Week 7: Origin and Evolution of Early Life

Question 1

What are the characteristics of life?

Answer 1

Homeostasis, structural organization, metabolism, growth + reproduction, response to environmental conditions

Question 2

How old is Earth?

Answer 2

4.5 billion years

Question 3

Describe the general timeline of the Earth

Answer 3

• 4.5 bya: Earth formed
• 4.2 bya: Earth cools
• 4.0 bya: Prebiotic chemical building blocks of life
• 3.8 bya: RNA world
• 3.5 bya: Cellular life
• 3.4 bya - present: Diversification

Question 4

What is the oldest evidence of life on Earth?

Answer 4

Microfossils from gold mine in South Africa - revealed life that was 3.2 by old (previously thought to be 2 by old), shows cell wall structure. Earth age estimated @ 4.5-4.6 bya

Question 5

What is LUCA and how long ago?

Answer 5

LUCA - last universal common ancestor - 3.8 bya - population of organisms, not the first or only early life form, but is the only one that left descendants. Represents a phylogenetic event horizon - point beyond which phylogenetic analysis is not possible.

Question 6

How did Eukaryotes arise from LUCA?

Answer 6

LUCA gave rise to bacteria and archaea which merged to form Eukaryotes.

Question 7

What did Louis Pasteur believe?

Answer 7

life only comes from life - disproved spontaneous generation

Question 8

What gave rise to protocell?

Answer 8

Inorganic abiotic molecules (water, methane, ammonia) lead to primitive self-replicating cell - protocell.

Question 9

What did Oparin and Haldane believe?

Answer 9

in absence of O2 - an energy source ie. UV light, cosmic rays, volcanic eruptions may have been source of energy to convert inorganic molecules into organic molecules. This is prebiotic/primordial soup hypothesis - early molecules in water gave rise to life with addition of energy. Oparin and Haldane proposed the idea of abiogenesis - chemical formation of life from abiotic compounds.

Question 10

What is the Miller-Urey experiment?

Answer 10

tested primordial soup hypothesis. Two important outcomes: 1. biomolecules can form under ancient earth like conditions. 2. Took speculation and transformed it into testable hypotheses.

Question 11

What is panspermia?

Answer 11

meteorites contain organic compounds including amino acids - may have enriched the prebiotic soup present on early Earth.

Question 12

How are lipids, nucleotides and amino acids possible based on Miller-Urey experiments but what isnt?

Answer 12

need vesicles → cell membrane, nucleotide chains → RNA, proteins → peptide chains

Question 13

Briefly describe early cell membrane

Answer 13

Simple fatty acids self-assemble into bilayer to form vesicles + micelles - created larger vesicles. Molecules flip flopped to transport things across membrane.

Question 14

How can nucleotides bind?

Answer 14

At high concentrations, nucleotides can bind into chains. ie. between microscopic layers of clay.

Question 15

How can amino acids bind?

Answer 15

Amino acids bind together under CO, water.

Question 16

What came first, nucleic acids or proteins?

Answer 16

Several scientists proposed that RNA originally did the role of both DNA and enzymatic molecule. Was experimentally tested in 80’s - enzymes don’t have to be proteins - these RNA enzymes are called ribozymes - RNA world phrase is used to capture idea that early life (up to 3.5 bya) was RNA based.

Question 17

What is the RNA world?

Answer 17

The RNA world (4-3.5 bya) - RNA performed biochemical function while self replicating

Question 18

What supports the RNA world hypothesis?

Answer 18

Support for RNA based life of early Earth - protein-based enzymes have mandatory non-protein cofactors, deoxyribonucleotides arise from synthesizing ribonucleic acid first, catalytic site of the ribosome (used for RNA translation to protein) is made entirely of RNA - is a ribozyme - suggesting it had a RNA-only precursor. Experiments suggest essential parts of RNA existed on early Earth - possible arriving on meteorites.

Question 19

What does natural selection require?

Answer 19

variability, heritability, differential reproductive success.

Question 20

What is Spiegelman’s experiment on the origin of NS?

Answer 20

1. Primer strand of RNA (4000 nucleotides) (green)
2. Other nucleotides A, G, C, U
3. Replicase enzyme (blue)
4. Incubation
5. Variability - mutations
6. Hertiability - induced via adding replicase
7. Differential reproductive success - short fragments were preferentially replicating

Question 21

Why does NS prefer DNA?

Answer 21

Natural selection would favour a more efficient system. DNA is more stable because of deoxyribose. DNA is double stranded which reduces interactions with other molecules. DNA has proofreading and repair systems to reduce errors.

Question 22

What is horizontal gene transfer?

Answer 22

Transmission of DNA between diff genomes - important in evolution of early life. Early life-forms are though to have readily swapped gene components.

Question 23

What are major transitions?

Answer 23

Fundamental changes in the history of life - represent large-scale changes in complexity

Question 24

What are the characteristics of major transitions as proposed by Maynard Smith and Szathmary?

Answer 24

1. Individuals give up ability for independent reproduction to join in a larger grouping that shares in reproduction
2. Once they join, individuals benefit from economies of scale, increased efficiencies
3. Once in groups, info is stored and transmitted more efficiently

Question 25

What are the steps in a major transition?

Answer 25

individuals join together, new combined individuals are at advantage because they can reproduce more readily due to: economy of scale (together can acquire more resources) and specialization efficiencies (division of labour), better info processing (ie. RNA to DNA)

Question 26

What are the differences between prokaryotic and eukaryotic cells?

Answer 26

Prokaryotic:

• DNA is circular, NOT membrane bound -No nucleus or membrane-bound organelles -Unicellular
• Evolved 3 bya

Eukaryotic:

• DNA is linear, in membrane-bound nucleus
• Nucleus and membrane-bound organelles -multicellular or unicellular -Evolved 1-2 bya

Question 27

What is endosymbiosis?

Answer 27

Symbiosis within cell - bacteria capable of producing energy and photosynthesizing started living inside other cells - symbiotically - relationship became obligate. Recall: transition characteristics 1 + 2

Question 28

Who is Dr. Lynn Margulis?

Answer 28

Dr. Lynn Margulis proposed the endosymbiotic hypothesis - suggested early evo got jump start by unicellular organisms joining and eventually evolving into multicellular organisms - idea was controversial - “Symbiosis in Cell Evolution”

Question 29

Describe the origin of the nucleus?

Answer 29

Prokaryotic host cell incorporates another prokaryotic cell, over time some exchange of DNA - eukaryote is mixture of genomes - membrane encloses the mixed genome and becomes the nucleus.

Question 30

Describe origin of mitochondria/chloroplasts

Answer 30

A eukaryotic cell engulfed an aerobic bacterium that became mitochondrion - then engulfed photosynthetic bacteria - evolved to be chloroplast - created an obligate relationship - initially mutualistic.

Question 31

What is the 3 Domain hypothesis and eocyte hypothesis?

Answer 31

Eukaryotes are most closely related to archaea than to bacteria. See image for specifics.

Question 32

What major transitions are shown with eukaryotic cell?

Answer 32

All 3

Question 33

What form of evolution is multicellularity?

Answer 33

Multicellularity has evolved many times within eukaryotes - supported by phylogenetic analysis of organisms. The distribution of multicellularity across the phylogenetic tree is convergent evolution.

Question 34

What are two evolutionary routes to multicellularity?

Answer 34

Non-clonal - independent cells of diff lineages coming together to produce non-clonal propagules and Clonal - cells of single lineage stayed together to produce unicellular propagules

Question 35

Describe the yeast multicellularity experiment

Answer 35

Genetically identical, asexually-reproducing replicates of yeast in separate test tubes; mutations allowed to arise by change. Removed yeast from bottom of tubes to put in new tube. After 7 days, snowflakes were dominant yeast form in all tubes. Mutations produced cells that remained attached after division rather than separate

Question 36

What major transition characteristics are shown in multicellularity?

Answer 36

All 3

Question 37

Describe meaning of individuals.

Answer 37

Individuals are integrated and indivisible wholes. They have variability that is heritable and results in differential survival/reproduction. NS can facilitate transitions from one level of individuality to another with incremental improvements that are responsible for evolution of complex traits.

Question 38

What is key to becoming an individual

Answer 38

• Separate cells for reproduction and for growth/maintenance, transfer reproduction from one cell to multicellular organisms
• Initially in group, all cells would have equal chance of reproduction - as groups grew - so did chance of mutation where cell reproduces more than the rest.
• Groups with some mutated individuals and mostly growth-only cells would be favoured because - point 2 of characteristics of major transitions.

Question 39

How do multicellular individuals arise?

Answer 39

Requires differentiation of cell lineages into specialized reproductive (germ) cells and those involved in maintenance and growth of organism (somatic). Multicellular group becomes large such that some cells turn into germ line but most remain somatic.

Question 40

What is evidence of individuality from Volvox algea and expression of regA gene?

Answer 40

When expressed it suppresses expression of genes that code for chloroplast proteins - cells remain small, produce flagella and become somatic cells. Where it is not expressed, cells photosynthesize, grow larger and lose ability to produce flagella - become germ cells.

Question 41

What characteristics of major transitions are shown in individuality?

Answer 41

1 and 2`

Question 42

What is a group?

Answer 42

a set of conspecific (same species) individuals who affect each others fitness.

Question 43

What is foraging associated with vs. coordinated hunting and foraging?

Answer 43

Foraging associated with economies of scale - passive benefit (ie. certain fish). Coordinated hunting (wolves), Coordinated foraging (honey bees - waggle dance)

Question 44

Describe honey bee waggle dance

Answer 44

the length of the waggle indicated distance, the angle at which the waggle indicates location in relation to where the hive is respect to the sun.

Question 45

What is the many eyes hypothesis?

Answer 45

more individuals, more likely predators will be noticed and group alerted. Anti-predator tactics: zebra stripes causing confusion, flash explosions from school of fish, large groups of Daphnia causing confusion for stickleback fish.

Question 46

What are costs of group living? Give example using cliff swallows

Answer 46

competition for food and resources, presence of cheaters (steal resources), transmission of disease and parasites ie. Brown nose bat and white nose syndrome.

As colony size increase, unhatched eggs decreases in cliff swallows. Bugs per nest increases as colony size increases. The number of ectoparasites increases which increases juvenile mortality but will as a whole be more likely to survive.

Question 47

What characteristics of major transitions does group living show?

Answer 47

All 3

Question 48

Summarize

Answer 48

Major transitions - fundamental change in complexity of life - emergence of eukaryotic cells, multicellularity, individuality and group living. Eukaryotes evolved via endosymbiosis. Two routes to multicellularity. Individuality supported by separate sex and somatic cells. There are costs and benefits to group living.