Early Hominins Flashcards
What are the 2 unique features that allow identification of hominins from the fossil record
Small monomorphic canine
Bipedalism (different types)
Who are the early
hominins?
Give the date of each
~ 7 Myr
•Sahelanthropus tchadensis
> 5 Myr
•Orrorin tugenensis
•Ardipithecus ramidus kadabba
~ 4 Myr
•Ardipithecus ramidus
Give features of the S. tchadensis skull from a superior view
SO torus wider than any other part of skull
Postorbital constriction
Give features of the S. tchadensis skull from a side view
Orthognatic face
Large SO torus
Similar to younger hominins than apes
Give features of the S. tchadensis skull from an inferior view
U shaped dental arcade
(cf U shaped arcade of ramipithecus who was originially thought to be hominin based solely off this feature)
Small molars
Small canine but not as small as later hominins (smaller than miocene apes)
FM – relative forward central position – between apes and later hominins
Give features of the S. tchadensis skull from a posterior view
Long low skull v small 350cc (similar to chimps)
Strong nuchal crest all the way round the occipital without any sagittal crests
How does the cranial capacity of Sahelanthropus compare to apes
Small brain – 360-370cc – within range of extant
What fossils do we have of Sahelanthropus
fossils include 1 partial skull, 3 fragmentary jaws, 3 teeth (one incisor, lower canine, and upper M3) from >6 individuals
How does the nuchal plane and crest of sahelanthropus compare to apes and hominins
Nuchal plane – less steep than in chimps but more steep than later hominins
Nuchal crest – slopes downwards – similar to OH5 boisei skull
What is important about Sahelanthropus’ Angle between base of skull and face
quad and bipedal primates have face facing forward so have small angle compared to humans which have an angle between FM and face of 103 degrees (almost right angle) – Sahelanthropus has angle of 95 degrees also close to right angle – evidence of bidepalism
What is interesting about the location of Sahelanthropus fossils
Found very far from east Africa with little in between – were there none between or are the bones only exposed in certain places eg theh rift valley?
Were there any other hominins in Chad?
1 other hominin in Chad - Australopithecus bahrelghazali
How was the Rift Valley created
Rift Valley created by geographical faulting as Africa was separated into the Nubian and Somali plates
What fossils do we have of Orrorin tugenensis (6)
- isolated teeth
- fragments of mandibles
- 3 fragments of proximal
femur, one preserving the full
femoral head - fragment of distal humeus
- distal thumb phalanx
- other small fragments
What are the key cranial features of the fossils of Orrorin
Thick dental enamel – like human, unlike other apes
Dentition is more ape-like – small and incisor-molar ratio differ from apes but otherwise very similar – M4 is apelike in character and canine is large, triangular and pointed at the apex with narrow anterior shallow groove
What is the postcranial evidence of bipedalism in Orrorin (7)
Femur:
• A large femoral head that is anteriorly twisted
• Elongated femoral neck
• A femoral neck that is compressed antero-posteriorly
• Inferiorly thickened femoral neck cortex (seen in humans - superior thickening seen in apes)
• An obturator externus groove
• A medially projecting lesser trochanter
• A well-developed gluteal tuberosity
What has been claimed about Orrorin based off the apparent changes to its functional anatomy?
Why has this claim not been further explored recently
Change in functional anatomy to make femur weight bearing
Originally claimed Homo evolved from orrorin not Australopithecus – NOT embraced by other researchers
Bones are stored in a secret location and have not been studied since
Where was Ardipithecus kadabba found
When did it exist
Middle Awash,
Ethiopia
Asa Koma Member (5.54 – 5.77 Myr)
Kuseralee Member (~5.2 Myr)
Discuss the dentition of kadabba
mosaic of human and ape features – lateral incisor similar in size to later hominins
Molars not increased in size as in later hominins
Canine – large and pointed at apex with posterior wear facet similar to apes – C/P3 honing complex
Discuss the upper limb of Kaddaba
2 humeri differ in size – sex dimorphism
Humerus and ulna are apelike in morphology
Clavicle – more robust than chimp
Phalanges show similar curvature to afarensis with deep lateral fossae for insertion of flexor digitorum superficialis – strong hand flexing musculature
Discuss the feet of kadabba
4th pedal phalanx – proximal articulation shows strong plantar curvature with a dorsally oriented articular surface (associated with toeing off while walking which is unique to bipeds)
This phalanx is 300ky younger than rest of material
What are the Hominin apomorphies of Ar. Kadabba? (2)
- Lower canines with developed distal tubercles and expressed mesial marginal
ridges - Derived morphology of the foot phalanx suggesting bipedal walking
When/ where did ramidus exist
Dated to ~ 4.4 Myr
From the site of Aramis, Middle Awash,
Ethiopia
What was the diet of Ard. ramidus
C3 - arboreal diet
Give the key features of ramidus’ skull
Less prognathic face
Intermediate FM
Short nuchal plane, but not as short as in chimpanzees
Teeth: thin dental enamel like chimp
Canine triangular with pointed apex and large but not as large as chimps
Molars are similar to chimps/ smaller than australipithecines
Narrower incisor row
Molar row length reduced cf. austr but longer than chimps
• Dental arcade ~ chimps
Give the key features of ramidus’ pelvis
What must be remembered about this evidence
Ilium shortened in height cf apes
Sagittally oriented (different to apes)
Has an incipient anterior inferior iliac spine – prominent in humans but not present in apes
Ape like ischium
Pelvis is crushed so limits reliability
Discuss the upper limb of ramidus
Radius:tibia – similar length – consistent with above branch quadruped
Primitive hands – see slide
Moved using palm of hand
Primitive grasping foot see slide
What did Lovejoy suggest about ramidus?
Little sex dimorphism from reduced variation and small canines – suggests little male-male competition – indicative of pair bonding – indicative of greater social adhesion
Suggests ovulatory crypsis – reduced visual signalling in oestrus
Thus provisioning would have been necessary and bipedalism to carry supplies
(Lovejoy, 2009)
What can we infer about the locomotion of ramidus
Efficient arboreal animal ith distinct clambering adaptations in hands and feet that could engage in bipedality terrestrially
Nature of bipedalism unclear
What was the habitat/ environment that ramidus lived in
Habitat: Woodland with forest patches
Humid and cooler than today
10 to 25% C4 plant diet
Isotopic values similar to contemporaneous Pliopapio
(arboreal, small, baboon‐like) and Tragelaphus
(woodland browser) – ie, arboreal‐dependent diets,
no grass‐eating animals.
What is the cranial capacity of ramidus
~ chimpanzees (300‐350 cc)
What is the near complete skeleton of ramidus
ARA‐VP‐6/500: 45% of (female) skeleton
estimated weight – 50 kg
estimated height – 120 cm
Describe the hands and wrists of ramidus
Primitive hands – short, grasping
thumbs and elongated fingers
• Wrist: arboreal palmigrady
Describe the feet of ramidus
• Primitive, grasping foot, with fully
abductable hallux
• Expanded base of 2nd metatarsal
• Robust plantar substructure
What are 4 key questions surrounding bipedalism as an adaptive change in locomotion
• Early or late?
• In a single change, or incrementally?
• Locomotor advantage or indirect benefit?
• Consequences for behaviour, technology, thermoregulation and life
history
What are the 3 different ways to be bipedal
- Obligate vs Facultative
- Lomotor vs Postural
- Habitual vs Occasional
How does the line of gravity change from quadrupeds to bipeds
passes just in front of the knees in humans
passes down halfway up the thorax of a quadrupedal ape
What do the forearm bones of Sahelanthropus suggest about its locomotion (3)
Which study is this from
curved ulnae suggest arboreality (curvature arises from repeated use of brachialis)
anterior facing trochlear notch (as in other hominins) - a proximally facing notch would suggest a posterior olecranon, favouring the action of triceps in quadrupedal terrestrial walking. However this is only inferred from proximal epiphyses - a humerus is needed for greater confidence
waisted trochlear notch - arboreality would require fully developed notch BUT this trait could not be selected for and independent of walking
Guy et al 2020
What can we learn from the Sahelanthropus femur
What study is this from
curved anteroposteriorly with an anteverted neck, as seen in later hominins and is important in modern humans to reduce horizontal turning movements of the femoral head
BUT anteverted neck is also seen in many quadrupeds
Guy et al 2020
Which groove is found in Orrorin’s femur, indicating bipedality?
Why is this important?
What is another important feature of Orrorin’s femur?
obturator externus groove
obturator externus muscle is required for adduction, flexion and external rotation of the thigh and is critically important in humans when the femur is hyperextended, such as when standing or running.
suggests Orrorin was involved in bipedal activities where the hip was hyperextended
a medially projecting lesser trochanter, which differs from the posterior position of the trochanter seen in australopithecines and is closer to the lesser trochanter seen in human femurs - insertion of iliopsoas (hip flexor) - suggests similarity of flexion in Orrorin and humans
BUT
australopithecines have posterior lesser trochanter - is Orrorin a more direct ancestor or is the medial postition of the trochanter just convergence?
also has well developed gluteal tuberosity for insertion of gluteal muscles (vital for bipedal walking) - insertion is more similar to humans than to bonobos
What arises from the anterior inferior iliac spine?
rectus femoris (hip flexor)
How is the ilium adapted in ramidus to suggest bipedal walking
ilium shifted to become more anterior and basin-like, away from the vertical ilium in Pan. This shift allowed the gluteal muscles to be positioned in a more advantageous position for centering ramidus’ centre of mass while walking
Give 10 key features of the functional anatomy of bipedalism
- Relative position of the head to the spine, involves a forward shift in the position of the foramen magnum
- Shape of the rib cage
- Increase in vertebral body
width from cervical to lumbar - “S” curvature of spine
(especially lordosis) - Changes in pelvis size (from long and narrow to short and wide) and shape (bowl-like)
- Valgus angle
- Knee lock
- Adduction of big toe
- Arches of the foot
- Flat phalanges
What are the pieces of evidence of bipedalism in hominins between 7-4mya
7.0 Myr – position of foramen magnum in
Sahelanthropus suggesting incipient changes to the
centre of gravity
- 0 Myr – femora of Orrorin tugenensis
- 8 Myr – pedal phalanx of Ardipithecus kadabba
- 4 Myr – different (?) bipedal adaptations in Ardipithecus ramidus
4.2 Myr – tibia of Australopithecus amanensis
showing bipedal adaptations
Give evidence of bipedalism 4-1mya
3.7 Myr – Laetoli footprints in volcanic ash
3.0 Myr – pelvic and other adaptations in
Australopithecus afarensis and all later
australopithecines
1.6 Myr – changes in body proportions (shorter arms than legs) and shape of rib cage in Homo erectus
Were the earliest hominins bipeds (5)
Sahelanthropus tchadensis: probably – based on relative position of foramen
magnum and size and position of neck/sternal muscle attachments
Orrorin tugenensis: probably – based on analyses of the shape of the femur, as
well as the size and position of pelvic‐femoral musculature insertions
Ardipithecus ramidus ‐ ? Unclear: position of foramen magnum suggests
bipedality; pelvis is crushed and reconstructions difficult; femur is missing critical
proximal end; foot is fully grasping, indicating arboreal adaptations.
Australopithecus anamensis – yes
Australopithecus afarensis ‐ yes
What are the key differences between australopithecine
bipedalism and our own bipedal adaptations?
Relative proportions of fore and hindlimbs Orientation of shoulder joint Curvature of phalanges Shape of rib cage Shape of pelvis
What kind of biped were australopithecines?
Australopithecines were habitual bipeds, but their ape‐like arms suggest maintenance of a degree of arboreality
What did bipedalism look like before the Pleistocene
Bipedalism: probably diverse until the Pleistocene
What do arboreal adaptations to the shoulder look like (3)
globular humeral head
cranially oriented glenoid
narrow scapula
What do arboreal suspensory adaptations to the wrist
reduced styloid process
distally displaced pisiform
What are ape features of the pectoral girdle/shoulder joint that enhance mobility?
- Features that increase the mechanical advantages of the deltoid muscle (main abductor at shoulder)
- Features that enhance the action of the two
main upward rotators of the scapula (trapezius and
serratus anterior
What are features of the shoulder joint that enhance its
mobility in multiple planes of movement (4)
- Wide, round and flat glenoid fossa
- No projecting supraglenoid tubercle
- Greater and lesser tubercles and inter‐tubercular groove of the proximal humerus placed well below the level of the humeral head
- Humeral torsion
What are features that enhance the mechanical advantages of the
muscles that raise the arm/ Features that increase the mechanical advantages of the
deltoid
laterally projecting acromion process
of scapula
• position of insertion of deltoid
muscle along humerus shaft
How does the pectoral girdle differ between humans and apes
- SCAPULA: Relatively small supraspinous fossa with a
relatively large infraspinous fossa in humans - SCAPULA: orientation of glenoid fossa
- CLAVICLE: no lateral cranial twist in clavicle in humans
Give 2 interesting features about A. africanus scapulae
• Cranially‐oriented glenoid fossa
• Some features of the scapula more similar to orang‐utans than either
humans or African apes
What is the importance of understanding extent of australopithecine bipedality?
‘If australopithecines and other early hominins were partly arboreal, postcranial changes seen in Homo erectus/ergaster might be due to a shift towards full bipedality. On the other hand, if the ancestors of Homo were already fully bipedal, the changes in the Homo skeleton must have been due to other factors, such as an increase in efficiency due to walking longer distances or running, and/or selection for body size, throwing, tool use, and transport, or any number of other possibilities’
Give 6 indirect explanations of bipedalism
Tool‐making Prey capture Predator avoidance Vigilance Threat displays Thermoregulation
What were the habitats of early hominins
Mostly woodland, riverine, with evidence for other habitats ranging from more closed forest, to tree covered grassland, bushland, and grassland
What are the thermoregulatory advantages of bipedalism
decreased solar radiation (only head and shoulders exposed superiorly)
more wind
What amount of time is needed to be spent on the ground for bipedalism to be beneficial
Bipedalism is more efficient on the ground but
more costly in the trees
Bipedalism is beneficial when 60% of time is
spent on the ground (i.e. even with 40%
arboreal activity)
Evolved in the context of greater day ranges
What are the causes, constraints and conditions leading to bipedalism
CONSTRAINTS:
• Water availability
• Speed
CAUSE:
• Locomotor energetics
• Free hands
• Thermoregulation
CONDITIONS:
• More open environments
• Change in resource base
• Increased predation
What are the evolutionary consequences of bipedalism
SHORT‐, MEDIUM‐ AND
LONG‐TERM CONSEQUENCES :
• Free hands (tool manufacture)
• Thermoregulation (loss of hair, sweating)
• Human walking/running
(dispersals, new foraging strategies)
• New subsistence (larger body size, encephalisation, life‐history)
Why is bipedalism so key to human evolution
Bipedalism became the adaptive precondition for the evolution of a series of traits that shape later hominin evolution ‐ encephalisation, tool use, climatic tolerance and geographical dispersals
