Week 1 Lectures: February 6-8, 2007 5 Major Questions

Last common ancestor looked very much like knuckle walker

“Compromised adaptation” – long fingers so they’re able to grab branches, but can then use knuckles to walk quadrupedally

Cost of locomotion – a human should cost 2 meters of oxygen per meters per gram of body weight, etc.

When you put a chimp on a treadmill, it cost 150% more than in should have for a mammal of its body mass, because of the knucklewalking

Chimps, knucklewalking has a locomotive energetic cost even though it’s useful

Now, bipedalism:

How much like us were the earliest bipeds?

Were they compromised or committed bipeds?

How much phylogenetic lag (how many features still remained in the body that were useful for arboreality and quadrupedalism remained anyway because they weren’t selected against?)

Bipedal posture – center of gravity is about behind one’s belt buckle

Quadrupeds – when it’s moving, it’s behind it’s shoulder; when they stand up, it’s way in front of the hip joint; it’s over a nice rectangle of support, makes it hard to fall over

Bipeds have a smaller support area, which is why we fall over more easily

Postural adaptations for bipedalism – ability to stand up w/o a lot of energy, and to not fall over

Standing up vs. lying down as a biped costs you about 7% more energy

Must get center of gravity right above hips to make standing more energetically efficient

Achieved this with our vertebral column, achieved by the curvature (lordosis) – the curve centers the center of gravity above the hips (also causes lower back pain)

Postural adaptations for bipedalism in the skull – need to be able to orient your head so it’s pointing forward; humans are able to look forward

For a chimp to stand upright, it would have to bend its knees the whole time

Close-packed hip and knee; can lock the hip in place, also the leg and hip

Bottom line – chimps don’t stand very well
Locomotion: gait (way in which you move your legs)

Two gaits; walking and running

Stride cycle = full cycle from heel strike to heel strike

1. 
   Heel strike
   
2. 
   Flat foot
   
3. 
   Heel off
   
4. 
   Toe off
   
5. 
   Swing phase
   
6. 
   Heel strike
   

When you walk, you always have one foot on the ground for about 60% of the stride cycle

Body acts as an inverted pendulum – center of gravity rises and falls as you move

Human gait is so effective because, you store up potential energy in first half of stance, then use it in the 2^nd half – walking is basically a series of inverted pendular motions, going up and down, and the potential energy of going up is used going down

While walking, several anatomical features minimize movement of center of gravity; only moves around within a 5cm cube (flexed knee and hip, feet/knees lateral to COG/pelvic tilt/thorax rotates with pelvis)

These features are visible in fossil record, which can help evaluate the way australopithecines walked

Hip adaptations for bipedal locomotion

Humans have bowl-shaped pelvis

Australopiths have a tall pelvis

Small gluteal muscles – they’re abductors; while you’re walking, they keep your body from falling over to the side; muscles originate on the hip, and articulate on the greater trochantor

Bipeds need larger femoral heads because all of our body weight goes on them, unlike quadrupeds, and we need a long femoral neck

Femoral head size can be indicative of body weight

When a chimp goes through the birth canal, it doesn’t have to twist

In a human, the baby starts in the birth canal with the nose sideways, then rotates

Lumbar column – human lumbar vertebrae are wedged (top and bottom of the vertebrae are not parallel)

In chimps, the width remains constant as you go down

In humans, the fifth lumbar vertebra is wider than the first

The knee – the femur is oriented inward toward the knee (bicondylar [carrying] angle]; angle is steeper in australopithecines than humans because their legs are shorter, and to get their knee under the center of gravity, angle must go inward quicker

Ankle and foot – arch in foot, which allows humans to stiffen the foot and use the toes to push off the ground; chimps have no arch in their feet

Human feet – weight transfer from lateral to medial side; short, straight toes, big adducted big toe (hallux)

Chimp feet – no weight transfer (mostly lateral), longer curved toes, smaller abducted big toe (useful for climbing trees)

Foot arch acts as an arch when humans run

Heel bone – because of the heel strike component of walking, all your body weight goes through this bones; generally there’s a linear relationship between size of heel bone (calcaneus) and body mass in mammals

Most incontrovertible evidence that australopithecines were bipeds is the Laetoli footprints (in Tanzania), about 3.6 million years old; consists of 3 individuals, 2 of whom walked in the footprints of the larger individuals

Footprints show that they were well-adapted for terrestrial, bipedal locomotion

Arboreal adaptations

Little foot – didn’t have a full arch like humans

Limb length – in apes, very long arms relative to legs; humans have short arms relative to legs; Lucy is right in the middle

Longer legs make locomotion more efficient; short legs are more efficient for climbing because they direct the center of gravity toward/into the tree

Limb mass – in a chimp, about 16% of mass is in arm, 24% in leg; in humans, 8% in arms, 30% in legs; australopithecines. 12% in arms, 28% in legs (closer to the chimp)

Shape of thorax – humans have relatively wide upper thorax, shoulders towards the back; apes and australopithecines have narrow upper thoraxes and permanently shrugged shoulders – more efficient for climbing; rotation of scapula isn’t needed with this design if you want to get your arms above your head

1. 
   Hip abductor
   
2. 
   Bicondylar angle
   
3. 
   Long femoral neck* - more efficient than humans
   
4. 
   Reinforced knee
   
5. 
   Lumbar curve
   
6. 
   Plantar arch
   
7. 
   Abducted hallux
   
8. 
   Wide sacrum
   

1. 
   Long curved toes
   
2. 
   More flexible ankle
   
3. 
   More flexible big toe
   
4. 
   Superiorly oriented shoulders
   
5. 
   Short legs
   
6. 
   Funnel shaped thorax
   

Much of this debate cannot be answered by the data we have

It’s been assumed that having a flexed hip and knee is bad, but most animals have that

There are costs and benefits to both sides –tradeoff between efficiency/mechanical advantage and speed; flexed hip/knee gives more speed (that’s why the natural position when you’re scared is to crouch; lowers your center of gravity, gives you more stability, prepares you to accelerate better should you need to run)

The force that muscles are able to produce do not scale with body size, so bigger animals tend to have more extended postures – otherwise, it would cost incredible amounts of energy to move

Humans weigh more than australopithecines, therefore we should have more extended limbs

Laetoli footprints – in the typical human, you go from the lateral size of your foot to the medial side, so the footprint is deeper in some areas; in the footprint, the deepest portions are roughly down the middle, hints at differences in force distribution of the foot

Australopithecines were probably compromised bipeds

Evidence suggests it was in sahelanthropus about 6 or 7 million years ago

Earliest hominids appear to be upright bipeds

Why did it evolve? Some hypotheses:

1. 
   Locomotor efficiency
   
2. 
   Postural hypothesis (not likely)
   

Seeing over tall grasses

3. 
   Carrying (food, tools/weapons, infants)
   
4. 
   Thermoregulation – standing upright reduces the amount of radiation your body gets
   
5. 
   Fast feeding on grass/seeds
   
6. 
   Provisioning mates
   
7. 
   Threat displays (throwing stuff at people)
   
8. 
   Swimming (aquatic apes hypothesis … ignore this, it’s stupid)
   

Adaptations – feature favored by natural selection for its present function

Exaptation – things that the feature later allowed for, but wasn’t selected for (i.e. we can type, but we didn’t become bipeds so we could type)

Running – early australopithecines were probably not good at running; they might’ve been good sprinters like chimps, but not good at distance running (as humans are)

Systematics – the species that we have and their evolutionary relationships

Early hominids, then 2 groups of australopithecines: gracile and robust; we don’t know if they’re really that different

Orrorin may not even be a hominid

What are the taxonomic units? (species)

Determine their evolutionary relationships

Cladogram (how closely things are related

Phylogeny (ancestor/descendant relationships)

Test hypotheses of causation (scenario of what might’ve been the selective forces that led to certain species, etc.)

Bones are influences by a combination of combination of genetic and non-genetic stimuli, so we can’t just look at DNA (there isn’t any left anyway); non-genetic stimuli doesn’t tell you anything about selection

Must not assume that the shapes of bones always show genetic information

Definition of species – no agreed upon definition

Biological Species Concept (BSC) – a set of actually or potentially interbreeding organisms

Phylogenetic species concept (PSC) – group of organism who share single common ancestor, distinguished by a unique combination of features (apomorphies – unique, derived features)

BSC – you can’t test it with fossil record

Doesn’t account for change over time

Phenotypic variation often a poor guide to reproductive isolation

PSC – convergence happens (similarities that evolve independently)

Doesn’t account for variation

Variation: We can measure variations in populations – sources of variations: sexual dimorphism, age, population, time, other; biggest source is sex. One standard deviation in an equally distributed group will give you 68% of the variation around the mean. So, if a feature falls outside the standard deviation of a species, it’s most likely from another

*But, the problem is that different features will give different answers

Why?

May not be a different species

Not all features change between species (e.g. having 2 eyes)

Typical heritability of most skeletal features is around 30%, which means there are lots of factors influencing the fossil record

Derived characters

Only things that have changed and exhibit modification provide information about evolutionary events

If you don’t have any genetic data, how do you know you’re right?

One issue is the number of species you’re willing to accept into your analysis – do you err on the side of too many (splitter) or too few (lumper)?

Logically, you’re better off erring on the side of too many, you’re better off being a splitter than a lumper

A lot of these species ideas are based on one sample  that’s a problem

Evolutionary relationships

Cladogram is a tree that simple tells you the evolutionary relationships between species, doesn’t tell you who is descended from whom (no time in a cladogram)

Phylogeny adds time to a cladogram, shows what is descended from what

How to make a cladogram

1. 
   Choose/define taxa (Operational taxonomic unit – what we think is a species, but not sure)
   
2. 
   Choose/define characters (aspects of organisms that we can compare)
   

Biological (i.e., the day of the week that you found the fossil doesn’t count)

Objective/quantifiable

Very more between than within taxa

Independent

Result from heritable processes

Homologous = similar because of common ancestry, not because of independent evolution  circular logic

Best character – DNA

Skeletal data as characters – complex, integrated, subject to non-genetic causes of variation

Vault thickness – not a good character

3. 
   Determine polarity (ancestral/primitive vs. derived) – 5 toes is ancestral, having hair is derived – how to figure this out? The outgroup method: imagine you have a group of creatures
   

Scientists use Occam’s razor – idea that the simplest solution is the most likely, so they select the most parsimonious tree (one that requires the fewest splits); if any trees are equally parsimonious, add another character

Problems with parsimony

1. 
   Function of # and type of characters used
   
2. 
   Non-independence
   
3. 
   Is evolution really parsimonious? No.
   

1. 
   Choose/define taxa (OTUs)
   
2. 
   Choose/define characters
   
3. 
   Determine polarity
   
4. 
   Identify synapomorphies (shared derived features) – problem, character conflict
   
5. 
   Pick the best tree (using parsimony/homology)
   

Example, with human/chimp/gorilla tree, we got it wrong because we got primitive vs. derived wrong – never would’ve figured that out without the genetic data

Strait and Grine (2004)

1. 
   Taxa included, chose their specimens
   
2. 
   Defined the characters to be used
   
3. 
   Used outgroups to figure out the character states (polarity)
   
4. 
   Most parsimonious cladogram – results in all robust australopithecines came together, most of the others got separated out
   

You can step back in the taxonomic hierarchy and look at genus

Teeth have a clock-like schedule of eruption that’s related to different phases of ontogeny

Humans first molar erupts around 6 years old, chimps and monkeys erupt closer to 1 year old

If you graph brain mass with the age at which your first molar erupts, you get a pretty good line, also, if you know when the first molar erupts, you can find when your brain stopped growing

Tong baby – first molar is just erupting; how old was he?

Teeth have striae of retzius (much like tree rings) – every 7 days, the emiloblast takes a break from squirting out enamel and leaves a line of hyper-mineralized enamel; lets you find out how long it took the tooth to erupt

Australopithecines have thicker enamel and larger teeth because they extruded enamel faster and grew their teeth faster than an African ape

Tong baby erupted its first molar around 3 years old

Macaques grow really quickly

Australopithecines are almost identical to chimps in their growth process

• 
  Primates have both big brains and long lives – the two features are correlated
  
• 
  Life history theory: focused on the evolutionary forces that shape trade-offs between the quantity and quality of offspring and between current and future reproduction
  
• 
  Aging and death result from trade-offs between reproduction at different ages and survivorship
  
• 
  The tradeoff between survival and reproduction is greatly biases against characteristics that prolong life at the expense of early survival or reproduction
  
• 
  The tradeoff between current and future reproduction and between quantity and quality of offspring generates constellations of interrelated traits
  
• 
  The benefits derived from current and future reproduction depend on a variety of ecological factors that influence prospects for survival
  
• 
  Natural selection not only shapes the relationship between life history traits, but it can also shift the values of these traits in response to changes in environmental conditions
  
• 
  Primates fall somewhere along the slow/long end of the life history continuum
  
• 
  The first adaptive change in the package of life history traits within the primate order involved the origins of the prosimians and their shift to an arboreal niche – 60 mya
  
• 
  The second major adaptive shift is associated with the appearance of anthropoid primates – monkeys and apes – 35 mya
  
• 
  The third adaptive shift is associated with the great apes who appeared during the Miocene epoch, about 20 mya
  
• 
  Great apes rely on complex extractive foraging techniques, sometimes using tools, to a greater extent than most other primates
  
• 
  Ecological hypotheses for the evolution of intelligence predict that specific characteristics of the diet or the environment of particular primate species will be correlated with their cognitive abilities
  
• 
  The social intelligence hypothesis predicts that there will be a positive correlation between the complexity of social life and the neocortex ratio
  
• 
  Comparitive analyses provide support for both the ecological hypotheses and the social intelligence model
  
• 
  Monkeys and apes construct mental maps of their home ranges, allowing them to move efficiently from one food source to another
  
• 
  There is evidence that monkeys and apes know something about the nature of kinship relationships among other members of their groups, and some evidence suggests they may understand the nature of rank relationships among individuals
  
• 
  Understanding of third-party relationships may be particularly useful in managing coalitions
  
• 
  Primates may understand more about the behavior of other animals than about their feelings, thoughts, intentions
  
• 
  The ability to predict what others will do might be based on a sophisticated ability to track contingencies between one event and another or on knowledge of other animals’ mental states
  
• 
  Great apes may have more knowledge of the minds of others than monkeys
  

• 
  Key terms
  
• 
  Neocortex – part of the cerebral cortex, generally though to be most closely associated with problem solving and behavior flexibility; in mammals, it covers the entire surface of the forebrain
  
• 
  Extracted foods – foods that are embedded in a matrix, encased in a hard shell, etc.
  
• 
  Social intelligence hypothesis (see above)
  
• 
  Neocortex ratio – the size of the neocortex in relation to the rest of the brain
  
• 
  Executive brain – composed of the neocortex and the striatum (see below), a structure in the basal ganglia that is functionally linked to the neocortex
  
• 
  Brainstem – the portion of the brain that lies between the cerebrum and the spinal cord, and provides the major route for communication between the forebrain, the spinal cord, and the peripheral nerves
  
• 
  Striatum –a structure composed of two of the basal ganglia of the forebrain: the caudate nucleus and the putamen
  
• 
  Tactical deception – the use of normal parts of an animal’s behavioral repertoire in an unusual context to achieve specific objectives that are beneficial to the actor
  
• 
  Cognitive map – a mental representation of the location of objects in space and time that allows for efficient navigation
  
• 
  Third-party relationships – relationships among other individuals
  
• 
  Redirected aggression – a behavior in which the recipient of aggression threatens or attacks a previously uninvolved party
  
• 
  Theory of mind – the capacity to be aware of the thoughts, knowledge, or perceptions of other individuals
  

This article discusses Lucy, the australopithecine fossil. It’s focused mainly on examination of the pelvis, and the features of the pelvic area that indicate that Lucy was adapted for bipedalism. It goes into a lot of detail about muscle motion, etc. that hopefully we won’t need to know, because there was a lot of it. Most of the article just reiterates the things we learned in lab 4, so see those handouts if you really want all the anatomical detail. The takeaway point, I’d say, is that Lovejoy is under the impression the Lucy was a dedicated biped, because he believes that without the species habitually walking upright, it never could have developed the adaptations that consistently made them better at it. Also, he feels that examination of her other features (upper body, knees, etc.) show that she was ill-suited for climbing – for example, the arms and fingers are shorter, which is bad for climbing. Because he thinks Lucy was a dedicated biped, he believes that bipedalism must have predated her by quite a lot, and that her ancestors must have left the trees and started walking around long before Lucy, which would explain why she’s such an awesome biped.

• 
  Why are we so interested in size and dimorphism?
  
  • 
    Tells us a lot about locomotion, diet and reproduction, competition, etc.
    
• 
  The Index of Dimorphism (ID) = (size of male)/(size of female)
  
• 
  CV = Coefficient of Variation, a standardized method of variation
  
  • 
    CV = (standard deviation)/(mean*100)
    
  • 
    One standardized deviation, standardized by the mean
    
• 
  The more dimorphism, the higher the CV
  
• 
  Regression equations: how we estimate body size and dimorphism form skeletons
  
  • 
    In order to do this, we needs lots of bones of australopithecines, and we haven’t had as many complete skeletons as we would like (i.e. male counterparts to Lucy)
    
    • 
      A lot found in Ethiopia, Hadar- helped with figures
      
  • 
    So, how do we estimate the ID?
    
    • 
      Estimate the ID from the CV
      
    • 
      Estimate the ID from likely males and females
      
    • 
      Both methods give similar results for A. afarensis
      
• 
  Australopithecines were similar in body mass to chimps, but more sexually dimorphic because of male (not female) body size.
  
  • 
    But how did we infer this? Induction v. Deduction
    
    • 
      Deduction: argument deduction valid if the conclusion makes no factual claim not (at least implicitly) made by the premise
      
      • 
        Most scientists comfortable with this type of argument
        
    • 
      Induction: argument induction strong if factual claims go beyond factual information in premises
      
      • 
        What most evolutionary biologists have to do
        
  • 
    Regression based arguments are inductive
    
• 
  How do we evaluate the strength of an inductive inference?
  
  • 
    Power of Prediction (how generalizable)
    
  • 
    Mechanistic basis behind assumption
    
• 
  What other inferences can we make?
  
  • 
    Brain size (encephalizaiton quotient) can show us brain size v. body size, but we need the estimated body mass. Then we can see how much larger its brain is in relation to its body.
    
    • 
      Based on knowing if we have the right brain and body size estimates
      
  • 
    Tooth dimorphism- looking at cheek and canine sizes, more dimorphic in molar size but less dimorphic in canine size
    
  • 
    Face dimorphism- pretty dimorphic face-wise, males had bigger faces than females. Dimorphism greater than chimps and closer to gorillas.
    
  • 
    Height from regression, legs shorter/arms longer than humans, apelike (stout) torsos
    

• 
  Know these 4 eras:
  

• 
  Holocene
  
• 
  Pleistocene
  
• 
  Pliocene
  
• 
  Miocene
  

• 
  Global shift from more forests to less forests in the last few million years
  
  • 
    In Africa the great rift valley systems also developed during this period
    
• 
  No apes/chimps live in areas with more than 4 months dry season, but hominid fossils found mostly in places with dry seasons longer than 5 months; no fossil apes found in these habitats
  
  • 
    Clear ecological separation
    
• 
  Global climates fluctuate continually from warmer to cooler mainly b/c of regular changes in the earth’s orbit and axis tilt
  
  • 
    Other factors also implicated (rifting, continental drift)
    
• 
  What happens when habitats become less forested and more open?
  
  • 
    Resources (food and safety) become dispersed but are sometimes clumped in large patches, distribution of food, safe sleeping spaces and predator changes
    
  • 
    Fruits more seasonal and well-protected, plants store more energy underground
    
• 
  Hypothesis: Hominids would be only one of many types of species that change over this period.