Biology

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Bipedalism

Considering we evolved from athletic arboreal primates, why are we so poorly adapted for life in the trees, weak, slow, and just overall awkward?

Bipedalism

First major transformation of the human body

Cataglyphis bicolor

Why be bipedal?

First major transformation of the human body

Carrying hypothesis, or tool use

Aquatic ape, or lookout

Thermoregulation hypothesis

Climate change hypothesis

“Man alone has become a biped; and we can, I think, partly see how he has come to assume his erect attitude, which forms one of his most conspicuous characters. Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to act in obedience to his will… But the hands and arms could hardly have become perfect enough to have manufactured weapons, or to have hurled stones and spears with a true aim, as long as they were habitually used for locomotion and for supporting the whole weight of the body, or, as before remarked, so long as they were especially fitted for climbing trees... If it be an advantage to man to stand firmly on his feet and to have his hands and arms free, of which, from his pre-eminent success in the battle of life, there can be no doubt, then I can see no reason why it should not have been advantageous to the progenitors to have become more and more erect or bipedal. They would thus have been better able to defend themselves with stones or clubs, to attack their prey, or otherwise to obtain food. The best built individuals would in the long run have succeeded best, and have survived in larger numbers.” – Charles Darwin

Spandrel: characteristic that is a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive selection.

Exaptation: characteristic that evolved to serve one function but subsequently may serve another (ex. feathers for thermoregulation, hollow-bones for increased speed, but both coopted for flight).

Must avoid “just so” stories

Climate change hypothesis

Apes spend 4X more energy to walk the same distance (2-3km vs 8-12km)

How we became bipedal

Compare modern bipedal and quadruped apes to infer capabilities of extinct species

Human

Chimp

The Pelvis

The Spine

The Foot

  • Heel pad for striking the ground
  • Then stiffen the arch to push upwards and forwards at the end of the stance with our shorter robust big toe (that can be hyper-extended).

When walking upright…

Chimp

Chimp

Human

Human

Evolutionary History of Hominini (Gray 1825)

Our results indicate that both hominin and modern great ape femora evolved in different directions from a primitive morphology represented by some fossil apes. Orrorin appears intermediate between Miocene apes and australopiths in shape space.

Ardipithecus ramidus aka “Ardi”

Brain size, Front teeth, arms, hands, legs and feet are similar to African apes, and likely spent most of the time in trees…

...but walked upright on the ground.

Ardi

Lucy

Laetoli footprints in northern Tanzania discovered by Mary Leakey and Paul Abell (1978) date back 3.6 million years. Two Au. afarensis individuals walked side-by-side along volcanic ash, preserving their footprints. They had an almost indistinguishable gate to modern humans.

What to eat

“You are what you’d rather not eat”

–Daniel Lieberman

El Niño

“You are what you’d rather not eat”

–Daniel Lieberman

Underground storage organs (USOs)

Diet

  • Larger thicker molars for eating tougher items like stems and leaves
  • Shorter snout, forward-placed cheekbones and more vertical faces which position the chewing muscles to produce higher bite forces
  • Smaller canines to better chew tougher more fibrous food

The combination of their locomotion, environment (partly wooded habitat), and diet, provides insights as to why the first hominins became bipedal

Huge molars, and temporalis muscle so large that some species possessed a sagittal crest and zygomatic arch displaced to the side to accommodate the muscle running underneath and larger site to attach the masseter muscle. More massive jaws and supporting bone pillars and sheets protect the jaw from deformation and damage (hence the wide faces).

Females 1.1m tall 28-35kg

Males 1.4m tall 40-50kg

What to do for food?

Oldowan tool culture

Acheulean tool culture

Dawn of the Ice Age

The First Humans

Australopithecus Homo habilis

Long-distance walking

Turning a liability into a strength

Hunting and gathering

Born to run

Why do we have cravings?

Which do you crave?

5X more calories
(equal weight)*

*p74

A bit more challenging to acquire than veggies

*

How to kill this without being killed?

Fastest human ever recorded

27.8 mi/hr

Average house cat

30 mi/hr

“…humans exhibit a significant bias toward

slow-twitch fibers in their skeletal muscle with measurements ranging from 69.2 ± 11.7% to 52.6 ± 7.9%. This is in contrast to 31.5 ± 11.4% in chimpanzees.”

“…hominin lineage experienced a decline in maximum dynamic force and power output … in response to selection for repetitive, low-cost contractile behavior”

The endurance jogging pace of humans is already less costly but is also just fast enough to force quadrupeds into their most costly form of locomotion.

https://www.toledoblade.com/attachment/2013/11/24/Toledo-Magazine-Outdoors-Hunting-On-The-Run.pdf

Efficient and endurance running species… Cool story

Mismatch