Anthropology Questions 2

Lecture #15—Bipedalism and Hominin Origins

Paleoanthropology

• παλαιός – Palaeos—“old” or “ancient”

• ἄνθρωπος – Anthrōpos—“human”

• λογία – Logia—“study of” or “discourse” or “word”

• The study of the hominin fossil record with the aim of reconstructing the behavior and ecology of our hominin ancestors and relatives

http://www.berkeley.edu/news/media/releases/2004/03/images/chart.gif

Hominins

http://www.berkeley.edu/news/media/releases/2004/03/images/chart.gif

Becoming Human

http://www.sciencenews.org/view/download/id/52262/name/Standing_tall

Becoming Human

• We will trace the evolution of the suite of characteristics that define humans over the last 6my.

• Human evolution is mosaic, i.e. not all of the traits that define humans evolved at the same rate or same time.

– E.g., bipedalism evolved much earlier than big brains.

Increasing Body Size

AVERAGE WEIGHT AVERAGE STATURE

SPECIES males females females as

% of males males females

females as

% of males

Australopithecus afarensis 92 lbs 64 lbs 64% 4 ft 11 in 3 ft 5 in 70%

Australopithecus africanus 90 lbs 66 lbs 73% 4 ft 6 in 3 ft 9 in 83%

Paranthropus robustus 119 lbs 88 lbs 74% 3 ft 9 in 3 ft 3 in 87%

Paranthropus boisei 108 lbs 75 lbs 69% 5 ft 4 in 4 ft 1 in 91%

Homo habilis 114 lbs 70 lbs 83% 5 ft 9 in 5 ft 3 in 92%

Homo sapiens 144 lbs 119 lbs 83% 5 ft 9 in 5 ft 3 in 92%

Adapted from http://anthro.palomar.edu/hominid/australo_2.htm

Decreasing Dental and Skeletal Robusticity

http://media.cleveland.com/metro/photo/teethjpg-47bb1717d9c1eb2b_large.jpg

Increasing Brain Size

http://psyc.queensu.ca/ccbr/Vol3/Figures/Tattersall/tattersall_f1.jpg

Stone Tool Use

http://ucsdnews.ucsd.edu/graphics/images/2007/11-07ChimpToolsBIG.jpg http://anthromuseum.missouri.edu/minigalleries/hand axes/1985-0235-handaxe-oldowan.shtm

http://anthromuseum.missouri.edu/minig alleries/handaxes/70-15-handaxe- acheuleanflake.shtm

http://anthromuseum.missouri.edu/minigalleries/handaxes/1980-1013-handaxe- acheulean.shtm

http://www.boneclones.com/images/ms-100-set-lg.jpg

Increasing Sophistication

Being Human

Culture Cooperation

http://www.humanrights.gov.au/sites/default/files/content/human_rights/UD HR/Amanda_Lim_cooperation_2003.jpg

What are fossils? Taphonomy

http://paleo.cortland.edu/tutorial/Taphonomy%26Pres/Taph%26Pres%20Images/pathwayscolor.GIF

Dead Remains

Exposed Remains

Living Organisms

Fossil Record

Buried Remains

Biostrainomy (sedimentary process)

Delayed Burial Biological Reworking

Delayed Burial

Death

Immediate Burial Diagenesis

http://humanorigins.si.edu/sites/default/files/imagecache/medium_banner_520px_height/images/banner/2.3.2-7_wo_bars.jpg

Traditional Classification

Family

Superfamily Hominoids

Hylobatids Pongids Hominids

Newer Classification

Tribe

Subfamily

Family

Superfamily Hominoids

Hylobatids Hominids

Pongines Gorillines Hominines

Panins Hominins

Why the Change?

• Taxonomies reflect evolutionary relationships

– Molecular data show that the great apes, including humans, are all very closely related to each otherall hominids in the new classification

– Molecular data also show that the two Pan species (bonobos and chimpanzees) are more closely related to humans than they are to gorillasPan spp. and humans (and our fossil ancestors) all in the subfamily hominines

Hominins

• Humans and our fossil ancestors since the split with the bonobo-chimpanzee lineage about 6mya

– Early Hominins

– Australopithecus spp. and Paranthropus spp.

– Humans—Homo spp. (spp. is an abbreviation for multiple species)

Characteristics of the Hominins

• BIPEDALISM

• Increasing brain size

• Reduction in overall robusticity (dental and skeletal)

• Increasing reliance on culture

• Tool-making

Mosaic Evolution

• Human evolution can be characterized as mosaic

– Not all of the human characteristics evolved at the same time or at the same pace

– Independent evolution of different functional systems:

• Locomotion—bipedalism and the locomotor anatomy

• Cranio-facial region—reduction in molar and canine size, increase in brain size

• Bipedalism is the most important defining characteristic of the hominins

Biocultural Evolution

• Human evolution is biocultural

– The mutual, interactive evolution of human biology and culture

• Biology makes culture possible and culture influences the direction of biological evolution

• Ratcheting Effect

Why Was Bipedalism Adaptive for the Earliest Hominins?

• Why did bipedalism evolve?

– Think about some of the possible explanations before moving the next slide…

• LCA (last common ancestor) of humans and the bonobo-chimpanzee lineage lived throughout equatorial AfricaWHY did the lineages diverge?

Why Was Bipedalism Adaptive for the Earliest Hominins?

• We have to think about the environment because the environment is the selection pressure.

• What was the environment like?

The Savanna Mosaic

The Savanna Mosaic

• 10-5mya, dense tropical forests of East Africa became fragmented, leading to savanna mosaic – ecologically mixed environment with grasslands,

shrubbery, clumps of discontinuous woodlands – drought-resistant plants evolved in these areas—

tubers and roots (USOs), well-protected fruits (melons)

• Apes in this environment were restricted to the forested areas – Note the overlap with extant (living) ape populations

in the next slide

pubs.usgs.gov/of/1999/of99-535/pliofebsst.gif

Distribution of Living Apes Today

Forest Coverage ~7mya

Savanna where first hominins evolved

Still forested area where LCA of bonobos and chimpanzees was evolving

• In East Africa, such as around the Great Rift Valley, early hominin sites pop up all over the place in savanna environments, usually close to water sources – Benefits of this environment:

• Great spots for plant foods and opportunistic small prey catching

– Challenges to this environment: • Drought and predation • Distance between food sources—clumped trees

– Travelling for calories • Bonobos, chimps, and gorillas have relatively small day

ranges (1-3km) • Early hominins had day ranges up to 7kmneed to travel

further in savanna mosaic to get enough food • Modern foraging populations travel up to 12km/day in

similar environments

• So, populations of the LCA of humans and Pan spp. In those in forested environments evolved into Pan (bonobos and chimpanzees), relying heavily on fruit

• But populations in savanna environments relied on long-distance travel to meet nutritional requirements

– This pressure, along with others, led to the evolution of bipedalism in these populations

– More efficient for long-distance travel and reduced stress on shoulder joints

Other Factors?

• Thermoregulation—heat dissipation

• Carrying things

• Seeing over tall grasses

• Feeding posture

Likely a combination of factors but efficient travel probably a BIG contributor

Anatomy of Bipedalism

Position of Foramen Magnum

Modern human Chimpanzee

Position of the Foramen Magnum

• Hole in the bottom of the cranium where the spinal cord comes out of the brain

– Directly under the skull in humans—head is vertical for upright posture

– More posterior (toward the back) in chimps

– Straight out the back in full quadrupeds like coyotes

Curvature of the Spine

Spinal Curvature

• S-curve in humans—two curves, cervical and lumbar—weight carried directly over knees and feet along the mod-line of the body

– Center of gravity pushes down toward pelvis

• Flatter curve in chimps—one curve, weight distributed onto limbs when knuckle-walking

– Center of gravity pushes down toward ground when quadrupedal, weight thrust forward when standing erect

Upper Limbs and Hands

Upper Limbs and Hands

• Humans have shorter arms than legs; chimpanzees have longer arms than legs

• The human humerus (upper arm bone) is not weight-bearing

• Humans have shorter fingers than chimps; chimps have very long fingers—brachiation

Rounded Pelvis

The Pelvis

• Humans have a short, rounded pelvis—center of gravity, weight distribution, basin to hold organs – Short illium—gluteal muscles attach to the bottom,

balanced weight distribution moving from one foot to the other (next slide shows muscle attachments)

– Narrower pelvic outlet makes for more efficient bipedalism

• Chimps have an elongated pelvis – Long, blade-like illium—cannot balance on one foot

for long

Homo A. afarensis Pan

Angle of the femur

Angle of the Femur

• Humans have an inward angle to femur

– brings knees into midline of body

– knees bend forward

– Australopithecus afarensis is more similar to humans—hominin ancestor

• Chimps have an outward angle to femur

– knees drop directly below hips

– knees bend outward

Human Skeleton

Chimpanzee Skeleton

Feet and Toes

• Human foot v. chimp foot

– Humans have shorter toes—non-grasping

– Humans have a non-divergent, non-opposable hallux (big toe)—used in propulsion

– Humans have a longitudinal arch—heel to toe

• Arches are for shock absorption and cushioning

Conflicting Selection Pressures through Human Evolution

• Biocultural Consequences of Bipedalism

• Compare openings in female and male pelves (plural of pelvis)…

Evolutionary Tradeoffs: The Pelvis, Bipedalism, and Babies

Human Female

Human Male

Chimpanzee

Female

Photo by David Jordan

Conflicting Selection Pressures

• Evolution of Bipedalism – Bipedalism evolved ~6mya and defines the

hominin lineage

– Bipedalism is more efficient with a narrower pelvic outlet (about the angle of the femur and weight distrubution)

• Evolution of Big Brains – Later in our evolution, ~2mya, brains started

getting a lot bigger

– Bigger brains = Bigger heads

Where Do Babies Come From?

Photo by David Jordan

Babies No Babies

Babies

Human Female

Human Male

Chimpanzee

Female

What happens when brains (and heads) get bigger but are trying to come out of a narrow

opening?

Photo by David Jordan

Baby

Giving Birth

• Head goes through first

– It goes sideways

• Widest part of pelvis is side-to-side when the pubic symphysis is relaxed, widest part of baby head is front- to-back

• Cranial Sutures Unfused—babies’ heads can smoosh on the way out

• How does the baby’s head get sideways?

– Someone has to help

Photo by David Jordan

Baby

Shoulders

Giving Birth

• Head goes through first

• Shoulders are next—shoulders are widest side-to-sideneed to turn that baby around

– Someone helps

– Most babies are born face down

Evolutionary Tradeoffs

• Narrower pelvis for more efficient bipedalism means narrower pelvic outlet for giving birth – add to that a big brain and you have a recipe for

ouch

• Chimpanzee females do not need assistance giving birth, humans do

• Conflicting Selection Pressures – Bipedalism  Narrower Pelvis

– Bigger Brains  Bigger Heads

The Solution?

• Humans are undercooked at birth – human brains are 25% adult size at birth, chimpanzee

brains are 40%

• Increasing Sociality • Assistance in birth

• Long Dependency—long developmental period, social learning

• Cooperative breeding

• Biocultural Evolution – Feedback system—Biology influences culture

influences biology…

Probably more complicated…

• A recent study suggests that babies are not only born at 9 months so their heads can get out but also because of the metabolic costs to mom

– It would cost too much nutritionally to keep supporting the offspring

http://www.npr.org/blogs/health/2012/08/28/1601 74038/why-does-pregnancy-last-9-months

http://humanorigins.si.edu/sites/default/files/imagecache/medium_banner_520px_height/images/banner/2.3.2-7_wo_bars.jpg

The Earliest Hominins

• Hominin Sites

– Mostly East Africa

– Each of these areas is discussed in the book

Hominin Sites

General Characteristics of Early Hominins

• Cranium – Cranial capacity like Miocene apes (450cc) – Prognathic face (jutting out of mid-face) – Intermediate foramen magnum

• Dentition – Large canines, shearing premolars – Thin molar enamel

• Post-Cranial Skeleton – Long arms and fingers—still hanging out in trees – Femur bipedal-like—travelling on the ground

Earliest Hominins

Sahelanthropus tchadensis 7mya,Chad

Orrorin tugenensis 6mya, Tugen Hills, Kenya

Ardipithecus 5.8-5.2mya

Ardipithecus ramidus 4.4mya Ethiopia

A few optional videos about bipedalism

• From a CARTA symposium a couple years ago

http://www.youtube.com/watch?v=DCPQK7Odh UY

http://www.youtube.com/watch?v=YTv5KhUtbx 0&feature=relmfu

http://www.youtube.com/watch?v=I2jo719D3Z w&feature=relmfu