1. History of Life on Earth
Chemical Evolution (prebiotic evolution) – most biologists believe that life developed from nonliving matter
Alexander Oparin (Russian) and John B. S. Haldane (England) were the first scientists (independently) to advance the idea that simple organic molecules could form spontaneously from more simple raw materials (1920’s)
they noted that the oxygen-rich atmosphere of today would not have permitted the spontaneous formation of organic molecules
they speculated that the Earth’s early atmosphere was very low in oxygen and rich in hydrogen in the form of hydrogen gas (H2), methane (CH4), and ammonia (NH3) – also contained carbon dioxide (CO2), water vapor (H2O), carbon monoxide (CO), and nitrogen (N2)

2. Conditions on primordial Earth
Earth is about 4.6 billion years old
Earth was very hot when first formed

3. four requirements must have existed for chemical evolution:
1. little or no O2 – Earth’s early atmosphere was probably strongly reducing which would cause any free oxygen to react and form oxides and be removed from the atmosphere
a source of energy – early Earth was a place of high energy
violent thunderstorms with torrential rainfall
widespread volcanic activity
bombardment from meteorites (caused cataclysmic changes in crust, ocean, and atmosphere)
intense radiation (including UV radiation, since there was no ozone layer and younger suns emit more UV light)

4. 3. presence of chemical building blocks – water, dissolved inorganic minerals (present as ions), and the gases present in the early atmosphere
4. time for molecules to accumulate and react with one another – Earth is approximately 4.6 billion years old, the earliest traces of life are approx 3.8 billion years old

5. Oparin and Haldane’s hypothesis is tested by Stanley Miller and Harold Urey in the 1950’s
they designed a closed apparatus that simulated conditions that presumably existed on early Earth
they exposed an atmosphere rich in H2, CH4, H2O, and NH3 to an electrical discharge to simulated lightening
analysis of the chemicals produced in a week revealed that amino acids and other organic molecules had formed

7. recent evidence indicates that organic polymers may have formed and accumulated on rock or clay surfaces (rather than in a “primordial soup” in the sea)
clay consists of microscopic particles of weathered rock and may have acted as a site for early polymerizations because it binds organic monomers and contains zinc and iron ions that might have served as catalysts
lab experiments using clay have confirmed that organic polymers form spontaneously from monomers on hot rock or clay surfaces

8. Protobionts – scientists have been able to synthesize several different protobionts (assemblages of abiotically produced organic polymers)
exhibit many characteristics of living cells – division after growth, maintaining an internal environment different from the external fluids

9. Microspheres protobionts formed by adding water to polypeptides
microspheres show an electrical potential, may absorb materials from the surrounding environment
microspheres may give clues as to the evolution of the cell membrane
membranes are made of phospholipid bilayers with proteins
scientists have heated amino acids without water and produced long protein chains – when water is added, stable microspheres (coacervates) are formed
microspheres can accumulate compounds inside them and become more concentrated than outside, they also attracted lipids and formed a lipid-protein bilayer around them

10. Protobionts
Microsphere Liposome

11. The first cells probably assembled from organic molecules
Cells were evident in microfossils 3.5 billion years old, perhaps even 3.8 billion years ago
The first cells were prokaryotic
Stromatolites offer more fossil evidence – rocklike columns composed of many minute layers of prokaryotic cells (usually cyanobacteria)
living stromatolite reefs are still found in hot springs and in warm, shallow pools of fresh and salt water

12. Fossilized Stromatolites – 3.5 billion years old
Modern day stromatolites

13. A crucial step in the origin of cells was molecular reproduction
both DNA and RNA can form spontaneously on clay, so… which came first?

14. RNA is self-catalytic and is believed to have appeared first (according to the proposed model of the “RNA World”)
chemistry of prebiotic Earth gave rise to self-replicating RNA that functioned both as enzyme and substrates for their own replication
RNA has catalytic properties – enzymatic RNAs are called ribozymes (in modern cells, ribozymes help catalyze the synthesis of RNA and process precursors into rRNA, tRNA, and mRNA)
ribozymes may have catalyzed the synthesis of RNA, and processed RNA molecules
RNA could also catalyze protein formation (catalyzes peptide bonds formation) – protein catalysis of RNA formation happen later

15. DNA probably evolved after RNA – it’s a more stable molecule
may have evolved from RNA making double stranded copies of itself
stability of DNA provides advantages as the information storage molecule

16. The first cells were probably heterotrophs
fermented organic molecules from the aqueous environment – appeared 3.1 – 3.4 billion years ago
first cells were anaerobes, free O2 not available
as concentration of free organic molecules in environment declined, photosynthetic organisms had a selective advantage

17. first photosynthetic organism were autotrophs which split H2S as a hydrogen donor (purple and green sulfur bacteria)
the first photosynthetic organisms to use H2O as a hydrogen donor were the cyanobacteria (released O2 as by-product)
source of the first free oxygen in aquatic environment and atmosphere – O2 existed in significant quantities by 2 billion years ago

18. Aerobes appeared after oxygen increased in atmosphere
aerobic respiration was “added” to glycolysis after free O2 became available
aerobic organisms are much more efficient in converting glucose to ATP
carbon dioxide produced helped to stabilize concentration of CO2 and O2 in atmosphere (by-product of each process – photosynthesis and aerobic respiration – are raw materials for other process)
O3 begins to accumulate in upper atmosphere to form ozone (protection from UV radiation) – allows organisms to live in more shallow water and ultimately on land

19. Evolution of Eukaryotic cells
evolved from prokaryotes about 2 billion years ago
Endosymbiont Theory – first proposed by Lynn Margulis – suggests that mitochondria were originally independent prokaryotic aerobic organisms which developed a symbiotic relationship with another prokaryote
aerobic prokaryote was engulfed by endocytosis but not digested
aerobic prokaryote continued to function and formed a symbiotic relationship with host
similar process occurred later with the host cell and photosynthetic prokaryotes (which became chloroplasts)

21. other evidence:
mitochondria and chloroplasts grow and divide like cells
they have a naked loop of DNA like prokaryotes
they synthesize some of their own proteins using 70s ribosomes, like prokaryotes
they have double membrane as expected since cells were taken into a vesicle by endocytosis
cristae are similar to mesosomes of prokaryotes
thylakoids are similar to structures containing chlorophyll in photosynthetic prokaryotes

22. Evidence for Evolution
Evidence from geographical distributions (biogeography)
if evolution did not occur, we would expect to find a given species everywhere that it could survive – in reality this does not occur
example – Australia (a separate land mass for millions of years) has distinctive animals (marsupials and monotremes) not found anywhere else
200 million years ago, Australia and other continents were joined together (Pangea)
Australian continent gradually separated from others
monotremes and marsupials continued to thrive and diversify in Australia
isolation of Australia prevented placental mammals (which arose elsewhere at a leter time) from competing with monotremes and marsupials
in other areas of the world, placental mammals outcompeted the marsupials and monotremes (which were eliminated)

23. Evidence from biochemistry
all living organisms use DNA (or RNA) as their genetic material
all use the same universal genetic code
all use the same 20 amino acids in their proteins
Variations in specific biochemical molecules can indicate phylogeny (line of evolutionary evidence)
phylogeny can be studied by comparing the structure of a protein or other biochemical
hemoglobin is often used – amino acid sequences of vertebrates are compared for the number of differences
the more difference there is, the longer the timespan since the two species had a common ancestor
evidence seems to show that differences tend to accumulate at a constant rate – differences can be used as an evolutionary clock

24. Evidence from homologous anatomical structures
at an early age, vertebrate embryos are very similar

25. limbs of vertebrates show striking similarities in their bones, despite being used in many different ways
ex. pentadactyl limb – this limb has a basic pattern of bones including 5 digits (fingers/toes) – pentadactyl limb is used differently by different mammals – appearance varies but same basic structure

26. Evidence from Paleontology – based on fossil record
fossils are difficult to explain without evolution
examples: evolution of horse, Archaeopteryx
fossil record provides direct evidence for evolution
fossils range from bone, teeth, shells, to actual body parts preserved in bogs, tar, amber or ice
Fossils most commonly form in aquatic environments and typically form in sedimentary rock

29. Various methods are used to determine the age of fossils
Radioactive dating – radioisotopes decay in a characteristic and known rate
Half-life is the amount of time required for 50% of the radioisotope to decay – at the end of this period of time (any where from fractions of seconds to thousands of years, depending on isotope) radioactivity will be half of what it was before

30. Carbon dating – excellent method of determining the age of organic material (relatively young, tens of thousands of years old)
uses radioactive isotope of carbon, Carbon-14
14C is unstable and spontaneously changes to 14N
all living organisms contain 14C in the same proportion as found in the atmosphere
when organisms dies, process of incorporating new carbon into body stops (ie, feeding)
half-life of carbon is 5730 years
Potassium-40 is used to date very old fossils (hundreds of millions of years old)
40K decays to form 40Ar
half-life of Potassium-40 is 1300 million years

31. Transient Polymorphism
a population may have more than one common form – polymorphism
Example – Peppered moth
Two forms – grey and black
Industrial melanism – natural selection resulting from industrial processes
Proportions of black and grey have changed over time (changes are temporary) – transient polymorphism

32. Balanced Polymorphism
Occurs when two or more alleles within a population are not transient and changing but are stabilized by natural selection
example – Sickle cell anemia
Condition is more common in areas where malaria is a problem
Homozygous (HbSHbS) condition is lethal
Heterozygous condition (HbAHbS) gives person slight resistance to malaria
Homozygous (HbA HbA) more likely to die from malaria
Allele for HbS remains relatively stable in population due to heterozygote advantage

33. Another example of natural selection:
Antibiotic resistance in bacteria

34. Speciation and Extinction have occurred through the Earth’s History
Speciation rates vary – ex. Adaptive radiation results in rapid speciation in response to new habitats becoming available (such as after a mass extinction)
Extinction rates vary – rapid extinction occurs during times ecological stress
fossil record indicates that the majority of species that ever lived are now extinct

35. Mass Extinctions Extinctions occur on a regular basis
At certain times, disruptive global environmental changes have caused the rate to increase dramatically – Mass extinction results
Five mass extinctions are documented in the fossil record over the past 500 million years

36. A Sixth Mass Extinction?
There is speculation that human activities such as habitat destruction are modifying the global environment and threatening many species with extinction
Some data suggest that unless dramatic actions are taken, a sixth (human-caused) mass extinction is likely to occur within the next few centuries or millenia