the invention of the baby sling artificially lengthened human gestation, said Dr Taylor. Formerly, gestation ended at birth with the most physically mature babies surviving as they needed to be carried by their mothers for less time. But their head and brain size was strictly limited by the width of their mother's pelvis.

"Courtesy of the baby sling, our ancestors got smarter," he added.