Differential reinforcement of an incompatible response (social attention for upright walking
) should decrease socially stigmatizing ambulation because the incompatible responses are functionally equivalent.
Some researchers argue upright walking
evolved as an efficient way to move between patches of shrinking forest, while others claim the need to free up the hands led to a two-legged stance.
But other foot features--including long, curved little toes--indicate that a skeletal system for upright walking
had not fully evolved in Lucy's kind, Jungers asserts.
Ardi also raises questions about the origin of two-legged upright walking
, considered a defining feature of the human evolutionary family.
ramidus was, in fact, a woodland with forest patches, where grasses were rare, then it's unlikely that the increased presence of grassy environments were the driving force behind the origin of upright walking
in early human ancestors.
If Ardipithecus adopted upright walking
in a big way and was a precursor of the human lineage, Hawks posits, "it could be the first hominid or perhaps even the common ancestor of humans and chimps--if we take genetic studies seriously." DNA analyses suggest that people and chimps split from a common ancestor between 5 million and 4.5 million years ago, around Ardi's time.
Ardi displays an unexpected mix of apelike and monkeylike traits suitable for both tree climbing and upright walking
. Overall, Ardipithecus looks unlike any living primate, White adds.
"It's supposed to be a powerfully built upright walking
If upright walking
originated in ancient apes, different styles of two-legged striding apparently evolved later in various hominid species, remark Paul O'Higgins and Sarah Elton, anatomists at Hull York Medical School in England, in an editorial published with the Thorpe team's paper.
"Orangutans have developed a unique way of coping with these problems; they move in an irregular way which includes upright walking
, four-limbed suspension from branches and tree-swaying, whereby they move branches backwards and forwards, with increasing magnitude, until they are able to cross large gaps between trees," according to Dr Susannah Thorpe, from the University of Birmingham's School of Biosciences.
He plans to examine the limb joints to test the theory--so far, based only on the foot bones--that this creature combined upright walking
with considerable tree climbing.
The arched 'rigid' foot of modern humans - thought to have appeared approximately 1.8 million years ago - is best adapted for upright walking
, but scientists have found that early humans once had 'flexible' feet and could have walked on the ground some years earlier.