So, Just Where Are We?Our course is concerned with US, where we came

from, why we look and behave the way we do.

In order to explore these issues, one of the first things that has to be done is to focus on us as humans, distinct from the rest of the animal world.

Thus, over the past two weeks, we have looked at the pattern of human distinctiveness, in biology, behavior and language. It is necessary that we begin our investigations of ourselves with the examination of just what it is that makes us:

Homo sapiens

Humans as Biologically Unique• Two weeks ago, we

explored the biological features that make us unique, including our erect posture, our large brains and our lack of projecting canine teeth.

• The chimpanzee film gives you an idea of just how different and similar we are to each other and sets in dramatic focus our evolutionary relationships with these African apes.

• Now, it is time to look at what we represent as a species.

Humans as a variable speciesAll modern humans are

part of a single species: Homo sapiens (we can all interbreed and produce fertile offspring). However, as we have already seen, we also differ in many biological features.

Today, we begin our second section, dealing with variation: what is the nature and causes of human biological, social and linguistic variation?

Human Variation 1.

There are all sorts of differences amongst humans, and we are all aware that people vary in facial and body features. We also know that these differences extend to the genetic materials themselves.

Indeed, there are parts of the human genome that are so hypervariable that examination of these specific locales can identify specific individuals with a 1 in 88,000 chance of mistaking one person with another.

Human variation I

Human Variation II

Human Population Variation• All humans vary (except for identical twins, but that is a

specific case, and usually by adulthood, there are differences resulting from environmental influences).

• This variation is the result of the complex interaction of genetic and environmental influences, but also random forces.

• It is clear that biological features are NOT randomly distributed across the human landscape but have specific geographic distributions. This is the basis for the simplistic notions of race that are used in social interactions.

• It is equally clear that most of the human variation that we observe is the result of human evolution to meet the needs of specific environmental demands, like climate, food, altitude and many other variables.

Genetics: The source of variability for evolution

The basis for much of the variability we see in modern humans originates in our genetic

material.

Like all living things, humans possess a genetic structure based on DNA and its ability

tocode for a huge class of functioning molecules: Proteins.

Our genetic structure has been formed over millions of years of evolutionary change.

Our focus today: how population survival strategies determine human biology and provide the basic background for human

variation

What does the genetic material do, anyway?

• The genetic material has a number of crucial functions:

1. Transmit genetic information from one generation to the next (humans produce human infants and not rats or elephants).

2. Since every cell in the body (with several exceptions) has more or less the same genetic material as the original cell (the fertilized egg), the genetic material must be able to reproduce itself when new cells are produced during growth and development as well as normal body maintenance.

3. The genetic materials are organized around a sequence of chemical ‘bases’ that encode for the synthesis of proteins, a huge class of chemicals that perform a wide range of functions in the body.

What determines cell structure and function?

• Proteins that are expressed

• Unique expression by cell type

• How is this controlled?

• Look to the cell nucleus

Chromosomes

• Carries information as part of their structure

• Name=colored bodies when stained and seen microscopically

• Species-specific number in each cell nucleus, with the chromosome number usually expressed in pairs (the complexity of the living thing is not reflected in the chromosome number (chimps, for example, have more chromosomes than humans).

Human chromosomes• Species specific number=46• 23 pairs of chromosomes• Specifially:• 22 pairs of autosomes

– or, homologous chromosomes

• 1 pair of sex chromosomes– XX female– XY male

• Question: Why are there pairs of chromosomes?

Where do the chromosomes come from?

• We are originally one cell:

• 23 of maternal origin– ova carry these

• 23 of paternal origin– sperm carry these

• If every cell has 46, how do these end up with only 23 and why?

Meiosis

Mitosis

• Cell Division• Chromosomal

Replication

What does the genetic material do?

• While the functions of the genetic material located on the chromosomes are numerous and complicated, for our purposes, we can examine the major function: that of the synthesis of proteins.

• Proteins are a very large class of molecules that perform a huge array of functions in living things. It has been estimated that there may be as many as 50,000 different proteins in the human body, only about 1500 of which have been identified.

• Proteins differ from one another, and thus perform differently, based on their organization and makeup.

Proteins: What are they?

Basic: You are what you eat! The ultimate source of your body’s protein is from your diet.

• Functions include:– Structure– Transport– Immune

• Function reflects their structure– Proteins have 3

dimensional structure– Folded chains

Differentiation

• Unique proteins in different cell types

• Keep in mind that every cell receives a complete set of chromosomes and thus the genetic materials.

• Thus, although every cell has the ability to produce every single protein in the body, specific cells only “turn on” very small segments of the total genetic material and only synthesize a very few proteins.

Proteins: what distinguishes one from another?

1. Proteins are composed of chains of amino acids (Polypeptide chains).

2. Polypeptide chains have variable lengths.

3. The sequence of amino acids along the chains vary.

4. Proteins can be made up of one or, more usually, two or more chains of amino acids.

5. Proteins have a folded three dimensional structure

Amino Acids: What are Amino Acids: What are they and where do they and where do they come fromthey come from??

• Chemical group based on their composition: an “amine” and an “acid”

• Of the 20 common amino acids:– 12 the body can

make– 8 must be eaten

be obtained from foods (these are the essential amino acids)

Glycine (gly) Glutamic acid (glu)

Alanine (ala) Aspartic acid (asp)

Valine (val) Isoleucine (Ile)Leucine (leu) Serine (ser)Threonine (thr) Proline (pro)Lysine (lys) Arginine (arg)Glutamine (gln) Aspargine (asn)Methionine (met) Cysteine (cys)Tryptophan(trp) Tyrosine (tyr)Histidine (his) Phenylalanine

(phe)

Proteins: How they are made: amino acids

• Polypeptide chains=aa• Sequence of aa crucial

to structure, and thus function

• Sequence determined by series of nucleic acids and the genetic code

• Determined by a gene

valhisleuthraspalaglulysalaalaval

sscysleutrpglylysvalasnseraspglu

met

What is a gene?• A “recipe” for a protein, or

more accurately, for a single polypeptide chain.

• Located at a specific region (locus) on a specific chromosome.

• A gene can have many alternative versions of itself. These are known as alleles.

• Implications:– different chromosomes carry

different information– Alleles are one of the major

sources of variation.

Diversity of form and function• The basis of evolution is variation• This variability in biological form

and function arises from genetic mutations, compounded by the random assorting of genetic materials during the production of eggs and sperm.

• There are two levels in evolution :1. Microevolution: the

generation by generation changes in a population.

2. Macroevolution: the evolution of populations over long periods of time, usually resulting in the appearance of a new species.

The origins of Variation I• Random changes in the structure of the

genetic material (DNA), which are called mutations, result in the appearance of new variations. This is one of the crucial pieces of information that Darwin lacked, and thus prevented him from actually directly addressing the name of his book: The Origin of Species (1859).

• It was an ongoing criticism of Darwinian evolution until the discoveries of how genetic material can randomly change provided the missing data.

The origins of Variation II• The second major source of variation is the

recombination of genetic traits that results from the transmission of genetic traits from male and female parents.

• During the process of producing sex cells, the genetic material carried in the chromosomes randomly assorts so that each offspring receives a random selection of parent’s genetic materials.

• Thus, each offspring confronts the environment with a slightly different set of genetic materials.

Genetics and EvolutionA Darwinian View

• Thus, genetic mechanisms produce random changes (mutations) in the genes. The changes are not in any way dictated by the environment or the needs of the animal.

• These mutations are acted upon by natural selection via behavior.

• In time, the changes may come to characterize the entire population.

Evolutionary Mechanisms• Evolutionary Mechanisms effect the frequency

of genetic variations (alleles) in a population, and thus represent the way by which evolution operates.

• There are four evolutionary mechanisms:

1. Mutation

2. Natural Selection

3. Gene Flow (Migration)4. Stochastic Forces (random genetic drift).

Evolutionary Mechanisms

1. Natural Selection:

reduces variation

2. Mutation:

increases variation

3. Gene Flow (Migration):

can reduce or increase variation

4. Stochastic Forces (Genetic Drift):

reduce variation

Action of Evolutionary Forces• These evolutionary forces do not work on

individuals, but rather on populations.• Individuals do not evolve, and in evolution,

the only role played by individual animals is to receive genetic materials from parents and pass them on to offspring.

• Evolution is differential reproduction, and not necessarily survival .

Human Variation and Adaptation• Human variation is very often related to the

evolutionary adaptation of a population to its environment.

• But, it is important to keep in mind that mutations are random events that do NOT appear as the result of some need that a population has to adapt.

• As a result, variations are NOT adaptive all of the time and in all places but are very much related to specific environmental conditions. When conditions change, it is often difficult for a species to modify to fit the new environmental conditions and they will often become extinct.

The Basis for Human Variation• Finally in this context, it should be kept in mind that

there are variations that may not have an adaptive basis. It is possible that in small populations, variations have become part of the biology by random changes in the genetic material. This process, known as genetic drift, can result in the appearance of features that are not part of an adaptive system. These random, or stochastic (means random) processes add a measure of uncertainty and complexity to the study of human variation.

• For example, when a particular trait is examined (hair color), should we always assume that the variations are adaptive and look for an environmental cause?

Human VariationThere are all sorts of differences amongst humans, and

we are all aware that people vary in facial and body features. We also know that these differences extend to the genetic materials themselves.

We will see from our examination of the human fossil record that during our evolution, humans became established in environments with differing conditions.

Thus, the origins of many of our biological variation appear to be directly related to our evolution as a geographically diverse species occupying a wide range of environments.

The Nature of VariationKeep in mind the following:

• The similarity of humans across all populations is very high, with numerous studies documenting that genetic variability within major geographic areas (i.e. Africa or Europe) is greater than the genetic variation between geographic areas.

• All modern humans share the unique attributes of the species, and thus are identical for this suite of features.

• What we can examine are those biological features, like skin color, which do vary amongst geographical populations, and which have often been used to characterize ‘races’.

The study of human variability:Description and process

• Populations vs individuals

• Nature vs nurture

• Stochastic vs adaptive

Variability in form and function in adults is the

result of the interaction of biocultural complexes

and reflects evolutionary scenarios:• Genetic

mechanisms– mutation– drift– selection– flow

• Development• Environmental

variables

The Interaction of genes and environment

In the development of an individual, a complex interplay of the genetic materials and environmental (which of course includes culture) factors is responsible for the outcome: a functioning (and reproducing) adult:

GENES + interaction with ENVIRONMENT = ADULT Form

(Genotype) (Phenotype)

Adaptation: The interplay of genotype and phenotype

• Genotype is the underlying framework– Environment molds and channels genotype

into the final phenotype– In humans, culture and the resultant

complex behavior are a unique basis for the environment to influence and shape final phenotype.(This represents a complex interplay that

few other animals have.)

Adaptation– Changes or modifications that enable a

person or group to survive in a given environment

This can be accomplished by means of:– Biological mechanisms

• Genetics• Physiology• Development

– Cultural, behavioral mechanisms

Environmental challenges

Human variation is related to environmental variation, with many biological complexes reacting to variations such as:

• Disease• Temperature• Altitude• Nutrition

Adaptation:

The basis of body shape and size?

Human variability:

populations vs individuals

The variable human species: Homo sapiens, The product of evolution and adaptation