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There are var­i­ous the­o­ries about when our ances­tors start­ed walk­ing upright

The abil­i­ty to walk upright on two legs is one of humanity’s defin­ing phys­i­cal characteristics.

Bone struc­ture

Our bones are organ­ised to help us walk on two legs. The pelvis, leg bones (femurs), knee joints and foot bones of ear­ly hominids give palaeoan­thro­pol­o­gists good indi­ca­tions as to the way they moved around.

Palaeoan­thro­pol­o­gists can also tell whether hominids walked upright from their skulls by look­ing at the fora­men mag­num – the point the spine enters the skull – and thus the nat­ur­al posi­tion of the head.

Pre­served footprints

At Lae­toli in Tan­za­nia, just south of Oldu­vai Gorge, a set of Aus­tralo­p­ithe­cus afaren­sis foot­prints of two indi­vid­u­als walk­ing along side by side, has been dat­ed to 3.5-million years ago.

The preser­va­tion of the foot­prints was due to a remark­able set of cir­cum­stances. Ini­tial­ly, a near­by vol­cano called Sadi­man erupt­ed, blow­ing a cloud of fine ash that set­tled over the sur­round­ing areas. Then rain fell, cre­at­ing some­thing sim­i­lar to wet cement.

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Fos­sil foot­prints of Aus­tralo­p­ithe­cus afaren­sis (Source: Flickr)

Many birds and ani­mals walked over this wet cement”, leav­ing their foot­prints in it. Among them, walked two Aus­tralo­p­ithe­cus afaren­sis hominids, a large one and a small one, side by side. The larg­er one was prob­a­bly car­ry­ing some­thing heavy, since it left a deep­er inden­ta­tion on one side. Per­haps it was a moth­er car­ry­ing a child.

Then Sadi­man erupt­ed again, leav­ing yet anoth­er lay­er of ash and seal­ing the foot­prints for the future. Ero­sion over mil­lions of years even­tu­al­ly exposed the foot­prints, which were found by researchers work­ing with Mary Leakey. They were exca­vat­ed in 1978.

The foot­prints are not ful­ly human and have ape-like fea­tures includ­ing a slight­ly diver­gent big toe. Researchers assume they were made by Aus­tralo­p­ithe­cus afaren­sis because they are the only hominids rep­re­sent­ed in the fos­sil record in East Africa for that period.

Fos­silised foot­prints from more than 3-mil­lion years ago tell us that our ances­tors walked upright, much like us.

How did bipedal­ism begin?

There are var­i­ous the­o­ries about when our ances­tors start­ed walk­ing upright, but a pop­u­lar view is that per­haps about 7-mil­lion years ago, ear­ly hominids began to adapt to a cli­mate that was cool­ing globally.

The huge rain­for­est expans­es in Africa were being replaced with savan­nah and patch­es of wood­land, requir­ing the tree-climb­ing apes to become more adept at walk­ing on land.

Our ances­tors who ven­tured out into the savan­nah were reward­ed with roots, shrubs and occa­sion­al ani­mal car­cass­es, ensur­ing that those who walked on two legs were more like­ly to survive.

The jump from trees to land wasn’t as big as some might see it. Some of the ear­ly hominids’ anatom­i­cal struc­tures may have already been pre-adapt­ed to bipedal­ism while climb­ing trees and stretch­ing for fruit.

The advan­tages

The host of advan­tages bipedal­ism brought meant that all future hominid species would car­ry this trait.

Bipedal­ism allowed hominids to free their arms com­plete­ly, enabling them to make and use tools effi­cient­ly, stretch for fruit in trees and use their hands for social dis­play and com­mu­ni­ca­tion. They could also see fur­ther over the savan­nah grass – but this also could have been a dis­ad­van­tage since preda­tors could prob­a­bly spot them more easily.

Bipedal hominids could spend more time for­ag­ing and scav­eng­ing out in the open savan­nah because their bod­ies would be exposed to less sun­light stand­ing upright.

Evolution Walk

Bipedal­ism allowed hominids to free their arms, allow­ing the use of tools

Walk­ing on two limbs was also more ener­gy effi­cient than walk­ing on four – giv­ing ear­ly hominids more ener­gy to repro­duce and there­fore more chance of pro­duc­ing off­spring bear­ing this unique trait.

But even with these advan­tages, these tran­si­tion­al hominids prob­a­bly spent time in the trees as well.

At least some Aus­tralo­p­ithe­cus species, includ­ing the one rep­re­sent­ed by Lit­tle Foot” at Sterk­fontein, which is as yet unnamed, were at least part­ly arbo­re­al between 4-mil­lion and 3-mil­lion years ago, when there was some for­est in the Cra­dle of Humankind environment.

Sim­i­lar­ly, fur­ther north in Africa, the Aus­tralo­p­ithe­cus species of Ethiopia and Tan­za­nia between 3-mil­lion and 2-mil­lion years ago would have been able to climb trees bet­ter than mod­ern humans, but were simul­ta­ne­ous­ly adapt­ing to more full-time upright walking.

Aus­tralo­p­ithe­cus afaren­sis, which pop­u­lat­ed the Afar Depres­sion in Ethiopia, would have lived in an envi­ron­ment typ­i­fied by wet­lands, wood­land and for­est. But the bipedal foot­prints of Aus­tralo­p­ithe­cus afaren­sis in Lae­toli, Tan­za­nia, are found in an area where the envi­ron­ment was prob­a­bly dri­er and sparse­ly wood­ed 3.6-million years ago.

Lit­tle Foot”, which rep­re­sents a species of Aus­tralo­p­ithe­cus more than 3.3-million years old, was most cer­tain­ly not a knuck­le-walk­er like some of the great apes. It prob­a­bly could have walked and climbed effectively.

Lit­tle Foot” and oth­er ear­ly aus­tralo­p­ithecines prob­a­bly climbed trees to escape preda­tors and maybe even to sleep in at night.

Did you know?

Many dinosaurs were bipedal!

How our joints help us to walk

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Ball-and-sock­et joint

The ball-and-sock­et joint is the most mobile type of joint, allow­ing us to swing our arms and legs in many dif­fer­ent direc­tions. The joint is made up of a ball-shaped bone which rotates in a cup-shaped cav­i­ty, as in our hips and shoulders.

Hinge joint

Hinge joints, such as the knee and elbow, act as a lever that enables our arms and legs to flex (bend) and straight­en. The move­ment is sim­i­lar to the open­ing and clos­ing of a hinged door.

Piv­ot joint

In a piv­ot joint, one bone twists against anoth­er. Piv­ot joints are found between the upper bones of the neck, allow­ing you to turn your head from side to side.

Semi-mov­able joint

Semi-mov­able joints, such as those found in the spine, only allow par­tial move­ment. The joints are con­nect­ed by pads of car­ti­lage that restrict movement.

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