Human Evolution

Human Evolution

What is a Hominid?

• Modern humans & our direct and indirect ancestors after our lineage split from the chimpanzee

• Until recently, earliest hominids were dated between 3.5 and 2.4 mya & placed in the genus Australopithecus

• In last few years, time range of Australopithecus pushed back to 4.2 mya, distribution expanded to include regions outside E. and S. Africa

• New finds from 4.5-7 mya are thought to be hominids that predate Australopithecines, although their status is debated

Hominid Sites• Earliest fossil hominid sites are in Africa

• They now span the latest Miocene to the early Pleistocene from about 6-7 mya to about 1.6 mya

•The major groups of sites are:1. Ethiopia = Middle Awash valley & Hadar (Australopithecus afarensis)

2. Kenya = Lake Turkana

3. Tanzania = Olduvai Gorge

4. South Africa = various sites in limestone caverns centered around Sterkfontein

What Makes A Hominid? - Bipedalism

• Primary feature distinguishing hominids from other hominoids is walking erect on two legs – erect bipedalism

• Adaptations for bipedalism in the the partial skeleton of “Lucy,” an australopithecine ( 3.2 mya) clearly seen in the hip, spine and leg bones

Why did bipedalism become the primary adaptation of hominids?

Carrying behavior

Reduction of overall heat stress - facilitates heat loss through convection by exposing body to air currents, only humans have sweat glands that produce moisture to cool body

Most energy efficient way to travel long distances

Allows for better vision in open environments & defensive action against predators by freeing hands to throw objects

Evidence for Early Bipedalism• The record of bipedalism is most graphically preserved in the fossilized footprints at Laetoli, Tanzania!

• 3.6 million year old tracks left by 2 individuals were uncovered in volcanic ash by Mary Leakey (1978-79)

• Footprints were left by 2 australopithecines in damp volcanic ash of Laetoli

• In the Laetoli trail, prints of the 2 individuals can be seen walking away from us

• Notice how close the tracks are!

Laetoli Footprints• Laetoli footprints

clearly show that the creatures who made them were fully bipedal

• Big toe hardly diverges from the rest of the foot, unlike in apes

• Gait = “heel-strike” followed by “toe-off” – the way modern humans walk

• In this reconstruction, the 2 early hominids, identified as Australopithecus afarensis, walk bipedally across an open ash field produced by an erupting volcano

• Region is wooded, but here trees are absent and the volcanic ash, wetted by a light shower of rain formed a flat shallow layer in which footprints are deeply implanted

• Footprints filled up with yet more ash, and were thus preserved

• Footprints reveal that, even at this early stage of human evolution, our ancestors walked upright with striding gait very similar to our own

Laetoli Reconstruction

Orrorin: Earliest Evidence for Walking on Two Legs?How far back in time does the record of bipedalism extend?• Fossils from Turgen Hills in

Kenya have been dated to about 6 mya

• Include: upper portion of a femur, lower portion of the humerus, some lower jaw fragments, & teeth

• Arm bone = virtually identical to that of a chimpanzee

• Femur = more human-like,most important for showing adaptations for walking on 2 legs

• Was Orrorin a direct human ancestor, or close to the common ancestor of chimps and humans?

Ardipithecus: Earliest True Hominid?• Between 4.5 and

5.5 mya from the Middle Awash valley site

• Remains very fragmentary: limb bones, toe bones, jaws & teeth

• Straight toe bones suggest it may have been bipedal

• Ardipithecus & Orrorin are candidates for last common ancestor of chimps & humans

• Each have mosaic of features seen in later hominids & modern chimpanzee

IT MAY WELL BE THAT THE LAST COMMON ANCESTOR OF CHIMPS AND PEOPLE HAD A MIX OF FEATURES: SOME RETAINED IN HUMANS, OTHERS RETAINED IN CHIMPS!

Hominid Evolution

Australopithecus anamensis

Turkana region of Kenya

Dated to 4.2-3.9 MYA

Probably walked upright

Teeth covered with enamel much thicker than that of Ardipithecus, so diet may have had hard-to-chew foods

•Lived roughly 4 mya

• Only a few anamensis fossils have been found

• Those shown here include: jawbone & part of the front of the face, parts of an arm bone (radius), fragments of a lower leg bone (tibia)

Australopithecus anamensis

Australopithecus afarensis

Eastern Africa 3.9 – 3.0 MYA Pelvis and leg bones resemble

modern humans in some ways Sexually dimorphic in body size May have been adept at tree

climbing, based on curvature of finger and toe bones

Ape-like features: Small brain case – averaging 430

cc Prognathic (jutting out) face U-shaped palate vs. the

parabolic shape of modern humans

Reduced canines

Australopithecus afarensis

A. afarensis Skeleton - Lucy

Paranthropus aethiopicus

Eastern Africa, dated 2.7-2.3 MYA Note widely flaring zygomatic arches (bones arching around side of skull

to join below eyes, forming cheeks). Prominent sagittal crest – largest ever discovered in human lineage – and cheek teeth are correspondingly large. Adaptations for heavy chewing

Small cranial capacity (410 cc) and prognathic face Black skull exhibits features more like A. afarensis

Paranthropus aethiopicus

Paranthropus boisei (KNM-ER 406)

Ethiopia, Kenya and Tanzania; dated 2.2 – 1.2 MYA Absolutely largest teeth found in any hominid species; referred to as

hyper-robust due to massive molar and premolar teeth Skull has extremely broad, short face with flaring zygomatic arches

(cheek bones), relatively small brain, pronounced sagittal crest in males Skull and dental features = adaptations for heavy chewing



Paranthropus boisei (OH 5)

Kenyanthropus platyops

Kenya, 3.5 to 3.2 MYA Ancestral features:

small ear canal more like that of chimps, A. anamensis & Ardipithecus

shape & small size of braincase

Derived features: relatively flat face and small molars

Importance: flat, human-like face appeared early in evolution alongside range of other facial forms; not outcome of progressive, linear evolution

Australopithecus africanus

3.3 – 2.5 MYA in Transvaal region of South Africa

First australopithecine to be described (1924)

More globular cranium & slightly higher ratio of brain to body size

Teeth & face appear less ancestral (reduced in size relative to earlier forms, face less prognathic)

May represent 2 species or one very sexually dimorphic species

Proportions of arm to leg lengths may be more ape-like than in A. afarensis

Australopithecus africanusAustralopithecus africanus

Taung Child STS 71 – Most complete A. africanus skull

Partial skeleton of A. africanus discovered in 1950s

Ms. Ples – best known A. Africanus cranium – front view

Ms. Ples – best known A. Africanus cranium – lateral view

Paranthropus robustus South Africa, 2.0-1.0 MYA

Short, broad face with deep zygomatic arches & large temporal fossa. Larger individuals (males?) have sagittal crests

Very large cheek teeth covered with thick enamel

Wear patterns on teeth indicate herbivorous diet of harder, more resistant, & perhaps smaller food items than A. africanus

Lived in secondary grasslands near rivers and wetlands

Paranthropus robustus

Paranthropus robustus

This recently discovered (1999) skull of Paranthropus robustus from Drimolen, South Africa is the most complete found so far

Note remarkable similarity

Note the remarkable similarity to the newly found but older skull of

A. africanus (above), reinforcing notion that

the 2 species represent a single ancestor-

descendent lineage

(reversed image)image)

Hominid Evolution

Australopithecus garhi

2-3 mya = lack of adequate hominid fossil record in Eastern Africa

2.5 mya = Recently discovered Hominid cranial & dental remains in Ethiopia

Recognition of new species of Australopithecus

Descended from Australopithecus afarensis = a candidate ancestor for early Homo


Discovered at Bouri, Ethiopia in 1999

Found with butchered bones – maybe oldest toolmaker

Oldest stone tools are also dated to about 2.5 mya

Ancestral to Homo? Right place at the right time…

One surprise in the A. garhi skull was enormous back teeth, instead of smaller ones seen in later Homo species (Video Image/UC Berkeley)


Not yet clear whether A. garhi falls on the direct lineage leading to modern humans, or

lies on an extinct side-branch


• EARLIEST BUTCHERS: Signs that hominids scraped and smashed animal bones, like this tibia, 2.5 mya

• Antelope tibia shaft has been shattered, smashed, and cut by stone tools

• The earliest documented percussion marks made by hominids, presumably extracting fatty marrow from these bones


Rise of the genus Homo

• Earliest come from the same African sites as Australopithecus

• Most date between 2.4 and 1.8 mya

• Homo habilis means “handy man”

• Growing consensus that there may be 2 or more species of Homo by 2 mya

“Homo habilis” (KNM-ER 1813)

East Africa, 2.4-1.6 MYA 3 traits define H. habilis

as transitional species: Expanded cranial

capacity (590-710 cc) Reduced post-canine

tooth size A precision grip,

provides anatomical basis for tool-making

One of many problems = obtaining accurate brain volume estimates from crushed and/or distorted skulls

May represent 2 or more species

“Homo habilis” (KNM-ER 1813)

“Homo rudolfensis” (KNM-ER 1470)

East Africa, 2.2-1.8 MYA

Distinguished from other specimens of early Homo by:

Slight supra-orbital brow ridge across the forehead with no trough behind

Less prognathic & longer face, squared off maxilla

Discovery of Kenyanthropus, similar facial features, have led some to see link between “platyops” and “rudolfensis,” representing a distinct hominid lineage – placing “1470” in the new genus as “Kenyanthropus rudolfensis”

“Homo rudolfensis” (KNM-ER 1470)

Hominid Evolution

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Human Evolution