Evolutionary Darwinian Medicine
Evolutionary (Darwinian) Medicine
“Nothing in biology makes sense except in the light of evolution” –Theodosius Dobzhansky
Proximate vs. Ultimate Questions
Biological causation: proximate vs. ultimate explanations
Physiology, genetics, biochemistry, etc. generally concentrate on * proximate causation
“What” & “How” questions
Ultimate causation: How evolutionary processes (natural selection; adaptation) and evolutionary history shape traits
“Why” questions
Traits have both types of causes—a complete biological explanation requires analysis of both
Examples: Childbirth, obesity, high blood pressure and asthma/allergy
The Big Picture: Hominid Origins
What are the key features that define us as humans & when did they …show more content…
arise?
Why did these features evolve?
How does an understanding of adaptation in earlier hominids help us understand contemporary human health issues?
Evolutionary history important for understanding modern human * condition
Two examples:
1) Bipedalism & low back pain
Why did it evolve? An extremely efficient way to travel long distances (also frees the hands for carrying)
But predisposed to back pains
Viewed as a product of design constraints & trade-offs
2) Bipedalism and brain size on infant growth and development (the “obstetric dilemma”)
Requires a shift in life history pattern
Early birth (risk for SIDS)
Requires high quality diets and ability to store large amounts of fat
Can identify times of increased risk for nutritional problems
Explains human propensity for consuming energy dense foods
A Key Concept - Novel Environments
Contemporary cosmopolitan, urban environments as novel
Widespread availability of energy dense foods (especially fats & refined carbohydrates)
Certain highly energy dense foods (especially liquids) “override” the system
Our metabolism is evolved to be thrifty, making it difficult to diet to lose fat
Links between chronic psychological stress & fat amount/pattern
Some Surprising Answers
New perspective on asthma, allergy, and autoimmune diseases
Allergy increasing in the US & developed nations
50 million in US with some type of allergy
Protective effect of microbial exposures
Decreased exposure to helminths (parasitic worms), viruses, and bacteria leads to increased risk of allergy, autoimmune disease, type I diabetes, and other diseases
Immune system tuned by environmental exposure
New treatments developed that use helminth (e.g., whipworm) introduction * * A New Perspective on Aging
Why do we experience more diseases with age?
Many physiological processes decline with age but not in all * Circumstances
How much is programmed vs. byproduct of our lives?
Are Humans Still Evolving?
Until only a few years ago most scientists would have said “No”
Food production & population expansion
Culture: Modern medicine
Evidence for evolution since beginning of food production
Including selection (sickle cell allele, lactose tolerance, etc.)
Evidence for ongoing evolution
Including selection (malaria resistance, skin color, skeletal development)
Does medicine without evolution make sense?
Evolutionary perspective complements existing biomedical frameworks—It does not replace it
Provides valuable information on:
Why diseases exist at all & our greatest disease vulnerabilities
Why disease is unequally distributed between individuals & groups
Not just an academic exercise but can influence:
How diseases are treated clinically
Public health practices & personal health decisions
Anthropology
The study of humankind using an integrative approach
Culture & society (human variation & universals)
Culture and society in the past (historic & prehistoric)
Culture and society in the past (historic & prehistoric)
Biological anthropology is the study of human biology and behavior within the framework of evolution, with an emphasis on the interaction between biology and culture
Two Focal Areas:
Human Origins & Evolution
Contemporary Human Biological Variation
The Scientific Method is a Way of Knowing
Not just collecting and cataloging facts, but instead a way of knowing based on a systematic search for explanations
A process based on:
Evidence (Asking questions & demanding evidence)
Observation (Collecting empirical evidence [data])
Generalization (Finding patterns & relationships)
Testing and continued testing (Refining explanations & correcting errors)
Self-correction – Gets better over time (Hopefully!)
Basic Principles of the Scientific Method
1) Nature is governed by natural laws
2) These natural laws can be discovered through inquiry and rational thought
3) Humans are part of nature and subject to these natural laws
These scientific principles are rooted in the works of Plato, Aristotle, and other Greek Natural Philosophers
Other Potential Principles
Or,
Asking lots of questions, being systematic in how you try to answer them, and tireless and open-minded in your pursuit of answers
Also,
“A way to ensure accountability for factual claims” – Sloan
Data Collection
Systematic & explicit in its methods
Often quantitative Set in a framework for understanding (Hypothesis Testing)
Hypothesis
Not ungrounded speculation—instead a statement about what might be true
Must be falsifiable—can be proven wrong with observations
Theory
Well-tested model that has survived repeated attempts to prove it false
Evolution is a theory (some use the term “fact”) that has stood the test of time and repeated tests and refinements
The collection of data and testing of hypotheses often leads to new and surprising questions & hypotheses
E.g., Rise in blood pressure not a natural part of the aging process
Great Chain of Being (Scala Naturae)
Basic idea dates back to Aristotle (in the 4th Century BC)
Influential for many centuries
All organisms exist in a universal hierarchical ladder
Linear (simple to complex) with continuity
Unchanging (Fixity of Species)
Humans at top but below gods & angels
“Ladder of progress”
Early Evolutionary Thinking & A First Attempt at Explaining Evolution
Key figure is French naturalist Jean Baptiste Lamarck (1744-1829)
Recognition of change in nature
Tried to explain how evolution worked
Postulated interaction of organism & environment
Inheritance of Acquired Characteristics (Use/Disuse)
Differential use of body parts; Could pass these traits to offspring
Geology and the Concept of An Ancient Earth
Key figure is the Scottish geologist Charles Lyell (1797-1875)
Founder of modern geology (Principles of Geology)
Explanatory Model: Uniformitarianism
Same gradual geological processes we see today were operating in the past
Earth is old & dynamic (geologic or deep time)
Resource Use & Competition
Key figure is the English economist and demographer T.R.
Malthus (1766-1834)
An Essay on the Principle of Population
Human population size can’t increase indefinitely because of limited …show more content…
resources
Overpopulation & constant competition for resources
Context of Darwin’s Ideas on Evolution
A “long” chronological view emerges
Geological evidence of old Earth
European contact with other civilizations with long histories
Monuments and antiquities
Human bones and artifacts with extinct animals
Earlier evolutionary ideas but:
Most speculative, not scientific
Most with no mechanism of change (or not tested with evidence)
No common descent
Charles Darwin (1809-1882)
Studied medicine & theology
Grandson of Erasmus Darwin
Zoonomia (evolutionary change & common descent)
Naturalist on HMS Beagle (1831-1836)
Biological & physical environmental variation
An Important Stop: Galapagos Islands
Fauna similar to South American mainland but with some key differences
…and no land mammals
Galapagos Birds & Reptiles: More variation within groups
Finches, Mockingbirds & Tortoises
Adaptive radiation
Diversification of one founding species into multiple species & niches
Often with new environments
Fill vacant niches
Competition may be limited
Example: Finches (Geospiza spp.)
13 species on islands
1 mainland species
Darwin’s Post-Voyage Years
Organisms not fixed but change over time
Fossils are the remnants of extinct life forms
All living organisms descend from a common ancestor
Processes today the same as those in the past (uniformitarianism)
These processes take lots of time
Natural Selection
First used by Darwin in 1842
Borrowed from “selection” from animal breeding
Influenced by Charles Lyell & T.R. Malthus
“Struggle for existence”—competition for limited resources (Malthus)
“Survival of the fittest”
Finally published in 1859
Alfred Russell Wallace was reaching similar, though not identical, conclusions
Two-step process
Variation (random process)
Selection of Variation (A non-random process, based on survival & reproduction – Differential reproductive rate)
Generate a variety of possible solutions & pick the one that works best for problem at hand
Evolution as tinkerer not engineer
Modification/transformation of what already exists Evolutionary change seen in populations, not individuals
Evolution by Natural Selection
Population variation that is heritable
Environmental pressure (expanding populations and limited resources)
Organisms with advantageous variation favored:
In their ability to survive, mate, and rear offspring to reproductive age (reproductive success)
Thus, they have higher fitness (measure of relative reproductive success): Better but not perfect
Darwin’s main contributions:
Provides evidence that evolution had occurred
Provides a naturalistic mechanism for evolution
Perspective of common descent/ancestry
Elimination of typological (essentialist) view of nature
Darwin’s Evidence
Geology & Paleontology
Old age of the Earth
Fossils of extinct animals
Some similarities with living species but the farther back in time the more different
But fossils of ancient humans almost completely unknown
Comparative Anatomy
Homologous Structures: Similar structures, but often with different functions
Indicative of common ancestry
Darwin’s Evidence (cont.)
Vestigial Structures: Retentions with no current function (e.g., pelvic bones in whales) (Appear to be “imperfections”)
Another imperfect adaptation: Testicular descent in mammals, including humans
Getting the testes out of the abdomen results in looping of the male reproductive tract around the ureter, and also increased risk of hernias
A constraint of development & evolutionary history
Comparative Embryology: Similarities in embryos but adult differences
Some traits appear early but are later lost (e.g., ape & human tails)
Could not explain certain traits if by design
e.g., Flounder eyes
Darwin’s Problems
Darwin could not explain:
How traits were inherited
Proposed that offspring inherited traits from both parents
How variation originated and was maintained
Genetics later provides us with information on:
How traits are inherited
DNA—As discrete units, not through blending
How variation originates and how it is maintained
Mutation
Understanding Evolution, Part II: Genetics
Genetics: The study of gene structure and action and the patterns of inheritance of traits from parent to offspring
Provides us with information on the following:
The inheritance of traits
Links between genes and physical traits
Links between genes and behaviors
Evolutionary relationships between organisms
The ability to measure evolutionary change in populations
Answering the questions that stumped Darwin
What is the ultimate source of variation?
Answer: Mutation
How are traits inherited from one generation to the next? Blending or discrete?
How are traits inherited from one generation to the next?
The basic principles of inheritance had been worked out by the 1860s but weren’t appreciated until 1900
Experiments by an Austrian monk named Gregor Mendel Key concepts:
Particulate Inheritance:
Not blending, but instead distinct and independent (effects can be masked but can reemerge in later generations)
The effects of some traits are dominant
They mask the expression of other traits (recessive traits) Traits are typically inherited independently of other traits
e.g., Seed color does not influence type of flower
This gives us information on variation and change (mutation)
An integrated approach?
Darwin outlines natural selection in 1859
Rediscovery of Mendel’s work in 1900
Natural selection & mutation were initially thought to be completely separate explanations for evolution and were competing hypotheses
The Modern Synthesis / Neo-Darwinism (begins in the 1930s)
Unites Natural Selection and Mendelian Genetics
Source of variation & principles of inheritance
Selection of variation based on environmental pressure
Bridges microevolution (evolution within a species) & macroevolution (evolution at or above level of species)
Given enough time, major evolutionary changes can occur
Important architects: Dobzhansky, Mayr, & Simpson
A new definition of evolution
Previously defined broadly as:
Descent with modification or change over time
A new definition…actually two:
A change in the genetic structure of a population over time
More precisely: A change in gene frequencies from one generation to the next
A Key Concept: Adaptation
A trait that increases the fitness (reproductive success) of an organism
Measured in terms of:
Differential survivorship (mortality)
Differential reproduction (fertility)
Produced by natural selection within the context of a particular environment
Environments are constantly changing
The Red Queen Effect– Leigh van Valen
Species have to “run” (evolve) in order to stay in the same place (extant / not extinct)
Evolutionary arms races (hosts vs. parasites)
However, adaptations are imperfect…
Evolution as an incremental process: No huge jumps, only small changes (each of which must be immediately beneficial)
Constrained by evolutionary history
Evolution modifies what is already there (“tinkering”)
Blind spots, retinal tears, choking, etc.
Environmental Pressure & Natural Selection
Multiple dimensions of the environment lead to adaptation
Physical: e.g., temperature
Biotic: e.g., predators or prey
Social dimensions of the environment
Social factors shape natural selection & adaptation
Members of your own species
Sexual selection: Type of natural selection that operates on only one sex; the result of competition for mates
Can lead to sexual dimorphism (physical differences between males and females)
Types: Male-male competition vs. female choice
The Discovery of DNA
1953 description of molecular structure of DNA (Watson & Crick) universal code- same basic structure and process in turnips, turtles, and humans complementary bases adenine (A) & thymine (T) cytosine © & guanine (G)
DNA directs cell replication and the creation of proteins
E.g., Collagen, hemoglobin, and hormones
Not all DNA codes for proteins (non-coding or “junk” DNA) * * Structure & Function of DNA
Gene: section of DNA with identifiable structure and/or function
Sequence of DNA that specifies order of amino acids in a protein ..
Chromosomes & DNA
Genes are found in long strands of DNA .. * * Genes: Location and variation
Locus: physical position of gene on chromosome
P as shorter arm and q as longer arm (e.g., Huntington’s disease locus: 4p16.3)
Alleles: different versions of the same gene
Found at the same locus on paired chromosomes
Mutation
An alternation in the DNA- may or may not alter function
E.g., point mutations: a single base change in the DNA
Can cause problems
Ultimate source of all genetic variation
Sickling hemoglobin, sickle cell anemia, and malaria
Types of genes
Humans have 25000 genes
Structural genes: code for proteins
Regulatory genes
Regulate expression & timing of other genes
All somatic cells contain same genetic info but not all DNA involved in protein synthesis (“master switch”)
Genes and the expression of traits
Typically not a one to one correspondence between genes and expression of traits
Most traits are influenced by multiple genes
Most traits are also influenced more or less strongly by the environment (developmental plasticity)
Genotype: specific allele composition of a certain gene or set of genes
Phenotype: the physical or biochemical expression of a genotype; the observable attributes of an organism
Single vs multi-gene inheritance
Single-gene inheritance (Mendelian traits)
E.g., curved thumb, dimples, earlobe attachment, blood type
Multi-gene (polygenic) inheritance
Traits influenced by two or more genes
E.g., height, skin color, & observable phenotypic traits
Often environmental influence & with continuous variation
Epigenetic effects
Nature (i.e., genes) vs nurture (i.e., environment): an unhelpful dichotomy
Environment affects expression of genes
E.g., fetal programming or high fat/calorie diet in childhood
Epigentics effects are differences in gene expression
Most epigenetic changes occur in early development (fetal life or infancy)
Human adaption and adaptability
Adaption often used in a general sense to refer to changes by which organisms surmount challenges to life
Types of adaptation
Cultural behavioral
Tools and technology
Medicine
Subsistence strategy and diet
Biological
There are multiple levels of biological adaption
Genetic (Darwinian) adaptation
Functional adaptations
Short-term acclimatization
Developmental acclimatization (plasticity)
Human emphasis on multiple simultaneous adaptive pathways
Darwinian or genetic adaptation: developed over many generations: reflects natural selection
Measured through differential fertility & mortality
E.g., climatic adaption
Body size/proportions and temperature
Cold: body size and proportions increase head production (large body weight) & maximize heat retention (short limbs)
Hot: reduce heat production (low body weight & long limbs)
Relatively high metabolic rates in cold environments, which produce more heat
Functional adaptations (adaptability)
Adjustments during the lifetime of an individual to help maintain homeostasis in response to a stressor
Homeostasis: maintenance of the internal physiological environment within tolerable limits
Stressor (stress): factor that interferes with homeostasis
Physical (cold, heat, etc) or psychological
Types of functional adaptations
Acclimatization: short-term, reversible
Ex:
Increased sweating ability in hot climates (within 10 days)
Tanning as an acclimatization response to exposure to high levels of UV
Developmental acc (plasticity)
Long term, irreversible changes that result from exposure to environmental stressors during growth & development
Ex:
Large lung volume at high altitude
“fetal programming” important environmental factors: nutrition disease exposure physical environment
..
Fetal programming effects of fetal environment on adult health (barker) developmental acc permanently changes structure, physiology, & metabolism (thrifty metabolism) later shift to environmental conditions (i.e., scarcity abundance), predisposed to metabolic, cardiovascular, and endocrine diseases in adulthood increased all-cause mortality, type 2 diabetes, heart disease, obesity, & hypertension
Pushing the limits of human adaption humans are an incredibly adaptable species bio-genetic and functionality cultural limits of adaption and consequences of extreme environment
Himalayan Sherpa- low levels of iodine in diet with resulting goiter and cretinism (developmental effects and mental retardation)
Siberian metabolic adaption and increased risk for hypertension and stroke
What makes us human?
What traits distinguish us from our closest relatives?
Culture
Tools and tech
Language and symbolic thinking
Art
Religion
Large brain size
Bipedalism
Reproductive bio
In fact, many of these traits are present in non-human primates
The traits that make us human did not all evolve at the same time
Humans evolved in a piecemeal fashion with different functional systems having different rates of evolution (mosaic evolution)
To answer the question what makes us human, we need to understand the evolutionary history of these traits
Humans are closely related to living great apes, especially chimps and gorillas
Share ancestor with chimps/bonobos 5-7 mya
Last common ancestor with chimps/bonobos and gorillas
Mixed arboreal and terrestrial behavior
Varied and opportunistic (omnivorous) diet
Complex social systems
Highly social with extensive learning
Limited tool use and other cultural behavior
Modern humans and hominid ancestors
Hominid: member of the group homininae, defined by the habitual bipedal adaptation
Later: increased brain size and reduced canine size (400cc vs 1300cc)
Most closely related to living African apes
Hominoid: group that includes apes and humans
We share 98.4% of our DNA with chimps
Earliest hominoids: 5-7 million yrs ago
Human evolution extremely complicated and species rich
Often, multiple species alive at one time
Multiple hominid adaptive radiations
Not one linear progression from simple to complex
Hominids evolved in Africa and were there exclusively until 2 million yrs ago
Earliest fossils in eastern and central Africa
Bipedal with small brains
Key developments: origin of the genus homo
First appears around 2.5 million years ago in east Africa
Expanded brain (500cc)
Smaller jaws and teeth
Ate higher quality foods (including meat)
First systematic stone tool use
Later, dispersal from Africa
Evolve in Africa around 160,00-200,00 yrs ago
Very large brain
Small and flat face with small teeth
Rapid cultural evolution
Global dispersal around 60,000 yrs ago
Bipedalism
Defining adaptation in hominids
Documented in early hominid fossils
E.g., Lucy (Australopithecus afraensis)
Anatomical rearrangement
Modified pelvis and lower limb
Also, vertebral column and base of skull
Positioning of foramen magnum
Energetically efficient walking in more open environments
Consequences
Low back strain, birthing issues, aging/balance
Brain expansion
Begins 2.5 million years ago with early homo (h.habilis & h.erectus)
Encephalization (increased relative size) and social factors/language
Volume increase from 400cc to 1250cc
Large body size
Appears – 1.8 mya with origin of homo erectus
Decreased predation risk
More efficient bipedalism
Relatively long legs
Larger daily range size, which facilitates dispersal
Requires high quality diet and more calories
Secondary altriciality
Children born developmentally young as a result of bipedalism and large brains
Life history shift and extended period of learning
Extremely pronounced in modern humans but started in homo
Increased dietary quality
Earliest hominids with lower quality diets
The hunted, not the hunters
Increasing diet quality in homo
Increased meat consumption and calorically dense plant foods
Facilitated by sophisticated tools
Later cooking, agriculture, etc
Reflected in decreasing size of face, jaws, and teeth and gl tract
Supports large brain size
Evolutionary medicine asks a series of answerable questions to understand this
Why hasn’t evolution eliminated the genes that make us susceptible to disease?
Why are there so many complications during birth?
Why do we lose our bone density with age?
Why do we age?
Three key players:
Paul Ewald: evolutionary biologist focused on the evolution of infectious disease
George Williams (evolutionary biologist) &
Randolph Nesse (psychiatrist): “The Dawn of Darwinian Medicine” (1991)
Application of evolutionary principles and knowledge of evolutionary history to understanding health and disease
Focuses on ultimate causes—the “Why” questions
Proximate vs ultimate questions
Biological causation in medicine: proximate vs. ultimate explanations
Physiology, genetics, biochemistry, and neurobiology generally concentrate on proximate causes
“What” & “How” questions
The immediate mechanisms that give rise to disease, disorders, and malfunctions
Ultimate causation: How evolutionary processes and evolutionary history shape traits
“Why” questions
How natural selection and other evolutionary forces shaped the trait under consideration
These questions can lead to new perspectives & treatments
Traits have both types of causes—a complete biological explanation requires analysis of both
Western biomedicine is largely based on proximate explanations of human disease
* An Example: Human Childbirth
Why is childbirth painful & dangerous?
Extremely dangerous & even modern medicine hasn’t eliminated the risk
Why? Doesn’t this seem crazy?
Proximate approach focuses on mechanisms:
The diameter of the human fetus’ head is very large and the diameter of the mother’s pelvic outlet is only slightly bigger
But, WHY???
The ultimate approach searches for evolutionary explanations
Why is the head so big?
Why isn’t the birth canal larger?
Why is the process so complicated?
Clues from using a comparative evolutionary approach:
Not terribly complicated in other primates
Solitary birth typical in other primates
From Rosenberg & Trevathan “The evolution of human birth” Scientific American, 2001
Birth in monkeys is less challenging not such a tight fit infant born facing forward (?) mother able to guide the process
An evolutionary approach to the problem
Humans are bipedal
Bipedalism led to changes in pelvic structure & a smaller and more complicated birth canal
Humans are highly encephalized
This combination leads to a complicated birthing process
Human birth is typically attended by others, especially women who have previously given birth
“Obligate midwifery”
A behavioral adaptation
A very complicated process in humans rotation through the birth canal infant typically born facing backwards mother requires assistance
Leads to more questions:
Why not a “solution” to the problem? Two potential solutions:
Smaller brain size of fetus at birth
Larger maternal pelvis
Why not a shorter gestation?
Already extreme—human newborns underdeveloped (extremely risky & require assistance)—CONSTRAINED
Why not a larger maternal pelvis?
It already is bigger than in males
Further changes CONSTRAINED by energetic efficiency of bipedal locomotion
Answer: An evolutionary compromise between efficient bipedalism & increased brain size
An Evolutionary Perspective on the Causes of Disease
1.
Defenses
2. Infectious Organisms & Evolutionary Arms Races
3. Novel Environments
4. Genes
5. Compromises
6. Constraints & Evolutionary Legacies
1. Defenses
Not actually causes but evolved defenses; can be confused with manifestations of disease
e.g., pain, fever, cough, nausea, vomiting, sneezing, anxiety, morning sickness
Decisions on when to block these defenses7
Evidence that fever (rise in the set point of the body’s thermostat) is an adaptation
Speeds up immunological reactions and reduces reproductive rate in many pathogens
Fever is adaptive against some pathogens
Lizards: move to warmer locations to increase body temperature
Humans: Some evidence that fever is adaptive against colds (rhinovirus) and syphilis
But fever has costs (e.g., metabolic & cell damage)
Bad idea to always suppress fever; very high fever can be bad
Reducing fever generally does not lead to faster recovery times or increased comfort while sick “Fever, as a mechanism that activates the immune system to cure us of the pathogen, is a defense, a tool on our side, and the best way to control an illness is to leave the fever alone, at least some and perhaps most of the time.” - Zuk
2007
Some conditions can be either defense or symptom, depending on the cause
Diarrhea from Shigella bacteria appears to be an evolved defense (Lomotil treatment prolongs illness)
However, cholera bacteria (Vibrio cholerae) has a toxin that causes diarrhea (anti-diarrheal treatment counters spread and minimizes dehydration)
Defense or disease manifestation— It’s not always easy to tell the difference!
2. Infectious Organisms & Evolutionary Arms Races
Infectious disease: Introduction of foreign organic substance (e.g., virus, bacteria, fungus)
Not just simple pests but sophisticated opponents
Most infectious agents reproduce relatively fast and mutations create different forms
Evolutionary arms race perspective helps us understand permanency of infectious disease & also some autoimmune disease
The Red Queen Hypothesis (Leigh van Valen)
We have evolved ways to counter their threats
Cultural strategies too (e.g., antibiotics & sanitation)
Cultural factors may contribute to disease burden & their evolution (antibiotic use & nosocomial [hospital acquired] infections)
Emergence of infectious diseases in modified form (e.g., MRSA [methicillin-resistant Staphylococcus aureus])
3. Novel Environments
Contemporary cosmopolitan, urban environments as novel
Different from the environmental conditions in which our species evolved
Nutritional environments
Change in food availability & physical activity levels
Relatively high activity levels in subsistence-level populations
Minimal use of extremely energy dense foods, which can “override” the system
Risk of cardiovascular disease & diabetes
Atherosclerotic plaque: fat, cholesterol, & calcium10
Infectious disease environments
Too few infectious exposures early in life can lead to allergy, asthma, and autoimmune disease
Psychological environments too
Some anxiety or depressive symptoms may be good
Sadness can lead us to stop a behavior that caused loss (e.g., money, reputation, etc.)
But, under certain conditions can lead to
Depression
Schistosoma mansoni parasitic flatworms
A little fear, anxiety, and depressive symptoms can be good, but there is also the risk of depression, anxiety disorder, etc.
New environments = New threats
Past (Hunter-gatherer populations):
Accidents
Hunger
Predation
Exposure
Parasites/Infectious disease
Now (food production, higher population density, industrialization, urbanization, globalization): Heart attack, stroke, atherosclerosis
Diabetes
Obesity
Cancer
Emerging infectious diseases/Epidemics11
It’s not a simple matter of going back to the past
“Just because our species evolved in a different environment does not mean…that following the ways of the past is automatically going to free us from the illnesses of modern life.” -Zuk 2007
It’s not a simple matter either
What “past” are we talking about?
Also, human variation in biology, diets, activity patterns, etc.
Dietary variation in hunter-gatherers by environment
4. Genes
Some genes are perpetuated despite the fact that they cause disease
Some because of novel environments (e.g., related to metabolism & fat deposition but now obesity)
A life course perspective is important (timing is everything!)
Gene variants related to increased calcium absorption can be adaptive early in life (increased bone density) but have negative effects later (increased arterial plaque formation)12
Some genes maintained because they have other benefits (sickle cell allele & malaria) Example: Sickle cell anemia: 1 in every
600 births among African-Americans
Sickle-shaped red blood cells block peripheral blood vessels
Without regular medical intervention few individuals live to adulthood
If sickle cell disease is so lethal, then why hasn’t it been eliminated?
Caused by an allele that can also be beneficial
(balanced polymorphism)
5. Compromises
Human bodies as “bundles of compromises shaped by natural selection in small increments to maximize reproduction, not health.”
–Nesse & Stearns 2008
Bipedalism: An extremely efficient way to travel long distances (also frees the hands for carrying)
But, predisposes us not only to birth complications but also back pain
Our joints are a compromise between efficient locomotion and stability13 6. Constraints & Evolutionary Legacies
Evolution as an incremental process:
No huge jumps, only small changes (each of which must be immediately beneficial)
Constrained by evolutionary history
Evolution modifies what is already there
(tinkering)
Human gut as modified primate gut (appendix)
Also, blind spots, retinal tears, & choking
Infant and child health
Disease and death rates for different age groups provide insights into causes
When are people getting sick and dying…
life expectancy measured at different ages extremely sensitive measure of population level health
* **epidemiological transitions * * 1st epidemiological transition begins about 10,000 years ago from nomadic hunting and gathering to sedentism and food production led to major changes in social organizations, demography..
origins of agriculture and animal domestication earliest 10,000-12,000 yrs ago
Neolithic period
Multiple locations independently
At least 10 separate centers of plant domestication
Massive population growth fueled by food production
Farm land: much higher rates of productivity per unit area than foraged land
Feed larger population
Ability to generate surpluses and allows storage
Domesticated species were originally low value foods for hunter-gatherers
Costs of higher productivity
Higher labor/energy investment
Narrower range of food species
More vulnerable to famines
..
health consequences of food production in general, population-level health decreases with food production paleopathology reduced height compared to hunter-gatherers thinned limb bone shafts evidence of iron-deficiency anemia evidence of increased infectious disease burden and interpersonal conflict (ex. War and organized violence) dental evidence tooth enamel defects increases cavities etc harris lines: bands of dense bone showing period of interrupted growth
occupational stress markers markers of repetitive stress kneeling facets: on distal metatarsals also “squatting facets” of tibia osteoarthritis: reflecting altered activity patterns * * causes of health change nutritional deficiencies narrower range of food species fewer animal foods, with lower protein and fat consumption more vulnerable to environmental insults higher infectious disease burden larger populations and higher population densities longer-term settlements (sanitation issues) modified landscapes proximity to domesticated animals and other species distribution of resources social stratification (variation structured by social status & sex)
2nd epidemiological transition begins in 19th cent Europe and n. America with industrialization accelerates post-wwII in developing nations sanitation, hygiene, and education medical technology (vaccines and antimicrobials) lifestyle changes, including diet and physical activity disease transition decline in infectious disease mortality increased burden of chronic, degenerative diseases some argue for 3rd epidemiological transition with emergence of new infectious diseases, resurgence of old diseases, and development of resistance
** infectious diseases, part 1
infectious disease in human evolution hunter gatherers: low burden of infectious disease mobile.. infectious disease burden food production leads to increased & relatively high infectious disease burden causes higher population density increased contact between.. *
“Nothing in biology makes sense except in the light of evolution” –Theodosius Dobzhansky
Proximate vs. Ultimate Questions
Biological causation: proximate vs. ultimate explanations
Physiology, genetics, biochemistry, etc. generally concentrate on * proximate causation
“What” & “How” questions
Ultimate causation: How evolutionary processes (natural selection; adaptation) and evolutionary history shape traits
“Why” questions
Traits have both types of causes—a complete biological explanation requires analysis of both
Examples: Childbirth, obesity, high blood pressure and asthma/allergy
The Big Picture: Hominid Origins
What are the key features that define us as humans & when did they …show more content…
arise?
Why did these features evolve?
How does an understanding of adaptation in earlier hominids help us understand contemporary human health issues?
Evolutionary history important for understanding modern human * condition
Two examples:
1) Bipedalism & low back pain
Why did it evolve? An extremely efficient way to travel long distances (also frees the hands for carrying)
But predisposed to back pains
Viewed as a product of design constraints & trade-offs
2) Bipedalism and brain size on infant growth and development (the “obstetric dilemma”)
Requires a shift in life history pattern
Early birth (risk for SIDS)
Requires high quality diets and ability to store large amounts of fat
Can identify times of increased risk for nutritional problems
Explains human propensity for consuming energy dense foods
A Key Concept - Novel Environments
Contemporary cosmopolitan, urban environments as novel
Widespread availability of energy dense foods (especially fats & refined carbohydrates)
Certain highly energy dense foods (especially liquids) “override” the system
Our metabolism is evolved to be thrifty, making it difficult to diet to lose fat
Links between chronic psychological stress & fat amount/pattern
Some Surprising Answers
New perspective on asthma, allergy, and autoimmune diseases
Allergy increasing in the US & developed nations
50 million in US with some type of allergy
Protective effect of microbial exposures
Decreased exposure to helminths (parasitic worms), viruses, and bacteria leads to increased risk of allergy, autoimmune disease, type I diabetes, and other diseases
Immune system tuned by environmental exposure
New treatments developed that use helminth (e.g., whipworm) introduction * * A New Perspective on Aging
Why do we experience more diseases with age?
Many physiological processes decline with age but not in all * Circumstances
How much is programmed vs. byproduct of our lives?
Are Humans Still Evolving?
Until only a few years ago most scientists would have said “No”
Food production & population expansion
Culture: Modern medicine
Evidence for evolution since beginning of food production
Including selection (sickle cell allele, lactose tolerance, etc.)
Evidence for ongoing evolution
Including selection (malaria resistance, skin color, skeletal development)
Does medicine without evolution make sense?
Evolutionary perspective complements existing biomedical frameworks—It does not replace it
Provides valuable information on:
Why diseases exist at all & our greatest disease vulnerabilities
Why disease is unequally distributed between individuals & groups
Not just an academic exercise but can influence:
How diseases are treated clinically
Public health practices & personal health decisions
Anthropology
The study of humankind using an integrative approach
Culture & society (human variation & universals)
Culture and society in the past (historic & prehistoric)
Culture and society in the past (historic & prehistoric)
Biological anthropology is the study of human biology and behavior within the framework of evolution, with an emphasis on the interaction between biology and culture
Two Focal Areas:
Human Origins & Evolution
Contemporary Human Biological Variation
The Scientific Method is a Way of Knowing
Not just collecting and cataloging facts, but instead a way of knowing based on a systematic search for explanations
A process based on:
Evidence (Asking questions & demanding evidence)
Observation (Collecting empirical evidence [data])
Generalization (Finding patterns & relationships)
Testing and continued testing (Refining explanations & correcting errors)
Self-correction – Gets better over time (Hopefully!)
Basic Principles of the Scientific Method
1) Nature is governed by natural laws
2) These natural laws can be discovered through inquiry and rational thought
3) Humans are part of nature and subject to these natural laws
These scientific principles are rooted in the works of Plato, Aristotle, and other Greek Natural Philosophers
Other Potential Principles
Or,
Asking lots of questions, being systematic in how you try to answer them, and tireless and open-minded in your pursuit of answers
Also,
“A way to ensure accountability for factual claims” – Sloan
Data Collection
Systematic & explicit in its methods
Often quantitative Set in a framework for understanding (Hypothesis Testing)
Hypothesis
Not ungrounded speculation—instead a statement about what might be true
Must be falsifiable—can be proven wrong with observations
Theory
Well-tested model that has survived repeated attempts to prove it false
Evolution is a theory (some use the term “fact”) that has stood the test of time and repeated tests and refinements
The collection of data and testing of hypotheses often leads to new and surprising questions & hypotheses
E.g., Rise in blood pressure not a natural part of the aging process
Great Chain of Being (Scala Naturae)
Basic idea dates back to Aristotle (in the 4th Century BC)
Influential for many centuries
All organisms exist in a universal hierarchical ladder
Linear (simple to complex) with continuity
Unchanging (Fixity of Species)
Humans at top but below gods & angels
“Ladder of progress”
Early Evolutionary Thinking & A First Attempt at Explaining Evolution
Key figure is French naturalist Jean Baptiste Lamarck (1744-1829)
Recognition of change in nature
Tried to explain how evolution worked
Postulated interaction of organism & environment
Inheritance of Acquired Characteristics (Use/Disuse)
Differential use of body parts; Could pass these traits to offspring
Geology and the Concept of An Ancient Earth
Key figure is the Scottish geologist Charles Lyell (1797-1875)
Founder of modern geology (Principles of Geology)
Explanatory Model: Uniformitarianism
Same gradual geological processes we see today were operating in the past
Earth is old & dynamic (geologic or deep time)
Resource Use & Competition
Key figure is the English economist and demographer T.R.
Malthus (1766-1834)
An Essay on the Principle of Population
Human population size can’t increase indefinitely because of limited …show more content…
resources
Overpopulation & constant competition for resources
Context of Darwin’s Ideas on Evolution
A “long” chronological view emerges
Geological evidence of old Earth
European contact with other civilizations with long histories
Monuments and antiquities
Human bones and artifacts with extinct animals
Earlier evolutionary ideas but:
Most speculative, not scientific
Most with no mechanism of change (or not tested with evidence)
No common descent
Charles Darwin (1809-1882)
Studied medicine & theology
Grandson of Erasmus Darwin
Zoonomia (evolutionary change & common descent)
Naturalist on HMS Beagle (1831-1836)
Biological & physical environmental variation
An Important Stop: Galapagos Islands
Fauna similar to South American mainland but with some key differences
…and no land mammals
Galapagos Birds & Reptiles: More variation within groups
Finches, Mockingbirds & Tortoises
Adaptive radiation
Diversification of one founding species into multiple species & niches
Often with new environments
Fill vacant niches
Competition may be limited
Example: Finches (Geospiza spp.)
13 species on islands
1 mainland species
Darwin’s Post-Voyage Years
Organisms not fixed but change over time
Fossils are the remnants of extinct life forms
All living organisms descend from a common ancestor
Processes today the same as those in the past (uniformitarianism)
These processes take lots of time
Natural Selection
First used by Darwin in 1842
Borrowed from “selection” from animal breeding
Influenced by Charles Lyell & T.R. Malthus
“Struggle for existence”—competition for limited resources (Malthus)
“Survival of the fittest”
Finally published in 1859
Alfred Russell Wallace was reaching similar, though not identical, conclusions
Two-step process
Variation (random process)
Selection of Variation (A non-random process, based on survival & reproduction – Differential reproductive rate)
Generate a variety of possible solutions & pick the one that works best for problem at hand
Evolution as tinkerer not engineer
Modification/transformation of what already exists Evolutionary change seen in populations, not individuals
Evolution by Natural Selection
Population variation that is heritable
Environmental pressure (expanding populations and limited resources)
Organisms with advantageous variation favored:
In their ability to survive, mate, and rear offspring to reproductive age (reproductive success)
Thus, they have higher fitness (measure of relative reproductive success): Better but not perfect
Darwin’s main contributions:
Provides evidence that evolution had occurred
Provides a naturalistic mechanism for evolution
Perspective of common descent/ancestry
Elimination of typological (essentialist) view of nature
Darwin’s Evidence
Geology & Paleontology
Old age of the Earth
Fossils of extinct animals
Some similarities with living species but the farther back in time the more different
But fossils of ancient humans almost completely unknown
Comparative Anatomy
Homologous Structures: Similar structures, but often with different functions
Indicative of common ancestry
Darwin’s Evidence (cont.)
Vestigial Structures: Retentions with no current function (e.g., pelvic bones in whales) (Appear to be “imperfections”)
Another imperfect adaptation: Testicular descent in mammals, including humans
Getting the testes out of the abdomen results in looping of the male reproductive tract around the ureter, and also increased risk of hernias
A constraint of development & evolutionary history
Comparative Embryology: Similarities in embryos but adult differences
Some traits appear early but are later lost (e.g., ape & human tails)
Could not explain certain traits if by design
e.g., Flounder eyes
Darwin’s Problems
Darwin could not explain:
How traits were inherited
Proposed that offspring inherited traits from both parents
How variation originated and was maintained
Genetics later provides us with information on:
How traits are inherited
DNA—As discrete units, not through blending
How variation originates and how it is maintained
Mutation
Understanding Evolution, Part II: Genetics
Genetics: The study of gene structure and action and the patterns of inheritance of traits from parent to offspring
Provides us with information on the following:
The inheritance of traits
Links between genes and physical traits
Links between genes and behaviors
Evolutionary relationships between organisms
The ability to measure evolutionary change in populations
Answering the questions that stumped Darwin
What is the ultimate source of variation?
Answer: Mutation
How are traits inherited from one generation to the next? Blending or discrete?
How are traits inherited from one generation to the next?
The basic principles of inheritance had been worked out by the 1860s but weren’t appreciated until 1900
Experiments by an Austrian monk named Gregor Mendel Key concepts:
Particulate Inheritance:
Not blending, but instead distinct and independent (effects can be masked but can reemerge in later generations)
The effects of some traits are dominant
They mask the expression of other traits (recessive traits) Traits are typically inherited independently of other traits
e.g., Seed color does not influence type of flower
This gives us information on variation and change (mutation)
An integrated approach?
Darwin outlines natural selection in 1859
Rediscovery of Mendel’s work in 1900
Natural selection & mutation were initially thought to be completely separate explanations for evolution and were competing hypotheses
The Modern Synthesis / Neo-Darwinism (begins in the 1930s)
Unites Natural Selection and Mendelian Genetics
Source of variation & principles of inheritance
Selection of variation based on environmental pressure
Bridges microevolution (evolution within a species) & macroevolution (evolution at or above level of species)
Given enough time, major evolutionary changes can occur
Important architects: Dobzhansky, Mayr, & Simpson
A new definition of evolution
Previously defined broadly as:
Descent with modification or change over time
A new definition…actually two:
A change in the genetic structure of a population over time
More precisely: A change in gene frequencies from one generation to the next
A Key Concept: Adaptation
A trait that increases the fitness (reproductive success) of an organism
Measured in terms of:
Differential survivorship (mortality)
Differential reproduction (fertility)
Produced by natural selection within the context of a particular environment
Environments are constantly changing
The Red Queen Effect– Leigh van Valen
Species have to “run” (evolve) in order to stay in the same place (extant / not extinct)
Evolutionary arms races (hosts vs. parasites)
However, adaptations are imperfect…
Evolution as an incremental process: No huge jumps, only small changes (each of which must be immediately beneficial)
Constrained by evolutionary history
Evolution modifies what is already there (“tinkering”)
Blind spots, retinal tears, choking, etc.
Environmental Pressure & Natural Selection
Multiple dimensions of the environment lead to adaptation
Physical: e.g., temperature
Biotic: e.g., predators or prey
Social dimensions of the environment
Social factors shape natural selection & adaptation
Members of your own species
Sexual selection: Type of natural selection that operates on only one sex; the result of competition for mates
Can lead to sexual dimorphism (physical differences between males and females)
Types: Male-male competition vs. female choice
The Discovery of DNA
1953 description of molecular structure of DNA (Watson & Crick) universal code- same basic structure and process in turnips, turtles, and humans complementary bases adenine (A) & thymine (T) cytosine © & guanine (G)
DNA directs cell replication and the creation of proteins
E.g., Collagen, hemoglobin, and hormones
Not all DNA codes for proteins (non-coding or “junk” DNA) * * Structure & Function of DNA
Gene: section of DNA with identifiable structure and/or function
Sequence of DNA that specifies order of amino acids in a protein ..
Chromosomes & DNA
Genes are found in long strands of DNA .. * * Genes: Location and variation
Locus: physical position of gene on chromosome
P as shorter arm and q as longer arm (e.g., Huntington’s disease locus: 4p16.3)
Alleles: different versions of the same gene
Found at the same locus on paired chromosomes
Mutation
An alternation in the DNA- may or may not alter function
E.g., point mutations: a single base change in the DNA
Can cause problems
Ultimate source of all genetic variation
Sickling hemoglobin, sickle cell anemia, and malaria
Types of genes
Humans have 25000 genes
Structural genes: code for proteins
Regulatory genes
Regulate expression & timing of other genes
All somatic cells contain same genetic info but not all DNA involved in protein synthesis (“master switch”)
Genes and the expression of traits
Typically not a one to one correspondence between genes and expression of traits
Most traits are influenced by multiple genes
Most traits are also influenced more or less strongly by the environment (developmental plasticity)
Genotype: specific allele composition of a certain gene or set of genes
Phenotype: the physical or biochemical expression of a genotype; the observable attributes of an organism
Single vs multi-gene inheritance
Single-gene inheritance (Mendelian traits)
E.g., curved thumb, dimples, earlobe attachment, blood type
Multi-gene (polygenic) inheritance
Traits influenced by two or more genes
E.g., height, skin color, & observable phenotypic traits
Often environmental influence & with continuous variation
Epigenetic effects
Nature (i.e., genes) vs nurture (i.e., environment): an unhelpful dichotomy
Environment affects expression of genes
E.g., fetal programming or high fat/calorie diet in childhood
Epigentics effects are differences in gene expression
Most epigenetic changes occur in early development (fetal life or infancy)
Human adaption and adaptability
Adaption often used in a general sense to refer to changes by which organisms surmount challenges to life
Types of adaptation
Cultural behavioral
Tools and technology
Medicine
Subsistence strategy and diet
Biological
There are multiple levels of biological adaption
Genetic (Darwinian) adaptation
Functional adaptations
Short-term acclimatization
Developmental acclimatization (plasticity)
Human emphasis on multiple simultaneous adaptive pathways
Darwinian or genetic adaptation: developed over many generations: reflects natural selection
Measured through differential fertility & mortality
E.g., climatic adaption
Body size/proportions and temperature
Cold: body size and proportions increase head production (large body weight) & maximize heat retention (short limbs)
Hot: reduce heat production (low body weight & long limbs)
Relatively high metabolic rates in cold environments, which produce more heat
Functional adaptations (adaptability)
Adjustments during the lifetime of an individual to help maintain homeostasis in response to a stressor
Homeostasis: maintenance of the internal physiological environment within tolerable limits
Stressor (stress): factor that interferes with homeostasis
Physical (cold, heat, etc) or psychological
Types of functional adaptations
Acclimatization: short-term, reversible
Ex:
Increased sweating ability in hot climates (within 10 days)
Tanning as an acclimatization response to exposure to high levels of UV
Developmental acc (plasticity)
Long term, irreversible changes that result from exposure to environmental stressors during growth & development
Ex:
Large lung volume at high altitude
“fetal programming” important environmental factors: nutrition disease exposure physical environment
..
Fetal programming effects of fetal environment on adult health (barker) developmental acc permanently changes structure, physiology, & metabolism (thrifty metabolism) later shift to environmental conditions (i.e., scarcity abundance), predisposed to metabolic, cardiovascular, and endocrine diseases in adulthood increased all-cause mortality, type 2 diabetes, heart disease, obesity, & hypertension
Pushing the limits of human adaption humans are an incredibly adaptable species bio-genetic and functionality cultural limits of adaption and consequences of extreme environment
Himalayan Sherpa- low levels of iodine in diet with resulting goiter and cretinism (developmental effects and mental retardation)
Siberian metabolic adaption and increased risk for hypertension and stroke
What makes us human?
What traits distinguish us from our closest relatives?
Culture
Tools and tech
Language and symbolic thinking
Art
Religion
Large brain size
Bipedalism
Reproductive bio
In fact, many of these traits are present in non-human primates
The traits that make us human did not all evolve at the same time
Humans evolved in a piecemeal fashion with different functional systems having different rates of evolution (mosaic evolution)
To answer the question what makes us human, we need to understand the evolutionary history of these traits
Humans are closely related to living great apes, especially chimps and gorillas
Share ancestor with chimps/bonobos 5-7 mya
Last common ancestor with chimps/bonobos and gorillas
Mixed arboreal and terrestrial behavior
Varied and opportunistic (omnivorous) diet
Complex social systems
Highly social with extensive learning
Limited tool use and other cultural behavior
Modern humans and hominid ancestors
Hominid: member of the group homininae, defined by the habitual bipedal adaptation
Later: increased brain size and reduced canine size (400cc vs 1300cc)
Most closely related to living African apes
Hominoid: group that includes apes and humans
We share 98.4% of our DNA with chimps
Earliest hominoids: 5-7 million yrs ago
Human evolution extremely complicated and species rich
Often, multiple species alive at one time
Multiple hominid adaptive radiations
Not one linear progression from simple to complex
Hominids evolved in Africa and were there exclusively until 2 million yrs ago
Earliest fossils in eastern and central Africa
Bipedal with small brains
Key developments: origin of the genus homo
First appears around 2.5 million years ago in east Africa
Expanded brain (500cc)
Smaller jaws and teeth
Ate higher quality foods (including meat)
First systematic stone tool use
Later, dispersal from Africa
Evolve in Africa around 160,00-200,00 yrs ago
Very large brain
Small and flat face with small teeth
Rapid cultural evolution
Global dispersal around 60,000 yrs ago
Bipedalism
Defining adaptation in hominids
Documented in early hominid fossils
E.g., Lucy (Australopithecus afraensis)
Anatomical rearrangement
Modified pelvis and lower limb
Also, vertebral column and base of skull
Positioning of foramen magnum
Energetically efficient walking in more open environments
Consequences
Low back strain, birthing issues, aging/balance
Brain expansion
Begins 2.5 million years ago with early homo (h.habilis & h.erectus)
Encephalization (increased relative size) and social factors/language
Volume increase from 400cc to 1250cc
Large body size
Appears – 1.8 mya with origin of homo erectus
Decreased predation risk
More efficient bipedalism
Relatively long legs
Larger daily range size, which facilitates dispersal
Requires high quality diet and more calories
Secondary altriciality
Children born developmentally young as a result of bipedalism and large brains
Life history shift and extended period of learning
Extremely pronounced in modern humans but started in homo
Increased dietary quality
Earliest hominids with lower quality diets
The hunted, not the hunters
Increasing diet quality in homo
Increased meat consumption and calorically dense plant foods
Facilitated by sophisticated tools
Later cooking, agriculture, etc
Reflected in decreasing size of face, jaws, and teeth and gl tract
Supports large brain size
Evolutionary medicine asks a series of answerable questions to understand this
Why hasn’t evolution eliminated the genes that make us susceptible to disease?
Why are there so many complications during birth?
Why do we lose our bone density with age?
Why do we age?
Three key players:
Paul Ewald: evolutionary biologist focused on the evolution of infectious disease
George Williams (evolutionary biologist) &
Randolph Nesse (psychiatrist): “The Dawn of Darwinian Medicine” (1991)
Application of evolutionary principles and knowledge of evolutionary history to understanding health and disease
Focuses on ultimate causes—the “Why” questions
Proximate vs ultimate questions
Biological causation in medicine: proximate vs. ultimate explanations
Physiology, genetics, biochemistry, and neurobiology generally concentrate on proximate causes
“What” & “How” questions
The immediate mechanisms that give rise to disease, disorders, and malfunctions
Ultimate causation: How evolutionary processes and evolutionary history shape traits
“Why” questions
How natural selection and other evolutionary forces shaped the trait under consideration
These questions can lead to new perspectives & treatments
Traits have both types of causes—a complete biological explanation requires analysis of both
Western biomedicine is largely based on proximate explanations of human disease
* An Example: Human Childbirth
Why is childbirth painful & dangerous?
Extremely dangerous & even modern medicine hasn’t eliminated the risk
Why? Doesn’t this seem crazy?
Proximate approach focuses on mechanisms:
The diameter of the human fetus’ head is very large and the diameter of the mother’s pelvic outlet is only slightly bigger
But, WHY???
The ultimate approach searches for evolutionary explanations
Why is the head so big?
Why isn’t the birth canal larger?
Why is the process so complicated?
Clues from using a comparative evolutionary approach:
Not terribly complicated in other primates
Solitary birth typical in other primates
From Rosenberg & Trevathan “The evolution of human birth” Scientific American, 2001
Birth in monkeys is less challenging not such a tight fit infant born facing forward (?) mother able to guide the process
An evolutionary approach to the problem
Humans are bipedal
Bipedalism led to changes in pelvic structure & a smaller and more complicated birth canal
Humans are highly encephalized
This combination leads to a complicated birthing process
Human birth is typically attended by others, especially women who have previously given birth
“Obligate midwifery”
A behavioral adaptation
A very complicated process in humans rotation through the birth canal infant typically born facing backwards mother requires assistance
Leads to more questions:
Why not a “solution” to the problem? Two potential solutions:
Smaller brain size of fetus at birth
Larger maternal pelvis
Why not a shorter gestation?
Already extreme—human newborns underdeveloped (extremely risky & require assistance)—CONSTRAINED
Why not a larger maternal pelvis?
It already is bigger than in males
Further changes CONSTRAINED by energetic efficiency of bipedal locomotion
Answer: An evolutionary compromise between efficient bipedalism & increased brain size
An Evolutionary Perspective on the Causes of Disease
1.
Defenses
2. Infectious Organisms & Evolutionary Arms Races
3. Novel Environments
4. Genes
5. Compromises
6. Constraints & Evolutionary Legacies
1. Defenses
Not actually causes but evolved defenses; can be confused with manifestations of disease
e.g., pain, fever, cough, nausea, vomiting, sneezing, anxiety, morning sickness
Decisions on when to block these defenses7
Evidence that fever (rise in the set point of the body’s thermostat) is an adaptation
Speeds up immunological reactions and reduces reproductive rate in many pathogens
Fever is adaptive against some pathogens
Lizards: move to warmer locations to increase body temperature
Humans: Some evidence that fever is adaptive against colds (rhinovirus) and syphilis
But fever has costs (e.g., metabolic & cell damage)
Bad idea to always suppress fever; very high fever can be bad
Reducing fever generally does not lead to faster recovery times or increased comfort while sick “Fever, as a mechanism that activates the immune system to cure us of the pathogen, is a defense, a tool on our side, and the best way to control an illness is to leave the fever alone, at least some and perhaps most of the time.” - Zuk
2007
Some conditions can be either defense or symptom, depending on the cause
Diarrhea from Shigella bacteria appears to be an evolved defense (Lomotil treatment prolongs illness)
However, cholera bacteria (Vibrio cholerae) has a toxin that causes diarrhea (anti-diarrheal treatment counters spread and minimizes dehydration)
Defense or disease manifestation— It’s not always easy to tell the difference!
2. Infectious Organisms & Evolutionary Arms Races
Infectious disease: Introduction of foreign organic substance (e.g., virus, bacteria, fungus)
Not just simple pests but sophisticated opponents
Most infectious agents reproduce relatively fast and mutations create different forms
Evolutionary arms race perspective helps us understand permanency of infectious disease & also some autoimmune disease
The Red Queen Hypothesis (Leigh van Valen)
We have evolved ways to counter their threats
Cultural strategies too (e.g., antibiotics & sanitation)
Cultural factors may contribute to disease burden & their evolution (antibiotic use & nosocomial [hospital acquired] infections)
Emergence of infectious diseases in modified form (e.g., MRSA [methicillin-resistant Staphylococcus aureus])
3. Novel Environments
Contemporary cosmopolitan, urban environments as novel
Different from the environmental conditions in which our species evolved
Nutritional environments
Change in food availability & physical activity levels
Relatively high activity levels in subsistence-level populations
Minimal use of extremely energy dense foods, which can “override” the system
Risk of cardiovascular disease & diabetes
Atherosclerotic plaque: fat, cholesterol, & calcium10
Infectious disease environments
Too few infectious exposures early in life can lead to allergy, asthma, and autoimmune disease
Psychological environments too
Some anxiety or depressive symptoms may be good
Sadness can lead us to stop a behavior that caused loss (e.g., money, reputation, etc.)
But, under certain conditions can lead to
Depression
Schistosoma mansoni parasitic flatworms
A little fear, anxiety, and depressive symptoms can be good, but there is also the risk of depression, anxiety disorder, etc.
New environments = New threats
Past (Hunter-gatherer populations):
Accidents
Hunger
Predation
Exposure
Parasites/Infectious disease
Now (food production, higher population density, industrialization, urbanization, globalization): Heart attack, stroke, atherosclerosis
Diabetes
Obesity
Cancer
Emerging infectious diseases/Epidemics11
It’s not a simple matter of going back to the past
“Just because our species evolved in a different environment does not mean…that following the ways of the past is automatically going to free us from the illnesses of modern life.” -Zuk 2007
It’s not a simple matter either
What “past” are we talking about?
Also, human variation in biology, diets, activity patterns, etc.
Dietary variation in hunter-gatherers by environment
4. Genes
Some genes are perpetuated despite the fact that they cause disease
Some because of novel environments (e.g., related to metabolism & fat deposition but now obesity)
A life course perspective is important (timing is everything!)
Gene variants related to increased calcium absorption can be adaptive early in life (increased bone density) but have negative effects later (increased arterial plaque formation)12
Some genes maintained because they have other benefits (sickle cell allele & malaria) Example: Sickle cell anemia: 1 in every
600 births among African-Americans
Sickle-shaped red blood cells block peripheral blood vessels
Without regular medical intervention few individuals live to adulthood
If sickle cell disease is so lethal, then why hasn’t it been eliminated?
Caused by an allele that can also be beneficial
(balanced polymorphism)
5. Compromises
Human bodies as “bundles of compromises shaped by natural selection in small increments to maximize reproduction, not health.”
–Nesse & Stearns 2008
Bipedalism: An extremely efficient way to travel long distances (also frees the hands for carrying)
But, predisposes us not only to birth complications but also back pain
Our joints are a compromise between efficient locomotion and stability13 6. Constraints & Evolutionary Legacies
Evolution as an incremental process:
No huge jumps, only small changes (each of which must be immediately beneficial)
Constrained by evolutionary history
Evolution modifies what is already there
(tinkering)
Human gut as modified primate gut (appendix)
Also, blind spots, retinal tears, & choking
Infant and child health
Disease and death rates for different age groups provide insights into causes
When are people getting sick and dying…
life expectancy measured at different ages extremely sensitive measure of population level health
* **epidemiological transitions * * 1st epidemiological transition begins about 10,000 years ago from nomadic hunting and gathering to sedentism and food production led to major changes in social organizations, demography..
origins of agriculture and animal domestication earliest 10,000-12,000 yrs ago
Neolithic period
Multiple locations independently
At least 10 separate centers of plant domestication
Massive population growth fueled by food production
Farm land: much higher rates of productivity per unit area than foraged land
Feed larger population
Ability to generate surpluses and allows storage
Domesticated species were originally low value foods for hunter-gatherers
Costs of higher productivity
Higher labor/energy investment
Narrower range of food species
More vulnerable to famines
..
health consequences of food production in general, population-level health decreases with food production paleopathology reduced height compared to hunter-gatherers thinned limb bone shafts evidence of iron-deficiency anemia evidence of increased infectious disease burden and interpersonal conflict (ex. War and organized violence) dental evidence tooth enamel defects increases cavities etc harris lines: bands of dense bone showing period of interrupted growth
occupational stress markers markers of repetitive stress kneeling facets: on distal metatarsals also “squatting facets” of tibia osteoarthritis: reflecting altered activity patterns * * causes of health change nutritional deficiencies narrower range of food species fewer animal foods, with lower protein and fat consumption more vulnerable to environmental insults higher infectious disease burden larger populations and higher population densities longer-term settlements (sanitation issues) modified landscapes proximity to domesticated animals and other species distribution of resources social stratification (variation structured by social status & sex)
2nd epidemiological transition begins in 19th cent Europe and n. America with industrialization accelerates post-wwII in developing nations sanitation, hygiene, and education medical technology (vaccines and antimicrobials) lifestyle changes, including diet and physical activity disease transition decline in infectious disease mortality increased burden of chronic, degenerative diseases some argue for 3rd epidemiological transition with emergence of new infectious diseases, resurgence of old diseases, and development of resistance
** infectious diseases, part 1
infectious disease in human evolution hunter gatherers: low burden of infectious disease mobile.. infectious disease burden food production leads to increased & relatively high infectious disease burden causes higher population density increased contact between.. *