D1 ORIGIN OF LIFE ON EARTH

D.1.1 Describe four processes needed for the spontaneous origin of life on Earth.

Include:

D.1.2 Outline the experiments of Miller and Urey into the origin of organic compounds.

They simulated conditions on early Earth by constructing an apparatus that contained a warmed flask of water simulating the primeval sea and an atmosphere of H20, H2, CH4 (methane), and NH3 (ammonia). Sparks were discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds back to the miniature sea. The simulated environment produced an abundance of amino acids and other organic molecules. However, it is now known that the environment did not contain a large amount of methane, so the results of this experiment are not so reliable.

Stanley Miller, a graduate student in biochemistry, built the apparatus shown here. He filled it with

• water (H₂O

• methane (CH₄)

• ammonia (NH₃) and

• hydrogen (H₂)

• but no oxygen

He hypothesized that this mixture resembled the atmosphere of the early earth. (Some are not so sure.) The mixture was kept circulating by continuously boiling and then condensing the water.

The gases passed through a chamber containing two electrodes with a spark passing between them.

At the end of a week, Miller used paper chromatography to show that the flask now contained several amino acids as well as some other organic molecules.

In the years since Miller's work, many variants of his procedure have been tried. Virtually all the small molecules that are associated with life have been formed:

• 17 of the 20 amino acids used in protein synthesis, and

• all the purines and pyrimidines used in nucleic acid synthesis.

• But abiotic synthesis of ribose — and thus of nucleosides — has been much more difficult.

One difficulty with the primeval soup theory is that it is now thought that the atmosphere of the early earth was not rich in methane and ammonia — essential ingredients in Miller's experiments.

D.1.3 State that comets may have delivered organic compounds to Earth.

Comets contain a variety of organic compounds. Heavy bombardment about 4,000 million years ago may have delivered both organic compounds and water to the early Earth.

The Murchison Meteorite

Representative amino acids found in the Murchison meteorite. Six of the amino acids (blue) are found in all living things, but the others (yellow) are not normally found in living matter here on earth. The same amino acids are produced in discharge experiments like Miller's.
Glycine	Glutamic acid
Alanine	Isovaline
Valine	Norvaline
Proline	N-methylalanine
Aspartic acid	N-ethylglycine

This meteorite, that fell near Murchison, Australia on 28 September 1969, turned out to contain a variety of organic molecules including:

• purines and pyrimidines

• polyols — compounds with hydroxyl groups on a backbone of 3 to 6 carbons such as glycerol and glyceric acid. Sugars are polyols.

• the amino acids listed here. The amino acids and their relative proportions were quite similar to the products formed in Miller's experiments.

The question is: were these molecules simply terrestrial contaminants that got into the meteorite after it fell to earth.

Probably not:

• Some of the samples were collected on the same day it fell and subsequently handled with great care to avoid contamination.

• The polyols contained the isotopes carbon-13 and hydrogen-2 (deuterium) in greater amounts than found here on earth.

• The samples lacked certain amino acids that are found in all earthly proteins.

• Only L amino acids occur in earthly proteins, but the amino acids in the meteorite contain both D and L forms (although L forms were slightly more prevalent).

The ALH84001 meteorite

This meteorite arrived here from Mars. It contained not only a variety of organic molecules, including polycyclic aromatic hydrocarbons, but — some claim — evidence of microorganisms as well.

Furthermore, there is evidence that its interior never rose about 40° C during its fiery trip through the earth's atmosphere. Live bacteria could easily survive such a trip.

Link to a discussion of the possibility of life on Mars and more on the ALH84001 meteorite.

Organic molecules in interstellar space

Astronomers, using infrared spectroscopy, have identified a variety of organic molecules in interstellar space, including

• methane (CH₄),

• methanol (CH₃OH),

• formaldehyde (HCHO),

• cyanoacetylene (HC₃N) (which in spark-discharge experiments is a precursor to the pyrimidine cytosine).

• polycyclic aromatic hydrocarbons

• as well as such inorganic building blocks as carbon dioxide (CO₂), carbon monoxide (CO), ammonia (NH₃), hydrogen sulfide (H₂S), and hydrogen cyanide (HCN).

Laboratory Synthesis of Organic Molecules Under Conditions Mimicking Outer Space

There have been several reports of producing amino acids and other organic molecules by taking a mixture of molecules known to be present in interstellar space such as:

• ammonia (NH₃)

• carbon monoxide (CO)

• methanol (CH₃OH) and

• water (H₂O)

• hydrogen cyanide (HCN)

and exposing it to

• a temperature close to that of space (near absolute zero)

• intense ultraviolet (uv) radiation.

Whether or not the molecules that formed terrestrial life arrived here from space, there is little doubt that organic matter continuously rains down on the earth (estimated at 30 tons per day).

D.1.3 Discuss the hypothesis that the first catalysts responsible for polymerization reactions were clay minerals and RNA.

• Polymers are chains of similar building blcoks or monomers, synthesized by condensation reactions (H and OH are removed from polymers and H20 is produced). Early polymerization reactions must have occurred without the help of enzymes.

Clay increases the rate of polymerization in these ways:

• Monomers bind to charged sites in clay, concentrating amino acids and other monomers

• Metal ions at binding sites in clay catalyze dehydration reactions

• These binding sites bring monomers close together. Thus the clay acts as a template with a specific structure.

Functions of RNA in polymerization:

• There is a type of RNA called a ribozyme that can catalyze its own replication. This may be very crucial to the origin of RNA and DNA and therefore for the beginning of life.

Another problem is how polymers — the basis of life itself — could be assembled.

• In solution, hydrolysis of a growing polymer would soon limit the size it could reach.

• Abiotic synthesis produces a mixture of L and D enantiomers. Each inhibits the polymerization of the other. (So, for example, the presence of D amino acids inhibits the polymerization of L amino acids (the ones that make up proteins here on earth).

Link to a discussion of enantiomers.

This has led to a theory that early polymers were assembled on solid, mineral surfaces that protected them from degradation, and in the laboratory polypeptides and polynucleotides (RNA molecules) containing about ~50 units have been synthesized on mineral (e.g., clay) surfaces.

An RNA Beginning?

All metabolism depends on enzymes and, until recently, every enzyme has turned out to be a protein. But proteins are synthesized from information encoded in DNA and translated into mRNA. So here is a chicken-and-egg dilemma. The synthesis of DNA and RNA requires proteins. So

• proteins cannot be made without nucleic acids and

• nucleic acids cannot be made without proteins.

The discovery that certain RNA molecules have enzymatic activity provides a possible solution. These RNA molecules — called ribozymes — incorporate both the features required of life:

• storage of information

• the ability to act as catalysts

Link to a discussion of ribozymes.


While no ribozyme in nature has yet been found that can replicate itself, ribozymes have been synthesized in the laboratory that can catalyze the assembly of short oligonucleotides into exact complements of themselves. The ribozyme serves as both

• the template on which short lengths of RNA ("oligonucleotides" are assembled following the rules of base pairing and

• the catalyst for covalently linking these oligonucleotides.

(The figure is based on the work of Green and Szostak, Science 258:1910, 1992.)

In principal, the minimal functions of life might have begun with RNA and only later did

• proteins take over the catalytic machinery of metabolism and

• DNA take over as the repository of the genetic code.

Several other bits of evidence support this notion of an original "RNA world":

• Many of the cofactors that play so many roles in life are based on ribose; for example:

• ATP

• NAD

• FAD

• coenzyme A

• cyclic AMP

• GTP

• In the cell, all deoxyribonucleotides are synthesized from ribonucleotide precursors.

• Many bacteria control the transcription and/or translation of certain genes with RNA molecules (Link to "riboswitches") , not protein molecules.

D.1.4 Discuss the possible role of RNA as the first molecule capable of replicating.

• Protobionts, or aggregates of abiotically produced molecules, accumulate organic materials from the environment. However, unless they possessed a mechanism for replicating their unique catalysts and other functional molevules, they could not replicate. A mechanism like RNA, however, would allow them to pass on their characteristics. RNA probably preceded DNA as DNA is too complex of a molecule to have formed in early protobionts. Short strands of RNA, perhaps most importantly the RNA called ribozymes, could have polymerized abiotically. They could then align nucleotides according to a certain pattern when bound to clay. Thus RNA could be replicated and passed on.

D.1.5 Discuss a possible origin of membranes and prokaryotic cells.

• Living cells may have been preceded by protobionts, aggregates (groups) of abiotically produced molecules. They are coated by a protein membrane and can maintain a fairly constant internal environment. Some protobionts in the presence of lipids form a molecular bilayer at the surface of the protobiont droplet. This resembles the lipid bilayer of modern cells. Lipid molelcules automatically assemble in a spherical bilayer, so if this formation caught some cell-stuff inside it, it could form a true life form.

To function, the machinery of life must be separated from its surroundings — some form of extracellular fluid (ECF). This function is provided by the plasma membrane.

Today's plasma membranes are made of a double layer of phospholipids. They are only permeable to small, uncharged molecules like H₂O, CO₂, and O₂. Specialized transmembrane transporters are needed for ions, hydrophilic, and charged organic molecules (e.g., amino acids and nucleotides) to pass into and out of the cell.

However, the same Szostak lab that produced the finding described above reported in the 3 July 2008 issue of Nature that fatty acids, fatty alcohols, and monoglycerides — all molecules that can be synthesized under prebiotic conditions — can also form lipid bilayers and these can spontaneously assemble into enclosed vesicles.

Unlike phospholipid vesicles, these

• admit from the external medium charged molecules like nucleotides

• admit from the external medium hydrophilic molecules like ribose

• grow by self-assembly

• are impermeable to, and thus retain, polymers like oligonucleotides.

These workers loaded their synthetic vesicles with a short single strand of deoxyguanosine (dC) structured to provide a template for its replication. When the vesicles were placed in a medium containing (chemically modified) dG, these nucleotides entered the vesicles and assembled into a strand of Gs complementary to the template strand of Cs.

Here, then, is a simple system that is a plausible model for the creation of the first cells from the primeval "soup" of organic molecules.

D.1.6 Discuss the endosymbiotic theory for the origin of eukaryotics.

• Forerunners of eukaryotic cells were symbiotic consortiums of prokaryotic cells with certain species living within larger prokaryotes (cells that live within another cell, termed the host cell). The endosymbiotic theory focuses on the origins of chloroplasts and mitochondria. The ancestors of these organisms originally entered the host cell as undigested prey or internal parasites. The host cell would then exploit these organisms for nutrition. The evidence that supports this theory is as follows:

• Mitochondria and chloroplasts have bacteria-like RNA and ribosomes that enable them to make thier own proteins.

• Mitochondria and chloroplasts both have double bilayer, as if the bacteria ancestors of these organelles had picked up an extra layer when entering the cell.

The 3 kingdoms of contemporary life — archaea, bacteria, and eukaryotes — all share many similarities of their metabolic and genetic systems [Link]. Presumably these were present in an organism (or organisms) that were ancestral to these groups: the "LUCA". Although there are not enough data at present to describe LUCA, comparative genomics and proteomics reveal a closer relationship between archaea and eukaryotes than either shares with the bacteria. (Except, of course, for the mitochondria and chloroplasts that eukaryotes gained later from bacterial endosymbionts [Link].)

D.1.1 Outline the conditions of pre-biotic Earth, including high temperature, lightning, UV light penetration and a reducing atmosphere.

• There was little oxygen, so there was no oxygen to steal electrons away from other atoms. Such a reducing atmosphere would have enhanced the joining of simple molecules to form more complex ones. The energy for forming the molecules was provided by lightning, volcanic activity, meterorite bombardment and UV radiation. At first, the earth was cold and later melted from heat produced by compaction, radioactive decay and the impact of meteorites. The molten material sorted into layers of varying density with the least dense material solidified into a thin crust. The present continents are attached to plates of crust that float on the mantle. The first seas formed from torrential (stormy) rains that began when Earth had cooled enough for water in the atmosphere to condense.

D.1.4 Discuss possible locations where conditions would have allowed the synthesis of organic compounds.

Examples should include communities around deep-sea hydrothermal vents, volcanoes and extraterrestrial locations.

D.1.5 Outline two properties of RNA that would have allowed it to play a role in the origin of life.

Include the self-replicating and catalytic activities of RNA.

D.1.6 State that living cells may have been preceded by protobionts, with an internal chemical environment different from their surroundings.

Examples include coacervates and microspheres.

D.1.7 Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphere.

D.1.8 Discuss the endosymbiotic theory for the origin of eukaryotes.

TOK: As with other theories that aim to explain the evolution of life on Earth, we can obtain evidence for a theory and we can assess the strength of the evidence. However, can we ever be sure that the theory explains what actually happened in the past? For something to be a scientific theory, we must also be able to test whether it is false. Can we do this if the theory relates to a past event? Is a special standard required for claims about events in

the past to be scientific? If they cannot be falsified, is it enough if they allow us to make predictions?

D2 SPECIES AND SPECIATION

D.2.1 Define allele frequency and gene pool

Allele frequency

Gene pool

D.2.2 State that evolution involves a change in allele frequency in a population's gene pool over a number of generations.

D.2.3 Discuss the definition of the term species.

D.2.4 Describe three examples of barriers between gene pools.

Examples include geographical isolation, hybrid infertility, temporal isolation and behavioural isolation.




D.2.5 Explain how polyploidy can contribute to speciation.

Avoid examples involving hybridization as well as polyploidy, such as the evolution of wheat.

D.2.6 Compare allopatric and sympatric speciation.

Speciation: the formation of a new species by splitting of an existing species. Sympatric: in the same geographical area.

Allopatric: in different geographical areas.

D.2.7 Outline the process of adaptive radiation.

D.2.8 Compare convergent and divergent evolution.

D.2.9 Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium.

Gradualism is the slow change from one form to another. Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution. Volcanic eruptions and meteor impacts affecting evolution on Earth could also be mentioned.

D.2.10 Describe one example of transient polymorphism.

An example of transient polymorphism is industrial melanism.

D.2.11 Describe sickle-cell anemia as an example of balanced polymorphism.

Sickle-cell anemia is an example of balanced polymorphism where heterozygotes (sickle-cell trait) have an advantage in malarial regions because they are fitter than either homozygote.

D.2.1 Outline Lamarck's theory of evolution by the inheritance of acquired characteristics.

• Inheritance of acquired characteristics - states that the modifications an organism acquired during its lifetime could be passed along to its offspring.

D.2.2 Discuss the mechanism of, and lack of evidence for, the inheritance of acquired characteristics.

• Use and disuse- those body organs used extensively to cope with the environment become larger and stronger while those that do not deteriorate. There is no evidence that acquired characteristics can be inherited. Characteristics that can be passed on are in your genes. If you are not born with genes programming for certain characteristics, you cannot pass down characteristics acquired during your lifetime. Example: a weighlifter will not pass down his/her larger, stronger muscles to children.

D.2.3 Explain the Darwin-Wallace theory of evolution by natural selection.

• While on his Beagle voyages, Darwin became intrigued with the different types of finches found in the Galapagos. All the species of birds differed in size and beak shape and Darwin found that the birds fed on different types of food. Their beaks are adapted to eat different types of leaves, worms and seeds and other types of diets. Darwin explained all his observations and thoughts about the origin of species by the concept of "Natural Selection". This theory states that great diversity in a species ensures that some members of a population will be more suited for their environment than others. These individuals will be more likely to live long enough to reproduce and pass on their well-suited genes. Therefore, because those that are best suited are the ones who have the most children, a population will, over time, become very well suited to its evironment. A change in population is evolution.

D.2.4 Discuss other theories for the origin of species including special creation and panspermia.

• Panspermia is the theory concerned with the arrival of material from outer space. Panspermia theory suggests that life was sent to earth from comets or meteors, and was not formed on earth. Special creation is mentioned by several religions; a study of all of them is not required. This theory states that creator(s) formed life directly.

D.2.5 Discuss the evidence for all these theories and the applicability of the scientific method for further investigation.

• Panspermia - organic compounds and amino acids have been recovered from modern meteorites. This theory is appealling to those who find little evidence for an environment on earth that could spontaneously produce life. Special Creation- most of the "evidence" for this "theory" is found in religious faith. There is no real scientific justification, other than lack of justification for alternate theories.

D3 HUMAN EVOLUTION

D.3.1 Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K.

Knowledge of the degree of accuracy and the choice of isotope to use is expected. Details of the

apparatus used are not required.

D.3.2 Define half-life

D.3.3 Deduce the approximate age of materials based on a simple decay curve for a radioisotope.

D.3.4 Describe the major anatomical features that define humans as primates.

D.3.5 Outline the trends illustrated by the fossils of Ardipithecus ramidus, Australopithecus including A. afarensis and A. africanus, and Homo including H. habilis, H. erectus,

H. neanderthalensis and H. sapiens.

Knowledge of approximate dates and distribution of the named species is expected. Details of

subspecies or particular groups (Cro-Magnon, Peking, and so on) are not required.

D.3.6 State that, at various stages in hominid evolution, several species may have coexisted.

An example of this is H. neanderthalensis and H. sapiens.

D.3.7 Discuss the incompleteness of the fossil record and the resulting uncertainties about human evolution.

Reasons for the incompleteness of the fossil record should be included.

TOK: Paleoanthropology is an example of the diverse aspects of science, in that it is a data-poor science with largely uncontrollable subject matter. Paradigm shifts are more common in a data-poor science. The discovery of small numbers of fossils has caused huge changes in theories of human evolution, perhaps indicating that too much has been constructed on too little.

Conversely, discoveries such as those made in Dmanisi, Georgia provide examples of falsification of earlier held positions, indicating why paleoanthropology can be considered a science.

D.3.8 Discuss the correlation between the change in diet and increase in brain size during hominid evolution.

D.3.9 Distinguish between genetic and cultural evolution.

D.3.10 Discuss the relative importance of genetic and cultural evolution in the recent evolution of humans.

TOK: This is an opportunity to enter into the nature/nurture debate. There is clear causation

when a genetic factor controls a characteristic. Cultural factors are much more complex, and

correlation and cause are more easily confused.

D.4.1 State the full classification of human beings from kingdom to sub-species.

• Kingdom - Animalia

• Phylum - Chordata

• Class - Mammalia

• Order - Primata

• Family - Hominidae

• Genus - Homo

• Species - sapiens

• subspecies - sapiens

D.4.2 Describe the major physical features, such as the adaptations for tree life, that define humans as primates.

• Shoulder joints that allow movement in 3 dimension

• Dexterous hands with opposable thumb and long fingers

• Sensitive fingers with nails

• Eyes closer together in front of the face for enhanced depth perception, excellent eye-hand coordination

• Shoulder joints and skull modified for upright posture

• Parental care with usually single births and long nurturing of offspring.

D.4.3 Discuss the anatomical and biochemical evidence which suggests that humans are a bipedal and neotenous species of African ape that spread to colonize new areas.

• Humans, like apes, care for their young for a long time. Offspring also have delayed puberty.

• Humans show more physical similarities with young apes than mature ones, so we may be neotenous - have evolved to retain juvenile ape characteristics.

• Humans and apes have dexterous hands and similar hips and muscles.

• 98% of human and chimpanzee DNA is exactly the same.

• Fossils have been found of intermediate species in the evolution of humans from apes.

D.4.4 Outline the trends illustrated by the fossils of Australopithecus including A. afarensis, A. africanus and A. robustis and Homo including H. habilis, H. erectus, H. neanderthalensis and H. sapiens.

A. afarensis: 3 - 3.9 million years - ape-like face

A. africanus: 2.3 - 3 million years - flatter face, larger molars for plant based diet


A. robustus: 1.4 - 2.2 million years - very large molars, bones and skull

H. habilis: 1.6 - 2.4 million years - smaller teeth and jaw for meatier diet, first with tools, size like humans

H. erectus: .4 - 1.8 million years - more complex tools so meat significant part of diet and changed teeth.

H. neanderthensis .5 million years - larger brains and bones, larger teeth and jaw, shorter limbs for the cold

H. sapiens: .1 million years - large brain, flat face, reduced teeth, reduced robustness, chin

• Enlargement of brain and taller and more erect structure.

• The spike connection to the skull becomes more central to balance centre of gravity.

• Pelvis changes to support organs in walking.

• Pelvis shorter and broader to attach walking muscles.

• Legs become stronger and longer while arms become shorter and weaker.

• Knee can now be fully straightened.

• Foot forms more of a platform and rigid shape without opposable toe.

D.4.5 Discuss the possible ecology of these species and the ecological changes that may have prompted their origin.

• The climate of Africa became drier with thin woodland instead of forests. The depletion of forests encouraged a ground-adapted species that could move on land for long distances and who could carry scattered food. Thus, the adaptation of a bipedal human.

• Later Africa became much cooler and there was mostly savanna. This may have prompted the evolution of the Homo genus which used tools and group work to hunt large animals for food.

D.4.6 Discuss the incompleteness of the fossil record and the resulting uncertainties with respect to human evolution.

• The fossil record of human ancestry is incomplete and thus we cannot certainly determine when certain species originated and became extinct.

• early fossil record is fragmented and scarce because: they were not buried, many killed by predators so bones were spread and few died in location where they would be preserved

• as there are `missing links' it can not be ensured that the hypothesised evolution of hominids is accurate

D.4.7 Discuss the origin and consequences of bipedalism and increase in brain size.

• Bipedalism - had to adapt to living on the ground and be able to look for food over longer distances.

• Allowed carrying of food and water and looking over brush.

• The increase in brain size allowed the production of tools. This allowed us to hunt larger animals for meat, changing the diet and the teeth structure towards smaller molars.

• Language becomes possible.

• We learned to limit the environment's influence (e.g. clothing, fire and housing).

• This comes at the cost of a longer development period and greater energy use by the brain.

D.4.8 Outline the difference between genetic and cultural evolution.

• Cultural evolution are changes in the behaviour of a species, for example, what tools they use, as where they live, the things they eat etc.

• Genetic evolution is changes in the genetic makeup of a species.

• Cultural evolution has spanned millions of years in three major stages: the nomadic (hunting), agricultural (settled), and industrial ages. However, we have not changed biologically in any significant way. We are probably not any more intelligent than the cave men. Our increased ability is due to the past experience we draw on.

D.4.9 Discuss the relative importance of genetic and cultural evolution in the evolution of humans.

• Evolution of the brain allowed cultural evolution to take off. Since then cultural evolution has been able to change humans far more quickly than genetic evolution has. Human behaviour and life has changed greatly without much genetic evolution at all. We are now able to change our environment instead of changing to suit our environment.

Option D.3 Evidence for Evolution

D.3.1 Describe the evidence for evolution as shown by the geographical distribution of living organisms, including the distribution of placental, marsupial and monotreme animals.

• Australia separated from the other continents, so animals there could no longer mate with animals from other continents, and Australian animals could not travel elsewhere. This is why marsupials and monotremes are found only on Australia. They evolved there and could not spread elsewhere. Australia has very few placental mammals, but this is because of geography, not because Australia is unfit for placental mammals.

ï‚· The two northern continents are only separated by a small sea, the Bering Straight which has been crossable at times in the past. They have very similar mammal life. The three southern continents have been far more isolated from one another and show far greater variety of mammal life.

ï‚· Looking at the way in which continents have drifted over time and examining the fossils in them we can map out the development of different sorts of mammals. Monotremes and marsupials developed in Gondwanaland 165mya before the three southern continents were separated. Placental mammals (developed life young) developed later 135mya and replaced almost all of the monotremes (egg laying mammals) and marsupials (undeveloped young born into pouches) in the other continents.

D.3.2 Outline how remains of past living organisms have been preserved.

• Sand eroded from the land is first carried by rivers, seas, swamps where the particles settle to the bottom. These deposits pile up and compress the older sediments into rock. Organisms swept into seas and swamps then settle to the bottom with the sediments when they die. The hard parts of the animal may remain as fossils because they are rich in minerals (bones and teeth).

• Study of past evolution - phylogeny

• Sand eroded from the land is first carried by rivers, seas, swamps where the particles settle to the bottom. These deposits pile up and compress the older sediments into rock. Dead organisms are swept into seas and swamps then settle to the bottom with the sediments. When the sediments turn to rock under pressure the hard parts of the animal may remain as fossils. Minerals also leach into the soft parts of the organism, leaving a petrified cast of its shape within the rock.

• Fossils can also be stored in:

• Resins, which become amber

• Frozen in ice or snow

• In acid peat in which they cannot decay

D.3.3 Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K.

• Fossils contain isotopes of elements that accumulated in the living organisms . If the isotopes are unstable, they will loss protons and break down over time. Since each radioactive isotope has a fixed half-life it can be used to date fossils. Half life is the amount of time it take for half of a sample of a certain substance to break down. Carbon-14 = dating fossils less than 50,000 years old. Potassium-40 = 1.3 billions year. Error of less 10%.

D.3.4 Define half-life.

• Half-life - The number of years it takes for 50% of the original sample to decay.

D.3.5 Deduce the approximate age of materials based on a simple decay curve for a radioisotope.

Look on the graph at how many times the concentration of the original isotope has halved then multiply that by the half-life.

D.3.6 Outline the palaeontogical evidence for evolution using one example.

• The existence of the many fossils similar but different to current species is expected under evolution.

• The Acanthostega fossil shows an amphibian from 365mya which has eight fingers and seven toes so is different from current organisms. It has four legs and a backbone like mammals, reptiles and amphibians but also a fish like tail and gills. It lived in water. It is a `missing link' fossil demonstrating an intermediate point in evolution between current amphibians and fish.

• In the horse, the number of toes they have has been reduced from four to one. A succession of fossils from the horse ancestor with four toes to a modern horse with one toe shows a trend towards reduced number of toes.

• A clear progression of species is visible in the fossil record over time, from bacteria, to simple water based organisms, to amphibians, to insects, to reptiles and finally to mammals.

D.3.7 Explain the biochemical evidence provided by the universality of DNA and protein structures for the common ancestry of living organisms.

• All living organisms have DNA, which suggests that all life forms had a common ancestor with DNA. To determine further relationships between organisms, comparing DNA and protein structure can be helpful. DNA - match two single stranded DNA from different species and see how tightly the DNA from one species can bind to DNA from another species. More links the species are more closely related.
  Protein strucure - proteins are genetically determined. Thus a close match in amino acid sequence of two proteins from different species indicates that the genes in those proteins evolved from a common gene present in a shared ancestor. For example, the hemoglonin of gorillas only differs by one one amino acid from human hemoglobin.

D.3.8 Explain how variations in specific molecules can indicate phylogeny.

• Check the protein and DNA of the organism - if they are similar then that means there is a close relationship. If they are dissimilar, they are distant relations.

• Proteins are genetically determined. Thus a close match in the amino acid sequence of two proteins from different species indicates that the genes in those proteins evolved from a common gene present in a shared ancestor.

D.3.9 Discuss how biochemical variations can be used as an evolutionary clock.

ï‚· Mutations are random changes in gene structure but they occur at a roughly predictable rate. In general the more differences between the amino acid sequence of a common protein, the further in the past two species had a common ancestor. For example, the haemoglobin of gorillas only differs by one amino acid from human haemoglobin whereas elephant haemoglobin differs from human haemoglobin by 26 amino acids. Therefore elephants separated as a species from a common ancestor with humans longer ago then did gorillas. Information like this can help to group organisms in trees of descent and suggest how long ago they had a common gene pool.

• First, check a specific protein and know that mutations occur at a certain rate. Count how many mutations there are in that specific protein and then calculate how many years the organism has evolved. For example, if there are 10 mutations and mutations occur every 1,000 years in this protein - 10 times 1,000 = 10,000 years.

D.3.10 Explain the evidence for evolution provided by homologous anatomical structures, including vertebrate embryos and the pentadactyl limb.

• Evolutionary transitions leave signs in fossil records in terms of descent with modifications.

• Homologous anatomical structure is a test for common ancestry. Descent with modification is evident in anatomical similarities between species grouped in the same taxonomical category. For example, the forelimbs of mammals have been modified to fit their function. However similarities in these structures demonstrate that they all originate from a common ancestor.

• A pentadactyl limb describes the same skeletal elements that make up the forelimbs of humans, cats, whales, bats and all other mammals. They have evolved for different functions but the relationships between the bones in the limb are all remarkably similar.

• Even distantly related organisms go through similar stages in their embryonic development. Many of them cannot be told apart in the embryonic stage despite looking entirely different as adults. All vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats. This is all easily explained by evolution.

D.3.11 Outline two modern examples of observed evolution. One example must be the changes to the size and shape of the beaks of Galapagos finches.

• The Galapagos Islands have dry years that alternate with wet years. In the wet years, large seeds are abundant. In dry years, small seeds are abundant. As the beaks of the fiinches increase, they are more able to eat larger and larger seeds. Because of natural selection, birds with larger beaks are more prominent in wet years, and smaller beaks are more prominent in dry years.

• In 1981 the finch Geospiza fortis had a short, wide beak and had a diet of mostly large, hard seeds. In 1982-3 there was an el Niño event which brought much rain and meant that the vegetation changed and over the next 5 years there were far more soft small fruits. The population boomed because of the rains and increase in food then dropped back when the rains stopped. By 1987 they had longer, narrower beaks than they had before because those beaks helped the birds to survive the population cutback after the rains.

• In industrial areas of Europe in 1960 more ladybugs showed the black melanic colour allele. The black absorbs more light, and so the black ones have an advantage staying warm when sunlight levels are low. After 1960 as smog levels decreased the proportion of black ladybugs decreased proportionally and both smog and black butterflies reached constant low levels from 1980. Instead they were replaced with a red colour which warned off predators and was more effective at improving survival.

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