Adaptation and Bipedalism Flashcards
Adaptation
evolved phenotypic traits that increase an organism’s reproductive success
Adaptationism
every aspect of an organism is the product of natural or sexual selection
Ardipithecus
One of the earliest known human ancestors; lived about 4.4–5.8 million years ago and walked upright but also climbed trees.
Great Rift Valley of East Africa
- creation and disappearance of lakes and streams during hominin evolution
- series of volcanoes (helped preserve fossils)
- shift to bipedality bc of climate change
- followed by increase in brainsize
- genetics show hominins appeared in late Miocene
- key anatomical changes: pelvis, spine, teeth
Holism
principle that emphasizes understanding human societies and cultures as interconnected and complex systems, rather than isolated parts
Oldowan Tools
- Earliest known stone tools (about 2.6 million years ago)
- mostly flakes and choppers
- Used for cutting meat, breaking bones
- Made by Homo habilis and other early hominins
Orrorin tugenensis
- One of the earliest known hominins (about 6 million years ago)
- Found in Tugen Hills, Kenya
- Likely walked upright (bipedal)
- Name means “original man”
- Evidence from femur suggests early bipedality
Pelvis
- Consists of Innominate bones (Os Coxae)
○ Each Os Coxa is made up of three bone that fuse during adolesence
○ Ischium - the bone you sit on
○ Ilium - the bone you feel when you put your hand on your hip
○ Pubis - the anterior boney portion of the pubic region - Pelvis is bowl shaped
- Easier biped balance
Reductionism
- focuses on each component to understand the whole
- each part of the organism is adaptive
- holists claim the reductionists oversimplify adaptation
Apparent adaptations
may be a by-product of natural selection process
Hominin
- all the species on our side of the line between apes and humans
- all fossils since the last common ancestor with chimps
Robusticity
- strength/thickness of bones and muscles
- Robust hominins (like Paranthropus): large jaws, strong chewing muscles, thick bones
- Suggests adaptation to tough, hard-to-chew diets
Sahelanthropus tchadensis
🗓️ ~7 million years ago
📍 Found in Chad (Central Africa)
🧠 Small brain (~350 cc), flat face, small canines
🚶 Possibly bipedal (based on skull position)
- Might be the earliest known hominin
- Shows early mix of ape & human traits
- Suggests bipedalism evolved before big brains
Tool Industry
🛠️ different types of tools made by early humans and hominins over time
🗓️ Includes Oldowan (simple flakes), Acheulean (handaxes), Mousterian (Neanderthals), and others
🔄 Each “industry” shows advances in thinking, motor skills, and adaptation to the environment
- Tracks the evolution of human intelligence and problem-solving
- Links tool use to diet, hunting, and social behavior
- Helps archaeologists date sites and understand cultural shifts across human evolution
Valgus angle
📐 The inward angle of the femur (thigh bone) from the hip to the knee
👣 Found in humans and some early hominins
🦵Problems at the knee
- Key evidence for bipedalism
- Helps position feet under the body’s center of gravity for upright walking
- Seen in fossils like Australopithecus afarensis (e.g., Lucy), showing that early hominins were walking upright millions of years ago
- Distinguishes bipedal hominins from quadrupedal apes, whose femurs do not angle inward
What are the anatomical changes related to the shift to bipedalism in humans?
What are the five hypotheses that have been put forward for the development of bipedalism? Why are there five and not just one accepted hypothesis?
- Savannah/Aridity Hypothesis
- Turnover Pulse Hypothesis
- Forest Hypothesis
- Variability Selection Hypothesis
- Pulsed Variability Selection Hypothesis
Why? human evolution is complex, different fossils show different traits, constantly changing environments, traits evolved at diff times, incomplete fossil record
Savannah/Aridity Hypothesis
🏜️Suggests that environmental aridification (drying) led to the expansion of savannahs -> pushing arboreal (tree-living) ancestors to adapt by becoming bipeds
🦓 Also linked to increased faunal (animal) diversification
✅Pros:
- evidence of increased aridity between 8-6 millions years ago (ma)
- loss of trees would lead to bipedalism
❌ Cons:
- early bipedal fossils (like Ardipithecus) were found in wetter, wooded, closed environments, not dry savannahs
- suggests bipedalism may have evolved before full savannah conditions
Turnover Pulse Hypothesis
🌍 Theory that during climate change, generalist species are more likely to survive than specialists
🌱 Generalists can adapt to changing environments and access new resources, leading to speciation and diversity
✅ Pros:
- Matches with periods of extreme glacial/interglacial change
- Explains multiple species turnover events in the fossil record
- Homos are a successful generalist that fits the pattern
❌ Cons:
- Doesn’t explain specific trait evolution (e.g., bipedality, brain size)
- Limited evidence it directly affected hominin evolution
Forest Hypothesis
🌳Argues that early hominins evolved in forested environments, contradicts open 🦍Savannahs hypothesis
suggests bipedalism started as a way to move through trees (arboreal locomotion), not just on the ground
✅Pros:
- pollen evidence shows early hominins lived in closed (forested) environments
- easy bipedal (like Ardipithecus) found in forests
- some fossils show opposable big toes
- no evidence of knuckle walking in LCA
❌Cons:
- fossils preserve better in wet, forested environments, so we might just be missing dry-habitat ones
- hard to prove/disprove knuckle walking in ancient species
Variability Selection Hypothesis
🌦️Hominins evolved traits for flexibility because climate conditions changed rapidly & unpredictably
🧠 Focus isn’t on a single environments or event, but the ability to survive across many conditions
✅Pros:
- similar to Turnover Pulse Hypothesis, but more focus on hominins
- explains generalist traits (like flexible diet, mobility, and brain development)
- recognized that climate was changing faster than hominins could fully adapt
❌Cons:
- based on “lumping” species together - may oversimplify evolution
- fossil records are limited, making it hard to track precise evolutionary pace
Pulsed Variability Selection Hypothesis
🌊 Cyclical environmental changes in deep lakes of East African Rift Valley like increasing/decreasing lake sizes every ~2000 years
🧬 This drove hominin evolution by creating changing habitats, isolating populations, & encouraging allopatric speciation
✅ Pros:
- evidence supports massive lake changes
- supports idea that environmental shifts caused population separation
- helps explain diversity of hominins through isolation and adaptation
❌Cons:
- doesn’t explain which traits variability would select for
- debate continues over how much lake changes actually drove speciation
What are the earliest known tools? Which species seems to be associated with them?
- Oldowan tools
🔹 Cores – stones struck to remove flakes
🔹 Flakes – sharp-edged fragments used for cutting
🔹 Hammerstones – used to strike cores or bones - Australopithecines are associated
