Early Hominin Fossils

Transition to Hominins

• Lectures covered occurred on February 28 and March 3, 2025, as part of ANTH 1210 A01.

Oldest African Hominin Fossils

• Sahelanthropus tchadensis is dated to around 7 million years ago in Chad.
• It is considered a hominoid due to a small brain, elongated skull, sloping face, and prominent brow ridges.
• It is considered a hominin because the foramen magnum is centrally located.
• Orrorin tugenensis is dated to around 6 million years ago in Kenya.
• It was about Chimpanzee sized with small teeth with thick enamel and overall human-like features.
• It was bipedal and partially arboreal.
• The neck length between human and Orrorin femora are similar and the proximal end of the Orrorin femur fits into the hip socket to allow movement (Clark et al 2020).
• Ardipithecus ramidus is dated to around 4.4 million years ago in Ethiopia.
• It is considered a possible hominin common ancestor.
• It had hominoid dentition and other hominin body characteristics like a big toe and rigid foot, a central foramen magnum, and it was bipedal/arboreal.

First Hominins: Great Rift Valley

• First hominins in the Great Rift Valley are dated from 4 to 1 million years ago.
• These include Australopithecines (Australopithecus) and Paranthropoids (Paranthropus).
• Their features included:
  • Small canines
  • Flat and thickly enameled molars
  • Fully bipedal; Partially arboreal

Hominin Diversity

• Hominin diversity can be seen as adaptive radiation to a dynamic environment.
• Forests were giving way to grasslands and savannahs.
• Hypotheses for hominin evolution include the Savannah or Aridity hypothesis, the Turnover Pulse, and Variable Selection.

Australopithecus anamensis

• Australopithecus anamensis dates to around 4.2 million years ago in Kenya.
• It was a small bipedal hominin with a human-like lower body.
• It was also partially arboreal.
• Its teeth were similar to Au. afarensis and it is thought to be ancestral to Au. afarensis.
• They had a small brain and a protruding face

Australopithecus afarensis

• Australopithecus afarensis dates to 4-3 million years ago and fossils have been found in Laetoli, Tanzania and Hadar, Ethiopia
• Laetoli is dated to around 3.6 million years ago.
• A large number of individuals have been recovered (n=24+).
• There is clear evidence of bipedalism in Australopithecus afarensis as well.
• "Lucy" is an Australopithecus afarensis skeleton from Hadar, Ethiopia, dating to 3.2mya that is 40% complete.
• As a species, They were completely bipedal but also occasionally arboreal and retained an ape-like skull, jaw and dentition

Australopithecus africanus

• Australopithecus africanus dates to around 3-2 million years ago in South Africa.
• The Taung Child was discovered was discovered in 1924 and is around 3.5 million years old.
• Australopithecus africanus had a rounder cranium, larger brain, smaller teeth, and an S-shaped lumbar curve.
• The bone morphology of the hand suggests it may have been a possible tool maker.

Australopithecus sp.

• Au. bahrelghazali dates to around 3 million years ago in Chad.
• It may be the same species as Au. afarensis.
• Au. garhi dates to around 2.5 million years ago in Ethiopia and is possibly the first species to have used tools.

Australopithicus sediba

• Australopithicus sediba was recently discovered in 1999 in Malapa Cave, South Africa dating to ~2mya.
• Fairly complete skeletons of juvenile and adult individuals were found.
• These individuals had a mix of Australopithecine and Homo traits.
• Physical features included small cranial capacity, dentition, and long arms. The chest was more Australopithecine-like.
• They had more regular, human-like bipedal locomotion.

Paranthropoids

• Paranthropoids were present in South Africa and East Africa from ~2.7m - 1mya.
• They had a body size similar to a robust Au. africanus.
• They are characterized by their thick jaws, larger molars, massive muscle attachments and well developed sagittal crests.

Paranthropus aethiopicus

• Paranthropus aethiopicus lived in southern Ethiopia and northern Kenya, & Tanzania 2.7 – 2.3mya.
• It was contemporary with A. afarensis.
• Paranthropus aethiopicus had very robust jaws and dentition, broad/dish shaped faces, and large sagittal crests for muscle attachments.

Paranthropus robustus

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

Paranthropus boisei

• Paranthropus boisei was found in Kenya, Malawi, and the Olduvai Gorge in Tanzania and dates to 2.4 – 1.4mya.
• 'Zinjanthropus' was discovered by Mary Leakey in 1959, which is the type specimen for P. boisei.
• This was the first evidence of early hominins present in East Africa.
• Paranthropus boisei can be characterized as a hyper-robust Parathropus sp. and they possibly used tools.

Genus Homo

• Genus Homo was present in western Kenya and Tanzania, Ethiopia and South Africa.
• Members of this genus have an increased brain size, flatter face, and a similar body to Au.
• Homo habilis lived around (~2.3-1.4mya).
• H. rudolfensis lived around (~1.9-1.8mya).
• Genus Homo was contemporary with P. boisei and possibly H. erectus.

Homo habilis 'Handy Man'

• Homo habilis had a larger brain with a brain size of 630-640cc.
• Homo habilis had reduced molars and premolars with thick enamel and were likely omnivorous.
• These individuals had hand bones capable of a precision grip and a body similar to australopithecines
• They were however still partially arboreal.
• Homo habilis fossils have been found in the same layers as butchered bone and the first stone tools, however, it is unconfirmed whether they actually made and used them.

Homo rudolfensis

• Homo rudolfensis had a brain size of ~775cc.
• Additional physical features includes a longer face and larger molars/premolars.
• They had a body similar to australopithecines and were still partially arboreal.

Hominin Trends: Bipedalism

• Bipedalism trends can be seen across hominins and include the following:
  • Spinal column centered on foramen magnum
  • Spinal column S-curved to act as a spring
  • Pelvis became more bowl-shaped
  • Legs angled inward at the knee
  • Arch in the foot

Why Bipedalism?

• Bipedalism may have evolved due to adaptation to life in savannahs, advantageous in tall grass for better vision. This is related to Savannah or aridity hypothesis for hominin diversity and evolution in general.
• Bipedalism may have evolved for energy conservation, because it is easier to walk on two legs than four.
• Bipedalism may have evolved for thermoregulation for better dispersion of body heat, which can have a cooling effect, for example, the Bergmann & Allen Rules.
• Bipedalism may have evolved for adaptation, allowing free hands while body moves, facilitating efficient foraging, and tool manufacture and use.
• Bipedal walking may have occurred ~ 1mya prior to tools in archaeological record.

Enlarging the Brain

• Brain development occurred after bipedalism, starting 2 mya.
• It was selected for tool making, and group living etc.

Reducing the Face, Teeth, and Jaws

• Reducing face, teeth, and jaws happened simultaneously with increased brain size.
• Homo species have narrower faces and smaller teeth

Other Trends in Hominin Evolution

• Hairlessness and sweatiness also contributed to changes (Shook et al 2023).
• Increased dependency of infants resulted in an evolutionary advantage for physiologically immature infants.
• Social living improved survivability of young.
• A larger brain was advantageous, but made birthing more dangerous.