Human Origins and Adaptations• Charles Darwin’s theory of natural selection

explains how species originate and change through time– On the Origin of Species by Means of Natural

Selection (1859)– The Descent of Man (1871) discussed human

evolution & our relationships to other animals

• Changed our view of our origin, our nature & our place in the universe

Evolution, Selection, and Adaptation

• Evolution is change in genetic composition of a population of organisms– development of bacterial resistance to antibiotics, new

strains of AIDS virus and emergence of new species

• Theory of natural selection– some individuals have hereditary advantages (adaptations)

enabling them to produce more offspring– if they pass these characteristics on it brings about a

genetic change in the population (evolution)– forces that favor some individuals over others are called

selection pressures -- climate, disease, etc.

Adaptations• Adaptations are useful features

that evolved in response to selection pressures

• DNA hybridization suggests a difference of only 1.6% in DNA structure between humans & chimpanzees

• Evolutionary developments help explain some aspects of our anatomy (vestigial organs)– piloerector muscle in the skin

have no use– auricularis muscles do not move

in most people• help us chose animals for

biomedical research

Primate Adaptations• Some human features can be

traced to the earliest primates• Squirrel-sized, insect-eating

mammals became arboreal probably due to safety, food supply & lack of competition

– shoulder became more mobile (reach any direction)

– thumbs became opposable to encircle branches with thumb & fingers (prehensile)

– forward-facing eyes provide depth perception

• judge distances accurately for leaping & catching prey

– color vision to distinguish ripe fruit– larger brains & good memory to

remember food sources

Walking Upright• African forest became grassland 5 million years ago• Bipedalism (standing & walking on 2 legs) evolved

– spot predators, carry food or infants• Adaptations for bipedalism

– pelvis, femur, knee, great toe, arch, skull, vertebrae, etc.• Australopithecus (2.5mya) gave rise Homo habilis

– taller, larger brain volume, speech, tool-making• Homo erectus (1.1mya) and Homo sapiens (.3mya)• Homo sapiens include Neanderthal & Cro-Magnon• Evolutionary medicine traces our diseases to evolutionary past

Primate Phylogeny

Human Structure• Hierarchy of complexity

– organism is composed of organ systems

– organ systems composed of organs

– organs composed of tissues– tissues composed of cells– organelles composed of

molecules– molecules composed of

atoms– Atoms compose molecules

• Reductionism versus holism

Perception; things aren’t always what they seem

Anatomical Variation

• No 2 humans are exactly alike

• Missing organs– palmaris longus or plantaris muscles

• More or less organs than normal– 2 spleens, single kidney, 6 or 4 lumbar vertebrae

• Variation in organ locations (situs inversus, dextrocardia, situs perversus)

The palmaris muscle

Dextrocardia

Human Function• Characteristics of life

– organization – cellular composition– excretion– metabolism– responsiveness and movement– homeostasis – development (growth or differentiation)– reproduction– evolution

How do you define Death?

• When the heart stops? What about CPR?• Brain activity? What about Persistent Vegetative

States (PVS) when people are awake, but unaware? Is there a particular part of the brain?

• Organs can survive after the Organism Dies• Cell Lines like those from Henrietta Lacks are

effectively immortal if cared for.• What evolutionary cause could there be?• Longest human life: 122 yrs • Clinical Death = no brain waves for 24 hours

Physiological Variation

• Differs with sex, age, diet, weight, degree of physical activity

• Typical human values– reference man

• 22 years old, 154 lbs, light physical activity• 2800 kcal/day

– reference woman• same as man except 128 lbs and 2000 kcal/day

Homeostasis• Hippocrates noted that body normally returns to a

state of equilibrium by itself – needs to detect the change & oppose it

• Walter Cannon (1871-1945) coined the term homeostasis indicating stable internal environment

• Internal environment described as dynamic equilibrium– fluctuates within a range around a certain set point

• Loss of homeostatic control causes illness or death

Negative Feedback Loops• Mechanism to keep a variable close to its set

point

• Body senses a change & activates mechanisms to reverse it

Negative Feedback, Set Point

• Room temperature does not stay at set point of 68 degrees -- it only averages 68 degrees

Human Thermoregulation

• Blood temperature sensing nerve cells in base of brain control shivering, sweating & vasomotor activity– vasodilation with heat & vasoconstriction with cold

• Evaporation of water & heat radiation occur

Control of Blood Pressure

• Rise in blood pressure detected – stretch receptors in wall of heart and

major arteries

• Nerve signals travel to cardiac center in brainstem

• Nerve signals slow heart and lower blood pressure

Structures Needed for Feedback Loop

• Receptor = structure that senses change– stretch receptors in heart & large blood vessels

send information of an elevated BP to integrator

• Integrator = control center– cardiac center in brainstem that signals heart to

slow

• Effector = structures that carry out commands of the control center– heart slows and BP decreases– sweating begins and evaporation cools the body

Positive Feedback Loops• Physiological change that leads to an even greater

change in the same direction (self-amplifying)• Normal way of producing rapid changes

– birth, blood clotting, protein digestion, generation of nerve signals

You have the weapons of knowledge go forth and conquer all