Anthropology 150

When and where did the first members of our family develop?

What were the selective pressures responsible for their origin?

The First Hominids

Defining hominids

 “hominid” is a term used to describe the line leading to H. Sapiens sapiens after the lineage split from gorillas and chimpanzees

 Unlike other apes, hominids are normally and customarily bipedal (walking on two legs)

 Other traits of hominids include increased brain size relative to weight, increased manual dexterity, and reduced sexual dimorphism.

 The hominid line split from chimps sometime between 8 mya and 6 mya

Problem Fossils

 Sahelanthropus tchadensis –probably a chimp, not a hominid

 Orrorin tugenensis – hard to tell, but perhaps it should be

classified as Ardiptihecus

Sahelanthropus tchadensis

Michel Brunet

Sahelanthropus tchadensis Some Consider Toumai to

be a hominid. Others

think it dates before the

ape-human split

An early gorilla or

chimp?

Early Hominids

 Ardipithecus kadabba (5.8-5.5 mya)

 Ardipithecus ramidus (4.4 mya)

 Australopithecus anamensis (4.2 mya)

 Australopithecus afarensis (3.8-3.0 mya)

 Australopithecus garhi (2.5 mya)

Later Australopithecines

 Gracile Australopithecines

 Australopithecus africanus

 Australopithecus garhi

 Robust Australopithecines

 Australopithecus (Paranthropus)

robustus

 Australopithecus (Paranthropus)

boisei

 Australopithecus (Paranthropus)

aethiopicus

Ardipithecus ramidus

 A 4.4 million year old ape-like

hominid from the Aramis site in

Ethiopia Its teeth are small, have

thin enamel, and are

intermediate in form between

those chimpanzees and later

australopithecines such as A.

afarensis. This suggests that

Ardipithecus had not yet adapted

to a tough, abrasive diet that

required heavy chewing. A milk

molar is especially primitive and

closely resembles a chimpanzee

tooth.

“Ardi” is the most complete specimen of Ardipithecus ramidus. Dated to around 4.4 mya, the fossil shows a bipedal ape That still spent considerable time in the trees.

Ecology of Ardipithecus

 Animal remains found in

association with

Ardipithecus suggest that it

lived in the forest. If this is

true, it is of great interest

because the selective

pressures that early

Pliocene apes faced in the

savannah are usually seen

as instrumental in the

evolution of bipedalism

and other hominid traits.

The relationship between Ardipithecus

and later hominids  It could be a sister species that was

contemporaneous with the group that gave rise to later hominids.

 Or, it may be the form at the base of the hominid lineage that gave rise to the later Australopithecines and eventually to modern humans. Chart by Tim White, UC Berkeley

Australopithecus anamensis

 Fossil remains

discovered at Kanapoi

and Allia Bay in Kenya

Dates: 3.9-4.2 million

years ago. Remains at

present consist of about

two dozens dental,

cranial and postcranial

specimens. The

cranium cannot be

reconstructed.

The Morphology of A. anamensis

 Has a mosaic of primitive and derived features that are intermediate between those of Ardipithecus and later Australopithecus afarensis.

 The shape of fragments of tibia (shin bone) shows that the A. anamensis was bipedal.

 The knee and ankle joints are more similar to those of a modern humans than they are to that of a chimpanzee.

 They show adaptations for absorbing the stresses generated during bipedal walking.

The knee of A. anamensis

 Buttressing of proximal end of the tibia

suggests an adaptation for absorbing forces

associated with bipedal locomotion

Chimp A. anamensis Modern human

Australopithecus afarensis

 Remains found at Laetoli

in Tanzania and Afar in

the Hadar region of

Ethiopia. Faunal,

Potassium argon and

geomagnetic dating

techniques indicate that

the Laetoli remain are

about 3.8 million years

old and the Afar 3.5-2.6

million years old

A. afarensis dental traits Reduction in the size of the incisors.

Reduction of canine interlock allows freer transverse motion than in apes or ramapithecines

Canines become integrated into grinding complex.

Long, narrow, straight-sided dental arch

There is a broadening of the post canine teeth and a buttressing the mandibular symphysis.

These features appear to be adaptations for heavy chewing

A. afarensis Cranial Features

 The face is prognathic

(forward projecting) and

the cheek bones heavily

developed A small brain

case and a cranial

capacity (380 and 530

cm3) is slightly larger

than that of modern

chimpanzees (282-300

cm3 )



Advanced Dental Traits of A. afarensis

 The molars and premolars are large

 Molars and premolars have thicker enamel than

Ardipithecus.

 In these respects they are much more like the teeth of the

later Australopithecines than they are those of modern apes.

Sexual Dimorphism in A. afarensis  Males appear to have been much larger

than those of females. Height estimates

for females are 3.5-4 feet and for males as

high as 5 feet. This amount of sex

difference is comparable to that found in

gorillas or baboons. There are marked sex

difference in the size and shape of the

canines

0

1

2

3

4

5

6

A.

afarensis

A.

africanus

A.

robustus

A. boisei H.

sapiens

H e

ig h

t (f

e e

t)

males

females

A. afarensis Locomotion

 Laetoli footprints indicate

striding bipedalism Afarensis

post-cranial remains provide

clear indications of bipedalism

A. Afarensis Hip-bones

 The ilium is short and broad

 It flares outward at the hip

 Femur angles in as in modern

humans

 Limb proportions appear

similar to those of humans

How much time did A. afarensis spend in the trees?

 A. afarensis has some ape-like features

such as curved fingers with attachments

for powerful flexor muscles. This evidence

of strong grasping capabilities suggests to

some that A. afarensis spent time feeding

and resting in trees. Others believe that A.

afarensis primarily a ground dweller. This

view is reinforced by the recent discovery

of a relatively short human-like A.

afarensis humerus that differs markedly

from those of African apes.

A. afarensis phylogenetic relationships: Ancestral to all later hominids?

Implications of A. afarensis brain size and bipedalism

 The presence of an ape

sized brain in fully bipedal

hominid such as A.

afarensis indicates that the

development of bipedalism

preceded expansion of the

brain.

 This is an example of

“mosaic” evolution

Australopithecus garhi  Recently discovered 2.5 m.y.a. hominid as a

site in Ethiopia

 Associated with crushed animal bones

 In the right place and dates from the right time to have given rise to later humans

 The skull of A. garhi looks very different from A. africanus, surprisingly primitive with a protruding apelike face.

 Looks like a scaled-up afarensis except its brain stayed small with a capacity of about 270cc





A. garhi phylogenetic relationships

The Gracile Australopithecines

Australopithecus africanus  Initially described by Robert Broom

as a new genus, Pleisanthropus

 Remains of this comparatively

lightly built or "gracile" species of

australopithecine have been

recovered from sites in South Africa

(Sterkfontein, Makapansgat and

Taung)

 Its presence in East Africa is subject

of dispute

Robert Broom

Australopithecus africanus dental features

 No sectorial canine

function, canines only wear

on the tip not on the back

edge as in A. afarensis No

gap (diastema) between

canines and premolars

Lacks a sectorial lower

premolars. premolars are

similar in shape and used

for grinding Molars are

bigger than in A. afarensis

•A. afarensis

•A. africanus

Cranial Features of Australopithecus africanus

 Brain case is small and

rounded Considerable

facial prognathism with a

"dished out" facial profile

The Brain of Australopithecus africanus

 Average cranial capacity around 450 to 500 cc

 Foramen magnum is located under vault for bipedalism

 Studies of internal casts of the braincase indicate an expansion of areas associated with higher cognitive functions. Whether or not lateralization was present is unclear from available material

Post-Cranial Features of Australopithecus africanus

 Pelvic and femoral

anatomy indicates full

bipedalism

 Some foot bones have

been interpreted as

indicating that the

feet were adapted for

climbing

The Robust Australopithecines

 A. aethiopicus

 A. robustus

 A. boisei

Australopithecus aethiopicus

 Most complete specimen is known as The "Black Skull"

 Heavily built, small brained australopithecine

 Found in an east African deposit that dates to around 2.5 million years ago

 This early date for a robust australopithecine has made paleontologists re-evaluate their theories of australopithecine evolution.

Australopithecus robustus

 Also know as Paranthropus robustus

 This is a heavily built species of

Australopithecus the remains of which

date later than those of the A.

africanus. It appears to have evolved

into a hyper-robust form known as

Australopithecus boisei that persisted

in Africa until as late as 1.3 million

years ago.

Sites containing robustus australopithecine remains

 South Africa (Kromdraai, Swartkrans,

Makapansgat) East Africa (Omo, Olduvai Gorge,

East Lake Turkana)

Trends in Australopithecine evolution

 Premolars loose their shearing function and take on the

grinding function of the molars The relative size of the

molars increases as grinding becomes more important

There is a reduction in facial prognathism and a

development of the cheek bones for heavy chewing

muscles Increase in brain size and perhaps internal

reorganization of the brain to accommodate higher

cognitive functions There is some evidence for an increase

in the length of the developmental period over that found

in apes

A. robustus Dental Features

 Parabolic dental arch

 Continuation of trend

toward increase in molar

size

 Thick enamel on molars

 Evidence of tooth

development indicates

rate of maturation was

intermediate between

that of modern humans

and great apes

Discovery of Australopithecus boisei

 Hyper-robust australopithecine

discovered in 1959 by Mary

Leakey at Olduvai Gorge,

Tanzania, Described a a new

genus: Zinjanthropus boisei K-Ar

dates on an overlying basalt

indicates a date of ca 1.8 mya.

A. boisei dental features

 Massive molars indicate an adaptations that involved heavy chewing. It probably had a diet similar to that of modern gorillas, heavy in vegetation such as bark and leaves.

 Reduced canines and incisors suggest foods consumed required little incisor preparation before ingestion

A. boisei cranial features

 Flat face and jaws placed under

the cranial base suggest a

masticatory adaptation that

emphasized heavy chewing

A. robustus Cranial Features

 Face is less prognathic than in

A. africanus Sagittal crest is

frequently present in males

Heavy cheek bones for

attachment of the masseter

muscle indicates heavy chewing

Cranial capacity between 500

and 540 cc is somewhat larger

than of A. africanus

Phylogenetic relationships of A. boisei

 Relatively late disappearance

suggest that it was a specialized

form that became marginalized

and eventually was driven to

extinction

 A. boisei was contemporary

with early Homo; unlike Homo,

A. boisei did not make tools and

did not have an extensive

cultural repertoire. It could not

adapt easily when confronted

with environmental changes.