The Valgus Angle: Hominin Traits Answers

Here are the answers to the problem set for this week on the evolution of the four key human traits: Bipedalism, Tool use, Dentition changes and Cranial Capacity changes. As required in this assignment, I have ordered them starting with the trait that appeared in the fossil record first and ended with the most recent trait to have evolved in humans. I have not put them in essay format to make it easier to process the information and to use as notes if you wish.

First Trait: Bipedalism

1. How is this trait expressed in humans compared to non-human primates?

· Placement of the foramen magnum forward, under the skull instead of toward the back. This is a trend over time with gradual changes visible in the fossil record, with the foramen magnum moving further and further under the skull.

· Secondary curvature in the lumbar region of spine. This is unique to bipedal organisms.

· Changes in the pelvis to a more bowl-shape. This reflects changes in muscle action but also the need to support internal organs, particularly the developing fetus in pregnant females.

· Development of the valgus angle which places the center of gravity of the body over the supporting foot during a walking stride for greater efficiency.

· Development of the platform foot, with the big toe becoming larger and moving in-line with the other toes (no opposable toe).

· Loss of locomotor ability in the hand; lighter bones and muscle markings, used only for manipulative purposes.

2. When did this trait appear in the fossil record?

Bipedalism evidence definitely appears by 3.5 MYA, but other evidence suggests that it appeared as early as 6 MYA.

3. What human ancestors first exhibited this trait?

Australopithecus afarensis (Lucy) definitely exhibited signs of being bipedal (3.5 MYA) but researchers who found Ardipithecus ramidus (5 MYA) offer fossil evidence of bipedalism in that individual as well and now we have Orrorin at 6 MYA.

4. Identify and describe 3 ways this trait was visible early in the fossil record.

a. Presence of a valgus angle

b. Loss of locomotor function in the hand

c. Foramen magnum has moved forward under the skull

d. Big toe moves in-line with other toe/loss of opposition

e. Pelvis becomes more bowl-like

f. Lumbar region of spine develops secondary curvature

g. Legs become much longer than arms

5. Was bipedalism dependent or influenced by any of the other three traits?

Since bipedalism seems to have appeared first in the fossil record, it is unlikely that it was dependent upon or influenced by the other traits.

Second Trait: Dentition

Chimpanzee, A. Afarensis, Modern human

1. How is this trait expressed in humans compared to non-human primates?

· The shape of the jaw changes from more elongated to shorter and wider, more parabolic or arched.

· Teeth in humans tend to be smaller than non-human apes.

· Enamel in humans is thicker than non-human apes.

· Canines are reduced in size and don’t exhibit sexual dimorphism

· Reduction in snout. This reflects a reduction in the use of the sense of smell. Shorter snout will shorten the jaw, leading to the parabolic shape.

2. When did this trait appear in the fossil record?

Dentition changes are evident by 4 MYA.

3. What human ancestors first exhibited this trait?

Australopithecus anemensis (4.2 MYA) and Australopithecus afarensis (3.5 MYA) definitely exhibited signs of dentition changes.

4. Identify and describe 3 ways this trait was visible early in the fossil record.

a. Thicker enamel

b. Reduction in size of the molars

c. Arched or parabolic jaw shape—didn’t appear definitely until H. habilis (2.5 MYA).

d. Reduction in snout

e. Reduced emphasis on smell

f. Occasional absence of third molars (wisdom teeth)

5. Were dentition changes dependent or influenced by any of the other three traits?

Dentition changes were influenced by two factors: Diet and the reduction in the reliance of the sense of smell in exchange for an emphasis on sight. As the sense of smell became less important, the human face became flatter which affect region of the nose and the mouth. Changing these areas affect the shape of the palate, leading to a parabolic jaw. Changes in diet would have changed the selection pressure on the size of teeth and the thickness of the enamel. They may have been influenced by change in the environment and this in turn is a possible candidate for the cause of the evolution of bipedalism. So while it is unlikely that bipedalism caused changes in dentition, the same forces that lead to bipedalism might have lead to the evolution of our teeth and palate.

Third Trait: Cranial Capacity Changes

1. How is this trait expressed in humans compared to non-human primates?

· Cranial capacity rises over time from 350 cc (non-human apes) to about 1200 cc in humans.

· Shape in the cranium changes, less protruding in the back and becoming larger above the eyes to reflect larger forebrain.

· Expansion of the cranium to eliminate the post-orbital constriction, the narrowing of the skull right behind the eyes (visible when you look down on the skull from above).

· Widest point of the skull (as seen from behind) rises, reflecting changes in brain shape.

2. When did this trait appear in the fossil record?

Changes in brain case by 3.5 MYA.

3. What human ancestors first exhibited this trait?

Australopithecus afarensis (3.5 MYA) exhibited signs of cranial changes with a cranial capacity increasing to 438 cc.

4. Identify and describe 3 ways this trait was visible early in the fossil record.

a. Larger cranial capacity

b. Changes in shape of frontal bone of skull to account for more forebrain.

c. Reduction in post-orbital constriction.

5. Were changes in cranial capacity dependent or influenced by any of the other three traits?

This is an interesting question. Regarding bipedalism, there are hypotheses that moving our bodies upright, leaving less surface area exposed to the sun, actually made it easier to keep the body cool. Cooler bodies make it easier to grow bigger brains. Really! So it is possible that bipedalism and cranial capacity are related.

Regarding dentition and its affect on cranial capacity: The brain is a very costly organ to grow, requiring nutrient rich resources and a reliable supply of them. Dentition changes may not have directly caused our brains to be larger, but they may both reflect changes in our diet, one perhaps richer in nutrients and more reliable, that didn’t require the large teeth to process and also provided the resources for large brain development.

Fourth Trait: Tool creation and tool use

1. How is this trait expressed in humans compared to non-human primates?

· Evidence of tools on site, i.e., stones or bones that have been intentionally altered by human manipulation.

· Garbage sites where tools are made, leaving behind chips and stone remains.

· Evidence of a fire pit… fire is a tool and it is also used in tool creation to heat and harden tools.

· Markings on animal bones suggesting tool use.

· Cooked animal bones indicating use of fire.

· Material goods that would have required tools to create, such as clothing with awl-holes or bore-holes.

2. When did this trait appear in the fossil record?

Contention between scientists between dates of 2.5 to 1.8 MYA.

3. What human ancestors first exhibited this trait?

Definitely by the time of Homo erectus (1.8 MYA) but support for Homo habilis was also a tool user (2.5 MYA).

4. Identify and describe 3 ways this trait was visible early in the fossil record.

a. Tool use markings on animal bones (H. habilis)

b. Tools

c. Fire
d. Trash pits with rock shards leftover from creating stone tools.

5. Were tool use changes dependent or influenced by any of the other three traits?

This may be the easiest relationship to see. Having a larger and more complex brain would have been conducive to the mental processing necessary for the complicated steps of tool creation.