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By Will Dunham

WASHINGTON (Reuters) -Bipedal locomotion - walking upright on two legs - is a fundamental trait underpinning humankind's success. Scientists now have identified two innovations that occurred long ago in the human evolutionary lineage that reshaped the pelvis and helped facilitate this defining characteristic.

The researchers examined the genetic basis for bipedalism, an ability that distinguishes humans from other primates, by studying stored samples of embryonic tissues from people and various other primate species, discerning a pair of genetic shifts that took place in our ancestors.

The first of these innovations involved cartilage formation during embryonic pelvic development. It allowed the ilium, the bone forming the upper part of the pelvis, to morph from being tall, flat and narrow, as it is in other primates, to short, wide and curved, helping to stabilize the body for upright walking and running.

The second innovation, representing a crucial complement to the growing brain size of our ancestors, allowed for a delay and rearward shift in pelvic bone formation during embryonic development. This preserved the new advantageous shape of the ilium while allowing for women to possess a birth canal large enough to accommodate big-brained babies.

"Without these changes, human walking likely wouldn't have been possible, and subsequent brain size increases would have been difficult to envision," said Harvard University human evolutionary biologist Terence Capellini, senior author of the study published on Wednesday in the journal Nature.

"Bipedalism is the form of locomotion that allowed our ancestors to traverse large areas and eventually the entire globe," Capellini added.

Bipedalism, replacing locomotion using all four limbs, freed the hands for other things such as using tools, gathering and preparing food, wielding weapons, creating art, carrying infants and toting materials long distances. An upright posture enabled better observation of the environment and lessened the body's surface area exposed to direct sunlight, helping to cool more efficiently in hot climates.

Humans employ a type of bipedalism not found in other living primates.

"It's a key step in what made us human," said study lead author Gayani Senevirathne, a Harvard postdoctoral fellow in evolutionary biology.

For instance, chimpanzees occasionally walk on two legs but primarily use quadrupedal locomotion. Human bipedalism differs anatomically from the bipedalism displayed by animals such as birds and kangaroos and previously by certain dinosaurs such as Tyrannosaurus.

"Our form of bipedalism is exceptionally efficient in its striding form, allowing us to walk or run long distances with limited energy expenditures. Other primates that try to walk bipedally use lots more energy, and this is taxing on them," Capellini said.

The researchers identified more than 300 genes involved in the two innovations behind human bipedalism, including three with outsized roles.

"We didn't find a single 'bipedalism gene.' It looked like many small DNA switches - regulatory elements - were working together," Senevirathne said.

Chimpanzees are the closest genetic relatives to our species, Homo sapiens, which arose roughly 300,000 years ago in Africa. The lineage that led to Homo sapiens split from the lineage that led to chimpanzees roughly 6-8 million years ago, according to Capellini.

The oldest fossilized pelvis in the human lineage, discovered in Ethiopia, comes from Ardipithecus ramidus and is about 4.4 million years old. This species was a hybrid upright walker and tree climber, and bore some humanlike pelvic features.

The celebrated fossil called "Lucy," dating to about 3.2 million years ago from Ethiopia and representing a species called Australopithecus afarensis that combined apelike and humanlike traits, showed further humanlike pelvic traits.

Capellini said these fossils indicate that the evolutionary change involving the shape of the ilium had already occurred by the time these species roamed the African landscape.

The pelvis forms through a process that starts when cartilage cells arrange into structures called growth plates, which later harden into bone, a transition known as ossification.

The first of the two pelvic innovations involved the reorientation of a growth plate by 90 degrees to make the ilium wide instead of tall. This rearrangement provided attachment points for gluteal muscles to maintain balance as a person shifts weight from one leg to the other during locomotion.

The second innovation, involving a delay in pelvic ossification, probably occurred by the time our ancestors achieved significant increases in brain size around 1.6 million years ago, Capellini said.

"This way, the pelvis could grow in size and maintain a shape important for walking - but also it retained the shape of a birth canal that eventually would be used to allow the passage of a big-brained baby," Capellini added.

(Reporting by Will Dunham, Editing by Rosalba O'Brien)