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 A105 Human Origins and Prehistory
Lecture 8

Lecture 8: Bipedalism and History of Palaeoanthropology

I.  Bipedalism

Advantages of Bipedalism 
Frees hands
Efficient at walking speed
Helps animal see further
Stay cool in hot environment

Disadvantages of Bipedalism 
Slower than quadrupedalism at running speed
Running uses twice the energy most quadrupeds spend
Further from the ground -- harder to see some resources, walk in wooded areas
Less stable due to higher center of gravity

Bipedalism Hypotheses

We can identify many hypotheses, in five main “groups.”
1. Carrying
2. Protection/Predator defense
3. Female provisioning
4. Efficiency of locomotion
5. Feeding

1.  Carrying Hypotheses
Propose bipedalism evolved to free hands in order to carry items: 
    Darwin: carry tools, weapons; he proposed this was after large brains 
          evolved.
    Carry infants: helpless infant must be carried.
    Carry meat home from scavenging/kill sites.
Problems: Darwin: big brains appeared well after bipedalism
                 Carry infants: Hairlessness, helpless infants uncertain
                 Meat carrying: Assumes efficiency; chimps carry but are not bipeds.
 
 

2.  Protection/Predator Avoidance Hypotheses
    Dart, others: Vigilance Hypothesis.  Locate predators.
    Throwing Hypothesis: Throw stones, spears at enemies.
    Threat Hypothesis: animal looks larger.
Problems: Dart, Throwing: Can stand to throw; doesn't explain habitual bipedalism
                 Vigilance: Not all habitat was grassland; standing doesn't help you see further in a wooded environment.
                  Threat: threat would become ineffective if animal always bipedal; threats in social setting only benefit dominant, adult animals.
 

3.  Female Provisioning Hypothesis
    Lovejoy. Loss of estrus, long maturation of infants force males into long-term 
        bond (monogamy) with females, who they guard.
       Ensure monogamy by provisioning female, so she reproduces more often.
       Carry food back to female.
Problems: Male cannong guard while collecting food.
                 Assumes female stays in small area -- but why couldn't she collect also?
                 Chimp dry habitat foods hard to carry
                 Monogamy goes with low dimorphism; early hominids highly dimorphic
                 Predicts bipedalism efficient from the first.

4.  Long Distance Travel Hypothesis
Proposes that as habitat dried out, trees further apart.
    Hominids had to spend more time walking between trees.
    Humans more efficient than chimps walking in such settings.
Problems: Again, predicts efficient walking from the first.

 Heat Stress Hypothesis
        Related to the previous idea.
        Heat is very dangerous to brains.
        Bipeds expose less surface area to the direct sunlight
        They are raised into cooler air.
Problems: Predicts maximum efficiency (generate less heat)
                 Predicts slender body build
                 Environment not uniformly hot/dry: rainy season, shade trees
                 Even other savanna animals not bipedal

5.  Feeding Hypotheses
Propose that bipedalism evolved primarily as a feeding behavior: Recall, primates 
          spend 50% of time getting food.
    Jolly: Seed-eating Hypothesis. Gelada baboons (savanna-living) as analogue.
            Parallels between them, hominids:
            Reduced canines (allow easier grinding) and retracted face.
            Expanded, thick-enameled molars.
            Erect trunk and improved dexterity to pick seeds quickly while standing.
Problems: Early hominids lived in woodlands, not just savanna
                 You can harvest seeds while sitting
                 Arboreal anatomical features in early hominids not explained

Postural Feeding Hypothesis
     Hunt (here at IU) has proposed a model based on anatomical comparisons        between A. afarensis and chimpanzees.
   Watched when chimpanzees were bipedal.
    Though bipedalism is relatively infrequent, 75% of the time chimps were bipedal was during feeding. No other context was more than 4% of the remainder.
   Chimps use an arm to stabilize, so it is arm-hanging bipedalism.
   Happened when feeding on small fruits in small trees.
   Thus, bipedalism is said to have evolved posturally, not as a locomotor behavior.
Problems: Would bipedalism have occurred often enough to be selected for?

How can we decide?
Functional morphology -- look at anatomy of early hominids
Comparative anatomy -- what chimps, humans have in common likely to be present in common ancestor AND early hominids
Environmental evidence -- just what WAS the habitat like?

II.  Early Palaeoanthropological Discoveries 

  Even after natural selection became more widely accepted, human fossils were
 still difficult for many people to accept.  The first humanlike fossils began to appear in the 1700s, and were often  dismissed as frauds or pathological (diseased) modern people.  But their numbers increased, and it became difficult to ignore this evidence. 

    A. First accepted human fossils, Cro-Magnon, France, 1868. 
                       An accidental find made by railroad workers, but excavated scientifically (by 1800s standards) and in full public view.  Increased credibility. 
                       5 fossil skeletons, extinct animal bones, and stone tools were found.  The skeletons looked modern, but were more robust than living people. 
 
 

                   B. Eugene Dubois, Indonesia, 1891. 
                      Dubois wanted to find early humans, and since theory of the day predicted that  they would be found in Asia, that's where he went. 
                      Found a skull cap, 1000cc, and a humanlike femur close to it. Skullcap had  big brow ridge, looked more primitive than Cro-Magnon. 
                      Dubois concluded that his find, which he named Pithecanthropus erectus, was an intermediate between apes (gibbons) and humans.  This was widely criticized, and eventually he recanted, agreeing with his critics.  Today we call specimens like his Homo erectus. 

                   C. Piltdown Fraud, "discovered" 1911, England. 
                     Matched Darwin's prediction that the brain should evolve before other human characteristics. 
                      Skull fragments estimated at 1400 cc brain size,  mandible chinless, face prognathic -- ape-like face. 
                      It turned out that the skull parts belonged to a 3,000 year old human, and the jaw belonged to an orangutan, stained, filed, and broken to look like they went together. 
                     Though there were doubters from the start, the fraud was not conclusively proven until the 1950s, which gave discoverers of genuine fossil hominids much trouble getting their finds accepted.