Hominin Evolution: Fossils, Bipedalism & Genus Homo

• These are study notes on the transition to hominins, focusing on hominin fossils, diversity, genus Homo, bipedalism, and other evolutionary trends.

Oldest African Hominin Fossils

• Sahelanthropus tchadensis is approximately 7 million years old and was found in Chad.
• Sahelanthropus tchadensis is a hominoid with a small brain, elongated skull, sloping face, and prominent brow ridges.
• As a hominin, Sahelanthropus tchadensis had a central foramen magnum.
• Orrorin tugenensis is approximately 6 million years old and was found in Kenya.
• Orrorin tugenensis was chimpanzee-sized.
• Orrorin tugenensis had small teeth with thick enamel.
• Orrorin tugenensis had overall human-like features.
• Orrorin tugenensis was bipedal and partially arboreal.
• Ardipithecus ramidus is approximately 4.4 million years old and was found in Ethiopia.
• The Ardipithecus ramidus species is a possible hominin common ancestor.
• Aridpithecus ramidus has hominoid dentition and hominin bodies.
• Other features of Ardipithecus ramidus include a big toe, rigid foot, central foramen magnum, and bipedal/arboreal locomotion.

First Hominins in the Great Rift Valley

• Australopithecines and Paranthropoids lived in the Great Rift Valley between 4-1mya.
• Hominin dentition included small canines and flat and thickly enameled molars.
• These first hominins were fully bipedal and partially arboreal.

Hominin Diversity

• Hominin diversity may be attributed to adaptive radiation to a dynamic environment.
• Forests gave way to grasslands and savannahs, influencing hominin evolution.
• Hypotheses for hominin evolution include the savannah or aridity hypothesis, turnover pulse, and variable selection.

Australopithecus anamensis

• Australopithecus anamensis lived around 4.2 million years ago in Kenya.
• It was a small, bipedal hominin with a human-like lower body.
• This species was partially arboreal.
• Australopithecus anamensis had teeth similar to Au. afarensis.
• Australopithecus anamensis is considered ancestral to Au. afarensis.

Australopithecus afarensis

• Australopithecus afarensis lived 4-3 million years ago in Laetoli, Tanzania, and Hadar, Ethiopia.
• Laetoli provides clear evidence of bipedalism, dating back to approximately 3.6 million years.
• A large number of individuals of this species have been discovered (n=24+).
• "Lucy" is an Australopithecus afarensis fossil that is 40% complete and dates back to 3.2mya in Hadar, Ethiopia.
• Australopithecus afarensis was completely bipedal with occasional arboreal behavior.
• This species retained an ape-like skull, jaw, and dentition.

Australopithecus africanus

• Australopithecus africanus lived in South Africa around 3-2 million years ago.
• The Taung Child, discovered in 1924, is an Australopithecus africanus fossil from about 3.5 million years ago.
• This species had a rounder cranium, larger brain, and smaller teeth similar in shape to humans.
• Australopithecus africanus had an S-shaped lumbar curve and bone morphology of the hand suggesting possible tool-making capabilities.

Australopithecus species

• Au. bahrelghazali lived approximately 3 million years ago in Chad, which broadened the Au. afarensis distribution.
• Au. garhi lived approximately 2.5 million years ago in Ethiopia.
• It is suggested that Au. garhi possibly was one of the first species to use tools.

Australopithicus sediba

• This species was discovered in 1999 in Malapa Cave, South Africa, dating back approximately 2 million years.
• Fairly complete skeletons of juvenile and adult individuals have been found.
• Australopithicus sediba had mixed Australopithecine and Homo traits.
• This species had a small cranial capacity, specific dentition, long arms, and a chest more like Australopithecines.
• Australopithicus sediba exhibited more regular, human-like bipedal locomotion, especially indicated by the leg bones and pelvis.

Paranthropoids

• Paranthropoids lived in South Africa and East Africa.
• They existed from approximately 2.7 million to 1 million years ago.
• Their body size was similar to robust Au. africanus.
• Paranthropoids had thick jaws, larger molars, massive muscle attachments, and well-developed sagittal crests.

Paranthropus aethiopicus

• Paranthropus aethiopicus lived in southern Ethiopia, northern Kenya, and Tanzania between 2.7 – 2.3mya.
• This species was approximately contemporary with A. afarensis.
• Paranthropus aethiopicus had a very robust jaw size and dentition, broad/dish-shaped faces, and large sagittal crests for muscle attachments.

Paranthropus robustus

• Paranthropus robustus was found in Kromdraai Cave, South Africa, dating back 2m - 1mya.
• This species is the type specimen for Parathropus.
• Paranthropus robustus had a robust jaw and teeth, and a likely vegetarian diet.

Paranthropus boisei

• Paranthropus boisei lived in Kenya, Malawi, and the Olduvai Gorge in Tanzania between 2.4 – 1.4 million years ago.
• "Zinjanthropus" was discovered by Mary Leakey in 1959 and is identified as the type specimen for P. boisei.
• The find provided the first evidence of early hominins present in East Africa.
• Paranthropus boisei is a hyper-robust Parathropus species.
• Possible tool use has been suggested for this species.

Genus Homo

• Genus Homo includes species found in western Kenya and Tanzania, Ethiopia, and South Africa.
• This genus is characterized by increased brain size, a flatter face, and a body similar to Australopithecus.
• Homo habilis lived approximately 2.3-1.4 million years ago.
• Homo rudolfensis lived approximately 1.9-1.8 million years ago.
• The genus Homo was contemporary with the P. boisei species and possibly H. erectus.

Homo habilis

• Homo habilis, known as 'Handy Man', had a larger brain with a volume of 630-640cc.
• This species had reduced molars/premolars and thick enamel.
• Homo habilis was omnivorous.
• The hand bones were capable of precision grip.
• Homo habilis had a body similar to australopithecines and was still partially arboreal.
• Fossils have been found in same layers as butchered bone and the first stone tools; though, it is unknown whether they made use of them.

Homo rudolfensis

• Homo rudolfensis had a larger brain which measured about 775cc.
• This species had a longer face and larger molars/premolars.
• Homo rudolfensis had a body similar to australopithecines.
• This species was still partially arboreal.

Hominin Trends: Bipedalism

• Bipedalism includes a spinal column centered on the foramen magnum, a S-curved spinal column, a bowl-shaped pelvis, legs angled inward at the knee, and arches in the foot.

Why Bipedalism?

• Bipedalism, as an adaptation to life in savannahs, led to a better vision in tall grass. It is related to the Savannah or aridity hypothesis for hominin diversity and evolution.
• Bipedalism is more energy conservation: easier to walk on two legs than four.
• Bipedalism is a thermoregulation adaptation for better dispersion of body heat and a cooling effect.
• Adaptation: free hands while body moves: Efficient foraging and Tool manufacture and use
• Bipedal walking appeared approximately 1 million years before tools in the archaeological record.

Enlarging the brain and reducing the face and teeth.

• Brain development occurred after bipedalism, starting 2mya
• It was selected for tool making, group living etc.
• There was a simultaneous increase in brain size as the face became flatter.
• There was also a reduction in tooth size, flatter molar cusps and thicker enamel.

Other trends in hominin evolution

• In included Hairlessness and sweatiness.
• There was an increased dependency of infants due to the evolutionary advantage for physiologically immature infants.
• There was then social living which improved survivability of young.
• Larger brains were advantageous, but made birthing more dangerous.