Lucy in the ground with diamonds

Surely one of the responsibles that you’re reading this post was the climate change that took place about 6 million years ago. The lifting of the Rift Valley caused a cooling and drying of sub-Saharan Africa, which favored the extension of the savannah at the expense of forests and the evolution of the first hominans who already walked by two feet. The most famous of them is undoubtedly Lucy. We encourage you to meet Australopithecus afarensis and the anatomy associated with bipedalism.

¿WHO WAS LUCY?

Just over 40 years ago, Donald Johanson discovered a partial skeleton (AL-288-1) 3.2 million old in Hadar, Ethiopia. It was the oldest hominan discovered and the bones belonged to an unknown species. At night, while celebrating the discovery with his team, the Beatles song Lucy in the Sky with Diamonds was playing in the cassette, and nicknamed the fossil remains. They belong to the species Australopithecus afarensis (Afar southern monkey).

Cast of Lucy in the Museum National d'Histoire Naturelle, Paris. (Photo: unknown author, Wikimedia)

Currently A. afarensis is one of the best known early hominans, as it have been found remains of hundreds of individuals, males, females and young ones.

ANATOMY

The average height and weight of A. afarensis was 1.05 m and 29 kg for females and 1.51 m and 42 kg for males, significantly smaller compared to us. Brain volume was also small, 387-550 cubic cm (similar to a current chimpanzee). The arms and fingers were longer than ours, which allowed them to climb easily in the trees, and the legs, though shorter, had characteristics that allowed bipedalism (walking on two feet) completely. The forehead was narrow and jaws were located forward (prognathism), with a large space for jaw muscles. Their diet was mainly herbivorous.

Reconstruction of Lucy by Elisabeth Daynès, with Laetoli footprint trails, in CosmoCaixa. (Photo: Mireia Querol)

ANATOMICAL FEATURES OF BIPEDALISM

A. afarensis already had the necessary adaptations to walk like us, though perhaps is not the older bipedal hominan: Orrorin tugenensis (6.2 to 5.6 million years) is aspiring to be one of the first members of the human race who walked upright .

Comparison between the skeletons of a current human (Homo sapiens), an A. afarensis and a chimpanzee (Pan troglodytes). (Photo: H. sapiens unknown author, A. afarensis John C. Phillips, chimpanzee Udo M. Savalli).

• Foramen magnum: the spinal cord goes through an opening in the skull, the foramen magnum. In the chimpanzee is located in the back of the skull, while afarensis has it in the base, which allows a vertical backbone settle.
• Backbone: lumbar and cervical area of the human backbone are more curved, we have a column with a “S” shape. The center of gravity is in the midline of the foot and allows spinal flexion during walking, therefore when chimpanzees walk on two feet, stagger for balance because they have a straight spine.
• Rib cage: A. afarensis still has a rather conical chest to accommodate a larger digestive system due to herviborous diet and better shoulder mobility to climb. H. sapiens have it shaped like a barrel, which facilitates the swing arm for better balance while walking and allows a better torso bending.
• Pelvis: human pelvis is shorter and wider than other primates, to allow better mobility with the base of the backbone, but the birth canal is narrow.
• Feet: A. afarensis toe, like ours, is aligned with the rest of the fingers, the sole is arched and a wide bead allows the foot to propel with the fingers and absorb shock when walking.
• Femur: due to bipedalism the joint surface is wide and the femur is angled toward the center of gravity. In the chimpanzee femurs are shorter, less inclined and with lower joints.

LAETOLI FOOTPRINT TRAILS

Laetoli footprint trails, Tanzania. (Photo: Sciencephoto Library)

2.000 kms further south where Lucy was found, in Laetoli (Tanzania), Mary Leakey discovered in 1978 the oldest known biped trail (3.6 million years) of probably 4 hominans who walked through the open savannah, with traces of other extinct animals like the horse Hipparion, a bird, a baboon and a centipede. The tracks are laid down in the ashes of the volcano Sadiman and are attributed to A. afarensis. There are 69 footprints, some overlapping others intentionally, perhaps as a strategy to leave no trace. The big toe is parallel to the rest of the fingers and a deep footprint and the bead is well marked, which confirms a completely bipedal stride.

But why has it been so important bipedalism in the process of humanization, towards the emergence of Homo sapiens? We’ll find out in the next article on human evolution.

Rreconstruction of A. afarensis by John Gurche. (Photo: Chip Clark)

 

REFERENCES

• Roberts, Alice. 2002. Akal. Evolución: historia de la humanidad.
• Turbón, Daniel. 2006. Ariel. La evolución humana.
• Cela Conde, Camilo José; Ayala, Francisco J. 2001. Alianza Editorial. Senderos de la evolución humana.
• Lucy, el fósil que reescribió la historia de la evolución humana
• Australophitecus afarensis (Smithsonian)
• Cambio climático motivó la evolución humana

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