Chapter 7 The First Bipeds

Chapter Preview What Is the Anatomy of Bipedalism, and How Is It Preserved in the Fossil Record? Who Were the Australopithecines, and What Were They Like? What Role Did Bipedalism Play in Human Evolutionary History?

Controversial Finds: The Earliest Bipeds? One of the main derived traits that mark hominids as different from apes is bipedalism.

Sahelanthropus tchadensis dated to 6-7 million years ago found in Chad (Central Africa) brain size of approximately 350 cc possibly bipedal (due to position of the foramen magnum underneath the skull) Ape-like features = brow ridges & hominid-like features = small canine teeth

Sahelanthropus tchadensis

Orrorin tugenensis dated to about 6 million years ago found in Kenya (East Africa) fossils include fragmentary arm and thigh bones, lower jaws, and teeth possibly adapted to both bipedality and tree climbing the femur indicates possible bipedality

Orrorin tugenensis

Ardipithecus ramidus dated to 5.8-4.4 million years ago found at the Aramis site in Ethiopia (East Africa) its teeth are small, have thin enamel, and are intermediate in form between those chimpanzees and australopithecines its deciduous molar (baby tooth) resembles a chimpanzee tooth possibly bipedal

Ardipithecus ramidus

The Anatomy of Bipedalism and Its Preservation in the Fossil Record

Why is bipedalism so important in distinguishing hominids from apes? - Remember, the term “hominid” = bipedal ape - Bipedalism has a number of anatomical consequences - These traits can be used to identify bipedalism in hominoid fossils and thus, can be used to distinguish apes from hominids

Why is bipedalism so important in distinguishing hominids from apes? - We now recognize the bipedalism is the first human- like trait to evolve among apes - Prior to the 1950s, this was not the case. Scholars had assumed that larger brains had developed first. - The best example of the persistence of this kind of thinking is the Piltdown Hoax.

Piltdown Hoax Many scientists of the 1920s believed the ancestor to humans had been found in the Piltdown gravels of Sussex, England, in 1910. The Piltdown specimens consisted of a humanlike skull and an apelike jaw that seemed to fit together though the joints connecting the two were missing. They were discovered along with the bones of other animal species known to be extinct.

Piltdown Hoax The Piltdown Hoax was accepted as ancestral to humans because it fit with expectations that the missing link would have a large brain and an apelike face. No one knows how many of the scientists who argued that this specimen was the missing link, were involved in the forgery.

Anatomy of Bipedalism Forward position of the large opening in the base of the skull (foramen magnum) Series of curves in the spinal column Basin-shaped structure of the pelvis Angle of the lower limbs from the hip joint to knees Shape of the foot bones

Foramen Magnum Bipedalism can be inferred from the position of the foramen magnum, the large opening at the base of the skull. Note its relatively forward position on the human skull (left) compared to the chimp skull (right).

Skeletons and Locomotion: Humans and Chimps

Upper Hip Bones and Lower Limbs Compare the upper hip bones and lower limbs of (from left) Homo sapiens, Australopithecus, and an ape. The similarities of the human and australopithecine bones are indicative of bipedal locomotion.

The Bipedal Gait The bipedal gait is really more like “serial monopedalism” or locomotion using one foot at a time through a series of controlled falls. Note how the body’s weight shifts from one foot to the other as an individual moves through the swing phase to heel strike and toe off.

Laetoli: The Fossil Record of Bipedalism Fossilized footprints were preserved in volcanic ash at the 3.6-million-year-old Tanzanian site of Laetoli. The foot of a living human fits inside the ancient footprint, which shows the characteristic pattern of bipedal walking.

The Australopithecines

Australopithecus afarensis dated to 3.5-2.6 million years ago found in Tanzania and Ethiopia (East Africa) males (5 ft. tall) appear to have been much larger than those of females (3.5-4 ft. tall) More evidence of sexual dimorphism = males have larger canines Possessed a protruding face with a low forehead

Lucy – an A. afarensis fossil Australopithecus afarensis Post-cranial anatomy and the Laetoli footprints indicate striding bipedalism ape-like features = curved fingers (adapted for arboreal life?) Lucy – an A. afarensis fossil

Lucy A 40% complete skeleton of “Lucy,” indicates that australopithecine ancestors were bipedal. This adult female was only 31/2 feet tall. Paleoanthropologists reconstructed the entire skeleton from the remains that were discovered.

Lucy’s Child – a juvenile A. afarensis fossil Australopithecus afarensis had a small brain case and cranial capacity (380 and 530 cc) - similar to that of modern chimpanzees Lucy’s Child – a juvenile A. afarensis fossil

Sexual Dimorphism in Canine Teeth Australopithecus afarensis should marked differences in the size of canine teeth among males and females.

Trunk Skeletons Compare the trunk skeletons of modern human, A. afarensis, and chimpanzee. In its pelvis, the australopithecine resembles the modern human, but its rib cage shows the pyramidal configuration of the ape.

Upper Jaws Upper jaws of an ape, Australopithecus, and modern human show differences in the shape of the dental arch and the spacing between the canines and adjoining teeth.

Australopithecus vs. Piltdown The Taung child, discovered in South Africa in 1924, was the first fossil specimen placed in the genus Australopithecus. In the early 20th century scientists were expecting the ancestors to humans to possess large brains and an apelike face and to originate from Europe or Asia rather than Africa.

Later Forms of Australopithecus The earliest forms preserve a number of features that indicate an ape-like ancestor. By 2.5 million years ago, new forms with a larger chewing apparatus and more massive head appeared, although brain size remained stable.

Australopithecine Fossil Locations Australopithecine fossils have been found in South Africa, Malawi, Tanzania, Kenya, Ethiopia, and Chad. Most have been found in the Great Rift Valley of East Africa. Why?

Australopithecine Fossils and Rifting In the Miocene, the Eurasian and African continents made contact at the eastern and western ends of what is now the Mediterranean Sea. As these land masses met, “rifting” occurred, gradually raising the elevation of the eastern third of Africa. The dryer climates that resulted may have played a role in human evolution in the distant past and provided excellent geological conditions for finding fossils.

Fossil Sites in South Africa Many of the fossil sites in South Africa were limestone caverns connected to the surface by a shaft. Over time, dirt, bones, and other matter that fell down the shaft accumulated in the cavern, becoming fossilized.

Species Of Australopithecus Location Dates Features/Fossil Specimens A. anamensis Kenya 3.9–4.2 mya Oldest australopithecine A. afarensis Ethiopia Tanzania 2.9–3.9 mya Well represented in fossil record. A. africanus South Africa 2.3–3 mya First discovered, gracile, well represented in fossil record.

Species Of Australopithecus Location Dates Features/Fossil Specimens A. aethiopicus Kenya 2.5 mya Oldest robust australopithecine (“Black Skull”) A.bahrelghazali Chad 3–3.5 mya Only australopithecine from central Africa

Species Of Australopithecus Location Dates Features/Fossil Specimens A. boisei Kenya 1.2–2.3 mya Later robust form co-existed with early Homo (“Zinj”) A. garhi Ethiopia 2.5 mya Later East African australopithecine with humanlike dentition A. robustus South Africa 1–2 mya Robust co-existed with early Homo

Kenyanthropus platyops dated between 3.5 and 3.2 million years ago found in Kenya (East Africa) skull has a broad flat face and small teeth the brain size is similar to that of australopithecines the lack of prognathism (like Sahelanthropus) may indicate its close relation to genus Homo it may be another australopithecine

Kenyanthropus platyops

Gracile Australopithecines Members of the genus Australopithecus possessing a more lightly built chewing apparatus. Best known example = Australopithecus africanus

Robust Australopithecines Several species within the genus Australopithecus, who lived from 2.5 and 1.1 million years ago in eastern and southern Africa. Known for the rugged nature of their chewing apparatus (large back teeth, large chewing muscles, and a bony ridge on their skull tops for the insertion of these large muscles).

Robust Australopithecines Also known by the alternative genus - Paranthropus.

Gracile and Robust Australopithecines Compared

Robust Australopithecines: The Sagittal Crest A crest running from front to back on the top of the skull along the midline to provide a surface of bone for the attachment of the large temporal muscles for chewing.

Foot Bones of Later Australopithecines Drawing of the foot bones of a 3- to 3.5-million-year-old Australopithecus from Sterkfontein, South Africa, as they would have been in the complete foot.

Australopithecus garhi In 1999, Ethiopian paleoanthropologist Y. Haile Selassie discovered fossil material placed into the new species Australopithecus garhi.

Australopithecus garhi a possible ancestor of genus Homo possessed an arched dental arcade and a ratio between front and back teeth more like humans and gracile australopithecines

Our Possible Evolutionary Relationship to Australopithecines

Later Australopithecines and Early Homo The gracile and robust forms of australopithecus appear to have developed in response to deforestation and the progressive drying of the environment These forms were ideally suited for plant foraging Predators posed a major problem and the yielding sticks, stones and bones as defensive weapons (manuports) may have paved the way for stone tool making among early Homo

Later Australopithecines and Early Homo When two closely related species compete for the same niche, one will out-compete the other, bringing about the latter’s extinction = law of competitive exclusion. early Homo and later Australopithecus did not compete for the same niche and appear to have co-existed for 1.5 million years

The Genus Homo

Homo habilis dated to 2.5 – 1.8 million years ago found in East and South Africa believed to have mastered the Oldowan stone tool industry there is ample fossil evidence that H. habilis was a major staple in the diet of large predatory animals; so it was probably not a hunter

Homo habilis its brain size is slightly larger than australopithecines, 500 - 650 cc its face is less prognathic than australopithecines

Homo habilis Compare the feet of apes (left), H. habilis (middle), and modern humans (right). How are they similar; how are they different?

Homo habilis Compare the digits of apes (left), H. habilis (middle), and modern humans (right). How are they similar; how are they different?

The Role of Bipedalism in Human Evolution

Bipedalism: Drawbacks Makes an animal more visible to predators. Exposes the soft underbelly. Interferes with the ability to change direction instantly while running.

Bipedalism: Drawbacks Quadrupedal chimpanzees and baboons are 30 to 34% faster than bipeds. Frequent lower back problems, hernias, hemorrhoids, and other circulatory problems. Consequences of a serious leg or foot injury seriously hinders a biped and they are an easy meal for some carnivore.

Reasons for Bipedalism A way to cope with heat stress. Allowed them to gather food and transport it to a place of safety for consumption. Mothers were able to carry their infants safely. They could reach food on trees too flimsy to climb. Allowed them to travel far without tiring.

Reasons for Bipedalism: The Savanna Model Food and water were easier to spot – Savanna Model. More likely to spot predators before they got too close for safety. Hands freed from locomotion provided protection by allowing them to brandish and throw objects at attackers.

Reasons for Bipedalism: The Carrying Model Hands freed from locomotion provided protection by allowing them to brandish and throw objects at attackers.

Reasons for Bipedalism: The Sexual Selection Model Advanced by Owen Lovejoy Hands freed from locomotion provided males the ability to carry more food to attract more mates Hands freed from locomotion provided females with another means to carry and protect infants.

Explaining Bipedalism: A Word of Caution When trying to explain the evolution of bipedalism we have to beware of teleological thinking (i.e. confusing a consequence of bipedalism for its cause). Review the previous hypotheses explaining bipedalism can you find evidence of teleological thinking? Can you think of counter-evidence from modern primate behavior?

Bipedalism and Environmental Change Since the late Miocene, the vegetation zones of Africa have changed considerably. The presence of bipedalism in the fossil record without a savannah environment indicates that bipedalism appeared without any particular adaptive benefits at first, likely through a random macromutation.

The Aquatic Ape Theory: For Class Discussion Argues that bipedalism developed within estuaries and water holes where hominids congregated to avoid predators. Since apes have lower levels of body fat, those who were bipedal did not drown and were able to move to deeper water. Can you spot any problems with this hypothesis?