1. Enduring Understanding 1.D
BIG IDEA I The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.D
The origin of living systems is explained by natural processes.
Essential Knowledge 1.D.1
There are several hypotheses about the natural origin
of life on Earth, each with supporting scientific evidence.

2. Essential Knowledge 1.D.1: There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence.
Learning Objectives:
(1.27) The student is able to describe a scientific hypothesis about the origin of life of Earth.
(1.28) The student is able to evaluate scientific questions based on hypotheses about the origin of life on Earth.
(1.29) The student is able to describe the reasons for revisions of scientific hypotheses of the origin of life on Earth.
(1.30) The student is able to evaluate scientific hypotheses about the origin of life on Earth.
(1.31) The student is able to evaluate the accuracy and legitimacy of data to answer scientific questions about the origin of life on Earth.
A number of experimental investigations have provided evidence that the conditions early in the Earth’s history provide an environment capable of generating complex organic molecules and simple cell-like structures.
For example, the “organic soup” model hypothesizes that the primitive atmosphere contained inorganic precursors from which organic molecules could have been synthesized through natural chemical reactions catalyzed by the input of energy.

3. Overview: Lost Worlds
Fossils in all parts of the world tell a similar, surprising story: past organisms were very different from those now alive.
The sweeping changes in life on Earth revealed by fossils illustrate macroevolution, the pattern of evolution over large time scales.
Specific examples of macroevolutionary change include the origin of key biochemical processes such as photosynthesis, the emergence of the first terrestrial vertebrates, and the long-term impact of a mass extinction on the diversity of life.
Taken together, such changes provide a grand view of the evolutionary history of life on Earth.

4. Early Earth Conditions
Conditions on early Earth made the origin of life possible.
The earliest evidence of life on Earth comes from fossils of microorganisms that are about 3.5 billion years old.
The current theory about how life arose indicates that chemical and physical processes on early Earth may have produced simple cells in a sequence of four main stages:
Primitive Earth provided inorganic precursors from which small organic molecules were abiotically synthesized due to the presence of available free energy and the absence of a significant quantity of oxygen.
These molecules served as monomers for the formation of more complex molecules, such as nucleic acids and nucleic nucleotides.
All these molecules were packaged into protobionts, membrane-containing droplets, whose internal chemistry differed from that of the external environment.
The joining of these monomers produced polymers with the ability to replicate, store and transfer information – which made inheritance possible.
These complex reaction sets could have occurred in solution (organic soup model) or as reactions on solid reactive surfaces.

5. Synthesis of Organic Compounds on Early Earth
There is scientific evidence that Earth and the other planets of the solar system formed about 4.6 billion years ago.
The first atmosphere was probably thick with water vapor; along with various compounds released by volcanic eruptions, including nitrogen and oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide.
As Earth cooled, the water vapor condensed into the oceans, and much of the hydrogen quickly escaped into space.
In the 1920s, Russian and British chemists Oparin and Haldane hypothesized that Earth’s early atmosphere was a reducing (electron-adding) environment, in which organic compounds could have formed from simple molelcules.
They suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

6. Abiotic Synthesis of Macromolecules
The presence of small organic molecules, such as amino acids, is not sufficient for the emergence of life as we know it.
Every cell has an assortment of macromolecules – including enzymes and other proteins and nucleic acids that are essential for self-replication.
Experiments suggest that such molecules could have formed in early Earth.
Some models suggest that primitive life developed on biogenic surfaces, such as clay, that served as templates and catalysts for assembly of macromolecules.
By dripping solutions of amino acids into hot sand, clay, or rock, researchers have been able to produce amino acid polymers. The polymers formed spontaneously, without the help of enzymes or ribosomes.
It is possible that such polymers may have acted as weak catalysts for a variety of reactions on early Earth.

7. Protobionts
The necessary conditions for replication and metabolism early in life’s history may have been met by protobionts.
Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure.
Protobionts may exhibit some properties of life, such as simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings.
Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds.
Two key properties of life are accurate replication and metabolism. Neither property can exist without the other.
DNA replication requires elaborate enzymatic machinery, along with supplies of nucleotides provided by the cell’s metabolism.
Miller-Urey type experiments have yielded some of the nitrogenous bases of DNA and RNA, but have not produced anything like nucleotides.
If the building blocks of nucleic acids were not a part of the early organic soup, self-replicating molecules and a metabolism-like source for building blocks must have happened together.
How did that happen?

8. Protobionts (A) This liposome is “giving birth” to smaller liposomes.
If enzymes – in this case phosphatase and amylase – are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products.
LIPOSOME: small membrane-bounded droplets

9. Self Replicating RNA and the Dawn of Natural Selection
According to the RNA World hypothesis, the first genetic material was most likely RNA, not DNA.
RNA molecules called ribozymes have been found to catalyze many different reactions:
For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA.
Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection.
The early genetic material might have formed an “RNA world”.
Once RNA sequences that carried genetic information appeared in protobionts, many further changes would have been possible.
RNA could have provided the template on which DNA nucleotides were assembled.
Since DNA is much more stable than RNA, it can be replicated more accurately. Accurate replication was a necessity as genomes grew larger through gene duplication and other processes.
After DNA appeared, perhaps RNA molecules began to take on their present-day roles as intermediates in the translation of genetic programs.
The stage would have now been set for a blossoming of diverse life forms – a change we see well documented in the fossil record.

10. Enduring Understanding 1.D
BIG IDEA I The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.D
The origin of living systems is explained by natural processes.
Essential Knowledge 1.D.2
Scientific evidence from many different
disciplines supports models of the origin of life.

11. Essential Knowledge 1.D.2: Scientific evidence from many different disciplines supports models of the origin of life.
Learning Objectives:
(1.32) The student is able to justify the selection of geological, physical, and chemical data that reveal early Earth conditions.

12. Geologic evidence provides support for models of the origin of life on Earth.
The Earth formed approximately 4.6 billion years ago, and the environment was too hostile for life until 3.9 billion years ago.
The earliest fossil evidence for life dates to 3.5 billion years ago.
Taken together, this evidence provides a plausible range of dates when the origin of life could have occurred.

13. The Fossil Record
The fossil record is the sequence in which fossils appear in the layers of sedimentary rock that constitute Earth’s surface.
The fossil record reveals changes in the history of life on earth. Fossils can also document how new groups of organisms arose from previously existing ones.
Sedimentary rocks are deposited into layers called strata and are the richest source of fossils.
Few individuals have fossilized, and even fewer have been discovered.
The fossil record is biased in favor of species that existed for a long time; were abundant and widespread, and had parts capable of fossilizing.

14. How Rocks and Fossils Are Dated
Sedimentary strata reveal the relative ages of fossils:
In relative dating, the order of rock strata is used to determine the relative age of fossils.
The absolute ages of fossils can be determined by radiometric dating
Radiometiric dating uses the decay of radioactive isotopes to determine the age of the rocks or fossils.
It is based on the rate of decay, or half-life of the isotope (the time required for half the parent isotope to decay).

15. Key Events in Life’s History
Key events in life’s history include the origins of single-celled and multicelled organisms and the colonization of land.
The study of fossils has helped geologists to establish a geologic record of Earth’s history.
Each era represents a distinct age in the history of Earth and its life.
The boundaries between the eras correspond to major extinction evens seen in the fossil record, when many forms of life disappeared and were replaced by forms that evolved from the survivors.

16. Key Events in Life’s History
Key events in life’s history include the origins of single-celled and multicelled organisms.
The earliest living organisms were prokaryotes.
About 2.7 billion years ago, oxygen began to accumulate in Earth’s atmosphere as a result of photosynthesis.
Eukaryotes appeared about 2.1 billion years ago.
Multicellular eukaryotes evolved about 1.2 billion years ago.
The colonization of land occurred about 500 million years ago, when plants, fungi, and animals began to appear on Earth.

17. The First Single-Celled Organisms
The earliest evidence of life (3.5 billion years ago) comes from fossilized stromatolites.
These are layered rocks that form when certain prokaryotes bind thin films of sediment together.
Early prokaryotes were Earth’s sole inhabitants from about 3.5 to 2.1 billion years ago.
These prokaryotes transformed life on our planet.

18. Photosynthesis and the Oxygen Revolution
Most atmospheric oxygen gas is of biological origin, produced during the water-splitting steps of photosynthesis.
When oxygenic photosynthesis first evolved, the free O2 produced probably dissolved in the surrounding water until it reached a high enough concentration to react with dissolved iron.
This would have caused the iron to precipitate as iron oxide, which accumulated as sediments. Once all of the dissolved iron had precipitated, additional O2 dissolved in the water until the seas and lakes became saturated.
After this, O2 began to “gas out” of the water and enter the atmosphere.

19. Photosynthesis and the Oxygen Revolution
This “gas out” change left its mark in the rusting of iron-rich terrestrial rocs.
These sediments were compressed into banded iron formations, red layers of rock containing iron oxide that are a source of iron today.

20. Effects of the Oxygen Revolution
The “oxygen revolution” had an enormous impact on life.
In certain chemical forms, oxygen attacks chemical bonds and can inhibit enzymes and damage cells.
As a result the rising concentrations of atmospheric O2 probably doomed many prokaryotic groups.
Some species survived in anaerobic habitats, where we find their descendants living today.
Among other survivors, diverse adaptations to the changing atmosphere evolved, including cellular respiration, which uses O2 in the process of harvesting the energy stored in organic molecules.

21. The oldest fossils of eukaryotic cells date back 2.1 billion years.
Endosymbiosis and the First Eukaryotes
The oldest fossils of eukaryotic cells date back 2.1 billion years.
The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells
An endosymbiont is a cell that lives within a host cell.
The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites
In the process of becoming more interdependent, the host and endosymbionts would have become a single organism
Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events

22. Endosymbiosis Theory Model for origin of eukaryotes
(A) endomembrane system of eukaryotes may have evolved from specialized infoldings of plasma membrane of ancestral prokaryotes
(B) chloroplasts are descendants of photosynthetic prokaryotes, probably cyanobacteria
proposed ancestors of mitochondria were endosymbiotic bacteria that were aerobic heterotrophs
may have first gained entry into larger cells as undigested prey or internal parasites
Be a good skeptic! Where’s the evidence?

23. Evidence Supporting the Endosymbiotic Theory
The endosymbiotic hypothesis proposes that mitochondria and plastids (chloroplasts) were formerly small prokaryotes that began living within larger cells. Evidence for this hypothesis includes:
Both organelles have enzymes and transport systems homologous to those found in the plasma membranes of living prokaryotes.
Both replicate by a splitting process similar to prokaryotes.
Both contain a single, circular DNA molecule, not associated with histone proteins.
Both have their own ribosomes which translate their DNA into proteins.

24. The Origin of Multicellularity
The evolution of eukaryotic cells allowed for a greater range of unicellular forms.
A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals.
Comparisons of DNA sequences date the common ancestor of multicellular eukaryotes to 1.5 billion years ago.
The oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago.
Molecular and genetic evidence from extant and extinct organisms indicates that all organisms on Earth share a common ancestral origin of life.
Common molecular building blocks (carbohydrates, lipids, proteins, nucleic acids)
Common genetic code (DNA or RNA)

25. The Rise and Fall of Dominant Groups
Anaerobic prokaryotes originated, flourished, and then declined as the oxygen content of the atmosphere rose.
Billions of years later, the first tetrapods emerged from the sea, giving rise to amphibians that went on to dominate life on land for 100 millions years – until other tetrapods (dinosaurs and later, mammals) replaced them as the dominant terrestrial vertebrates.
These and other major changes in life on Earth have been influenced by large-scale processes such as continental drift, mass extinctions, and adaptive radiations.
The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations.

26. Continental Drift
Continental drift is the movement of Earth’s continents on great plates that float on the hot, underlying mantle.
Plate movements rearrange geography slowly, but their cumulative effects are dramatic. In addition to reshaping physical features of our planet, continental drift has a major impact on life on Earth.
Formation of the supercontinent Pangaea about 250 million years ago had many effects:
A reduction in shallow water habitat; a colder and drier climate inland; changes in climate as continents moved toward and away from the poles; changes in ocean circulation patterns leading to global cooling.
Continental drift alters habitats in which organisms live and promotes allopatric speciation on a grand scale:
1. CD alters habitats in which organisms live – which can drive many species to extinction and provide new opportunities for groups of organisms that survive the crisis.
2. CD facilitates climate change which can cause organisms to adapt, move to new locations, or become extinct.
3. CD promotes allopatric speciation – when continents break apart, regions that were connected become geographically isolated. This geographic isolation provides a barrier to gene flow between organisms that may have once been a single species. As they drift further apart, each continent becomes an evolutionary arena.
Continental drift can explain puzzles about the geographic distribution of species:
Australia flora and fauna contrast sharply with those of the rest of the world. Marsupial mammals fill ecological roles in Australia analogous to those filled by eutherians (placental mammals) on other continents.
Marsupials probably originated in what is now Asia and N. America and reached Australia via S. America and Antarctica while the continents were still joined.
The break up set Australia afloat – like a great ark of marsupials. In other areas, eutherians diversified.

27. The “Big Five” Mass Extinction Events
The fossil record shows that the overwhelming majority of species that ever lived are now extinct.
Although extinction occurs on a regular basis, at certain times disruptive global environmental changes have caused the rate of extinction to increase dramatically – and a mass extinction results – in which large #s of species become extinct.
The five major extinctions illustrate that species extinction rates are rapid at times of ecological stress.
In each of the “Big Five” mass extinctions, 50% or more of Earth’s marine species became extinct.

28. Consequences of Mass Extinctions
By removing large numbers of species, a mass extinction can reduce a thriving and complex ecological community to a pale comparison of its former self.
In addition, when an evolutionary lineage disappears, it cannot reappear. This changes the course of evolution forever.
By eliminating so many of Earth’s species, mass extinctions can pave the way for adaptive radiations, in which new groups of organisms rise to prominence.

29. Adaptive Radiations
Adaptive radiations are periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological niches.
Large-scale adaptive radiations occurred after each of the big five mass extinctions, when survivors became adapted to the many vacant ecological niches.
Fossil evidence indicates that mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs .
The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size.
Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods.

30. Major Changes in Body Form
The fossil record tells us what the great changes in the history of life have been and when they occurred.
Our understanding of continental drift, mass extinction, and adaptive radiation provides a picture of how those changes came about.
We now must seek to understand the intrinsic biological mechanisms that underlie changes seen in the fossil record.
For this, we focus on genetic mechanisms of change, paying particular attention to genes that influence development.

31. Evolutionary Novelty
Evolutionary novelty can arise when structures that originally played one role gradually acquire a different one.
Structures that evolve in one context but become co-opted for another function are referred to as exaptations.
For example, it is possible that feathers of modern birds were co-opted for flight after functioning in some other capacity, such as thermoregulation.

32. “Evo-Devo”
“Evo-devo” is a field of study in which evolutionary biology and developmental biology converge.
This field is illuminating how slight genetic divergences can be magnified into major morphological differences between species.

33. Heterochrony
Heterochrony is an evolutionary change in the rate or timing of developmental events.
Change relative rates of growth even slightly can change the adult form of an organisms substantially, thus contributing to the potential for evolutionary change.
Example:
The fetal skulls of chimpanzees and humans are similar in shape.
However, the growth in the jaw is accelerated in chimpanzees, relative to humans.
This produces the characteristic of elongated skull and sloping forehead of an adult chimpanzee.

34. Homeotic Genes
Homeotic genes are master regulatory genes that determine the location and organization of body parts.
Hox genes are one class of homeotic genes.
Changes in Hox genes and in the genes that regulate them can have a profound effect on morphology, thus contributing to the potential for evolutionary change.

35. Hypothetical vertebrate ancestor (invertebrate)
Fig
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Duplication of the single Hox complex occurs and provides genetic material associated with origin of first vertebrate. Dulpicate set of genes takes on new roles – such as development of backbone.
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Figure Hox mutations and the origin of vertebrates
The vertebrate Hox complex contains duplicates of many of the same genes as the single invertebrate cluster – in virtually the same linear order on chromosomes, and they direct the development of the same body plans.
Scientists infer that the four clusters of the vertebrate Hox complex are homologous to the single cluster in invertebrates – indicating relatedness and common ancestry.
Second duplication of Hox complex may have allowed the development of even greater structural complexity – such as jaws and limbs.
Vertebrates (with jaws)
with four Hox clusters

36. Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya Drosophila Artemia
Fig
Hox gene 6
Hox gene 7
Hox gene 8
Ubx
About 400 mya
Figure Origin of the insect body plan
New morphological forms likely come from gene duplication events that produce new developmental genes
A possible mechanism for the evolution of six-legged insects from a many-legged crustacean ancestor has been demonstrated in lab experiments
Specific changes in the Ubx gene have been identified that can “turn off” leg development
Drosophila
Artemia

37. Evolution is Not Goal Oriented
Evolution is like tinkering—it is a process in which new forms arise by the slight modification of existing forms:
Most novel biological structures evolve in many stages from previously existing structures.
Natural selection can only improve a structure in the context of its current utility.

38. Evidence for the Origin of Life Hypotheses
Chemical experiments have shown that it is possible to form complex organic molecules from inorganic molecules in the absence of life.
In the 1920s, Russian and British chemists Oparin and Haldane hypothesized that Earth’s early atmosphere was a reducing (electron-adding) environment, in which organic compounds could have formed from simple molecules.
They suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

39. The Miller-Urey experiment http://bcs. whfreeman
In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by creating laboratory conditions comparable to those that scientists at the time thought existed on early Earth. Their apparatus yielded a variety of amino acids found in organisms today, along with other organic compounds.
However, it is unclear as to whether or not the atmosphere of young Earth contained enough methane and ammonia to be reducing – and growing evidence suggests that the early atmosphere was primarily nitrogen and CO2 – and organic molecules have NOT been produced in such atmospheres.
Perhaps instead of forming in the atmosphere, the first organic compounds formed near submerged volcanoes and deep-sea vents, where hot water and minerals gush into the ocean from Earth’s interior. These regions are also rich in inorganic sulfur and iron compounds, which are important in ATP synthesis by present-day organisms.
The Miller-Urey experiments demonstrate that the abiotic synthesis of organic molecules is possible – and support for this idea also comes from analyses of the chemical composition of meteorites – which indicate the same proportions of amino acids produced in the Miller-Urey exp.

40. Evidence for the Origin of Life Hypotheses
Molecular and genetic evidence from extant and extinct organisms indicates that all organisms on Earth share a common ancestral origin of life.
Scientific evidence includes molecular building blocks that are common to all life forms (carbohydrates, proteins, lipids, amino acids).
Scientific evidence includes a common genetic code (DNA and RNA).