The hips of females from the species Homo erectus, a primitive relative of modern humans, have been found to be wider than was previously thought.
That means they were well equipped for delivering babies with a larger cranial capacity which ultimately allowed intelligent human beings to evolve.
Scientists came to their conclusion after reconstructing a 1.2 million-year-old fossil pelvis discovered in Gona, Ethiopia.
The new pelvis fossil suggests that Homo erectus infants were more than 30 per cent larger than has been believed until now.
The need to give birth to large brained infants was probably the primary driver behind the shape of the pelvis.
Previous research based on the skeleton of a young male, named Turkana Boy, had led experts to believe that Homo erectus had a tall, thin body shape adapted to a hot climate or endurance running.
A narrow pelvis for females would have caused infants to be born with relatively small brains.
The new discovery, published in the journal Science, means the speed at which their brains grew before birth would have been similar to that of modern humans.
However, scientists believe that after birth the brain growth rate would have been somewhere between that of humans and chimpanzees.
Homo erectus was a highly effective species which used stone tools, may have controlled fire, and is thought to have been the first human species to leave Africa.
It was the dominant human species on Earth from about 2 million to about 500,000 years ago.
Its brain size expanded over time from about 850 cubic centimetres to about 1,100 cubic centimetres.
Males were nearly 6ft tall and much stronger than their modern descendants.
They are thought to have lived in small hunter gatherer societies, to have engaged in large scale hunting using complex weapons, and to have cared for weaker individuals in the group.
However, despite their large brains they are not thought to have developed language abilities.
Modern humans of the species Homo sapiens typically have a brain size of 1,400 cubic centimetres, which is more than twice the size of a modern chimpanzee or gorilla.
Homo sapiens originated in Africa about 200,000 years ago and the species is descended from Homo erectus.
Homo sapiens displaced Homo neanderthalensis, and other species descended from Homo erectus, partly through reproducing more successfully.
Dr Sileshi Semaw, from Indiana University, who led the research into Homo erectus, said: "This is the most complete female Homo erectus pelvis ever found from this time period.
"This discovery gives us more accurate information about the Homo erectus female pelvic inlet and therefore the size of their newborns."