Lecture 1: Evolution Of Early Life Flashcards
Fundamental conditions for life on earth
Essential elements:
- building blocks of organic molecules C, H, O, N, S, P (we made of these)
Continual source of energy:
- for our system
- solar (from sun) / geothermal (supports individual habitats)
Temp allowing liquid H2O
temp of H2O effected by:
- nature & distance from the sun
- atmospheric heat retention (greenhouse effect/climate change)
Earth formation: Hadean eon
- ~4.5 BYA (bill years ago) (10^9 YA (years ago))
- earth elements formed from death of previous stars
- Key to life today is the cycling of elements present when Earth formed
- intense geological activity (volcanic etc)
- red supergiants: massive stars, role in synthesis of heavy elements (e.g. C, N, O) & all elements on periodic table through nuclear fusion processes in their cores.
- black holes: dense objects formed from remnants of massive stars that have collapsed, part of life cycle of massive stars (which form elements)
- prestellar nebulae: clouds of gas & dust in space where stars born, gravitational forces cause collapse forming protostars, material surrounding protostars forms a rotating disk & planets form from debris in disk.
- supernovae: catastrophic explosion at end of massive stars’ life cycle. Responsible for dispersing heavy elements (synthesised in stars core). These elements contribute to material available for forming planets e.g. earth.
- CNO Cycle (carbon-nitrogen-oxygen cycle): a nuclear fusion process (2 light atomic nuclei combine to form a single heavier one while releasing massive amounts of energy). Converts hydrogen into helium using C, N, O as catalysts. This powers the star & synthesises heavier elements e.g. C, N, O (essential building blocks for earth).
During Hadean eon, these processes contributed to formation of earth.
Heavy elements synthesised in cores of red supergiants & through CNO cycle were dispersed into space through supernova explosions, elements then become part of prestellar nebulae where they combined to from Earth.
Earth formation: Hadean eon
- mostly iron core & mantle, lost H early in formation: During Hadean eon, denser materials like iron sank to form planet’s core, while lighter materials rose to form mantle & crust. This led to Earth having a predominantly iron core & mantle. Hydrogen, being a light element, was likely lost early in Earth’s formation due to intense heat from accretion process & solar wind.
- crust mostly silicon dioxide (SiO2) - also carbonates & nitrates: Earth’s crust, particularly during Hadean eon, was primarily composed of silicon dioxide (SiO2), commonly known as quartz, & carbonates & nitrates.
- repeated meteor bombardment: Hadean eon characterized by intense meteor bombardment, as the early solar system was still littered with debris from planet-forming process. These impacts played role in shaping Earth’s surface & may have contributed to redistribution of materials & formation of some geological features observed today.
- bacteria in rocks 3km down?: potential presence of bacteria deep within Earth’s crust, (3 km). This refers to endolithic bacteria & archaea, which are microorganisms capable of surviving in extreme environments, such as deep within rocks. If confirmed, this would suggest that life on Earth may have existed much earlier than previously thought, potentially thriving in environments that were shielded from the harsh conditions on the surface. So maybe life started below the surface.
- Crust: Composed primarily of quartz (SiO2), with a depth ranging from 5-50 km.
- Mantle: Mainly composed of iron magnesium silicate ((Fe, Mg)2SiO4), extending ~2,900 km deep.
- Core: Predominantly composed of iron (Fe), with a solid inner core & a liquid outer core, extending to center of the Earth at approximately 6,370 km deep.
How did our atmosphere form?
- early processes dominated by volcanic release of CO2, N2, H2O (water Vapor):
During early stages of Earth’s formation, volcanic activity played a role in shaping atmosphere. Volcanoes released gases e.g. CO2, nitrogen (N2), & water vapor (H2O) into atmosphere. These gases likely sourced from interior of Earth & emitted during volcanic eruptions. - thin atmosphere: In early stages of Earth’s history, atmosphere was much thinner & less dense compared to current state. composition of early atmosphere was predominantly CO2, similar to present-day atmosphere of Mars.
- early life released CO2 & converted it into biomass: As life began to emerge on Earth, early organisms, e.g. certain types of bacteria, engaged in processes like photosynthesis (absorbed CO2) from atmosphere & converted into biomass, releasing O2 as byproduct. This process played a crucial role in altering composition of atmosphere over time.
- further processes evolved, releasing O2 & N2: Over billions of years, as photosynthetic organisms proliferated & evolved, levels of O2 in atmosphere ^ significantly. O2 began to accumulate as a result of photosynthesis & other biochemical processes, eventually becoming a major component of atmosphere. Nitrogen (N2) likely became more abundant through various geological & biochemical processes, contributing to composition of Earth’s atmosphere as we know it today.
Temperature & atmosphere
Temp: atmospheric density & composition
- atmosphere absorbs light & converts it to heat: atmosphere acts as a blanket around planet, absorbing sunlight & trapping heat to regulate planet’s temp. When sunlight reaches Earth’s surface, some is absorbed & converted into heat energy, warming atmosphere. This process helps maintain a relatively stable climate conducive to supporting life.
- CO2 is a greenhouse gas that plays a crucial role in regulating Earth’s temp. In moderate amounts, CO2 helps trap heat in atmosphere, creating a greenhouse effect that keeps planet warm enough to support life. However, if there is an excessive amount of CO2 in atmosphere, can lead to a runaway greenhouse effect, as seen on Venus, where temps ^ to extreme levels. Conversely, if there is too little CO2 in atmosphere, as on Mars, planet can become cold & inhospitable.
- high CO2 levels on early Earth may have become too hot if species had not evolved to use CO2: During early stages of Earth’s history, atmosphere likely contained high levels of CO2, Without presence of organisms capable of utilizing CO2 through processes such as photosynthesis, these high CO2 levels could have led to excessive heating of the planet, potentially making it uninhabitable. However, evolution of photosynthetic organisms played a crucial role in reducing CO2 levels over time by converting it into biomass & O2, helping to stabilize Earth’s climate & make it suitable for life.
How did our atmosphere form?
- light-driven CO2 fixation (photosynthesis) evolved at least 3.4 BYA: Photosynthesis = plants, algae, & certain bacteria convert CO2 & H2O into organic compounds, such as glucose, using sunlight as energy source, evolved at least 3.4 BYA, likely in primitive (basic) single-celled organisms.
> oxygenation of atmosphere: These early photosynthetic organisms played a role in altering composition of atmosphere by reducing CO2 levels & releasing O2 as a byproduct. - species evolved to cope, then thrive, on the toxic gas: ^ in atmospheric O2 presented a challenge for early life forms that were not adapted to an O2-rich environment. O2 is highly reactive gas & can be toxic to organisms that lack mechanisms to cope with its presence. But, overtime, certain species evolved physiological adaptations to utilize O2 for respiration, leading to emergence of aerobic metabolism.
- Stromatolites: layered structures formed by trapping & cementation of sediment by microbial communities, primarily cyanobacteria. These structures are among earliest forms of life on Earth & provide evidence of ancient microbial ecosystems.
- Cyanobacteria: played role in shaping Earth’s early environment through photosynthesis, contributing to oxygenation of atmosphere & formation of stromatolites. group of photosynthetic bacteria. among 1st organisms to evolve photosynthesis & release O2 as byproduct.
Lessons from the past
- living organisms continue to determine atmosphere of Earth: living organisms impact composition & dynamics of Earth’s atmosphere, through processes e.g. photosynthesis, respiration, & decomposition. E.g. photosynthetic organisms release O2 contributing to the O2-rich atmosphere today.
- underlying principles of the biochemical cycling of elements formed early in Earth history: Earth is dynamic system where elements e.g. C, N, O2, & phosphorus are continuously cycled between atmosphere, biosphere, geosphere, & hydrosphere.
Lessons from the past
- products from 1 type of metabolism are intimately linked to other biological processes: (Metabolism = biochemical processes by which organisms convert molecules into energy & essential components for growth, maintenance,& reproduction). products generated by 1 metabolic pathway often serve as substrates or regulators for other pathways, creating a complex network of interactions. E.g. products of photosynthesis, e.g. glucose & O2, not only used for energy production but also as building blocks for cellular structures & as substrates for cellular respiration.
- elements key to life cannot become unavailable: Life on Earth depends on availability of certain key elements, e.g. C, N, O, hydrogen, phosphorus, & sulfur. These elements are essential components of biomolecules like proteins, nucleic acids, lipids, & carbohydrates, which form basis of biological structures & functions, importance of biogeochemical cycles, e.g. C cycle, N cycle,& phosphorus cycle, which continuously recycle these elements between atmosphere, biosphere, geosphere, & hydrosphere.
- small scale processes combine to global effects: small-scale biological, chemical, & physical processes can combine to produce global-scale effects. E.g. uptake of CO2 by photosynthetic organisms at microscopic level contributes to regulation of atmospheric CO2 levels on a global scale.
How does life evolve?
Where is the evidence?
- life evolved ~3.8BYA, during Archaean eon: Archean eon, lasted from ~ 4 to 2.5 BYA, was characterized by emergence of early life forms & significant geological changes on Earth.
- stable oceans formed (volcanic water Vapor): formation of stable oceans during Archean eon is supported by geological evidence indicating presence of large bodies of H2O on Earth’s surface. One proposed mechanism for formation of these oceans is release of H2O vapor from volcanic activity. As volcanoes erupted & released gases, including H2O vapor, into atmosphere, these gases condensed & accumulated on Earth’s surface, forming bodies of H2O e.g. oceans, seas, & lakes. presence of stable oceans provided suitable environment for emergence & evolution of early life forms.
EVIDENCE:
- Fossil Evidence: fossilized microorganisms dating back to Archean eon been discovered in sedimentary rocks. These microfossils, e.g. stromatolites, provide clues about types of organisms that inhabited Earth during this period.
- Geochemical Evidence: Isotopic analysis of ancient rocks & minerals has provided indirect evidence of biological activity during Archean eon. E.g., presence of certain isotopic signatures, such as carbon isotopes, can indicate activity of photosynthetic organisms & cycling of organic carbon in ancient environments.
How does life evolve?
Where is the evidence?
Why is H2O so important for life?
- remains liquid over relativity wide temp range: H2O is unique among common liquids as remains in liquid state over broad range of temps typically encountered on Earth’s surface. Without this stability, life as we know it would struggle to survive, as liquid H2O is essential for various biochemical reactions & for maintaining cellular function.
- dissolves many organic & inorganic chemicals: H2O is excellent solvent (dissolve range of substances, both organic (carbon-based) & inorganic). allows H2O to serve as medium for biochemical reactions by facilitating transport of nutrients, ions, & other molecules within living organisms.
- allows membranes to form: role in formation & structure of biological membranes, which are essential components of cells & organelles. amphiphilic nature of phospholipid molecules (compose lipid bilayer of membranes), allows them to spontaneously assemble in aqueous environments. lipid bilayer serves as barrier that separates internal contents of cells & organelles from external environment, enabling cellular compartmentalization & regulation of biochemical processes.
How does life evolve?
Where is the evidence?
Can’t rely on just 1 evidence
- fossils: Stromatolites & Mircofossils
- Isotope ratios
- Biosignatures
- Oxidation state
Stromatolites & Mircofossils
Stromatolites:
- Shark bay, Australia
- observed today in high saline pools (no predation)
interdependent microbial mats
> photosynthetic Cyanobacteria
> sulphide oxidising photobacteria
> sulphate reducing bacteria
Mucilage traps CaCO3, cementing biofilm & creating structure
Stromatolites & Mircofossils
- fossil date to the Archaean (3.4 BYA) - no predators
- sediment-trapped by mucilage forms rock over time
BUT cannot see cells in fossils, may not be biological?
Stromatolites & Mircofossils
Microfossils:
convincing microfossils in rocks - 2 BYA (eukaryotes ~ 1.2 BYA)
- mineral precipitate making fossil form, must be 3D
- cell forms are visible & measurable
- but still subjective & some may form from abiotic factors
- why are there no earlier fossils?
Archaean rock not compatible
Isotope ratios
Bio signatures in fossil rock
- enzymes can select isotopes e.g. CO2 fixing Rubisco prefers 12C over 13C
- fixed C in cells converted to CaCO3 over time
- difference in 12C & 13C can be measured - δ13C:
(13C/12C (experimental rocks) - 13C/12C (standard rocks)) / (13C/ 12C (standard rock)) X1000
- similar observations with 34S/32S ratios ~ 3.5 MYA
Isotope ratios
Carbon isotope depletion
Isotope ratios
Advantages
+ highly reproducible, provide a physical measure
+ strong evidence for dating earliest life
+ can be used to calibrate phylogenetic trees
Disadvantages
— only tells us life existed, not what looked like or what it did
— cannot guarantee they are not produced abiotically
Biosignatures
- some compounds in rock can only be made by biological processes
- hopanoids - steroid-like membrane lipids
- derivative 2-methylhopane in 2.5 BYA rocks
But no cells present & compounds removed by metamorphic processes in earlier rock
Oxidation state
- the precipitation of iron ores linked to O2 production
- 2.5 BYA gradual O2 increase; actual levels rose & fell
> crustal ferrous (Fe2+) iron soluble
> oxidised ferric iron (Fe3+) insoluble - fine layers Fe3+ (e.g. Fe203) in sedimentary rock
- alternating oxic & anoxic conditons
- ultimately led to low iron levels in ocean
Combining data
Hamersley Basin - Western Australia
Sedimentary rock show to contain:
- 2-methylhopane
- 13C depletion
- extensive iron oxide banding
Conclusion: life was here at least 2.5BYA
How did life form?
We don’t know .. but can postulate
Early life
- had to evolve metabolism with elements available
- NB no O2 gas, no photosynthesis
Possible sources of energy in early Archaean oceans:
- oxidised forms of iron, nitrogen (NO3-) or sulphur (SO4-) interacting with H2 driven by UV light reactions
- light driven iron pumps creating gradients across membranes
- methanogenesis: 2H2 + CO2 CH4 + H20
Early life
How do energy generating reactions become life?
3 models proposed:
1. Prebiotic soup / pizza model
2. Metabolist model
3. RNA world model
Prebiotic soup model
Oparin & Miller’s stimulation experiment
- heat & pressure»_space;> Biomolecules e.g. glycine»_space;> proto-cells
- amino acids & nucleobases also found on meteorites
- NOx FeOx & SOx present in early oceans may generated energy reactions
- large leap from biomolecules to a cell, membrane key
Forming the early cell
- biochemical reactions compartmentalised by membranes
- Micelle
- shake Micelle and it gives a bilayer
- formed from amphipathic fatty acid glycerol esters
- ultimately evolved into phospholipid bilayers
Metabolist models
- self-sustaining CO2-based metabolism similarities to the citric acid (TCA) cycle
- catalysed by metal sulphides
- other models: amino acids from TCA cycle acids & dinucleotides
- suggest link to translation
- ultimately the models aim to explain evolution of biosynthetic processes
The RNA World Model
- facilitating complex info (nucleic acids & proteins) based on RNA (ribonucleic acid)
- Adenine formed from NH3 & CO2 in ancient oceans
- ribose-base nucleotide formed by chemical reaction in lab
- energetically easy to form and degrade than DNA
- thymine biosynthesis comes after uracil in the cell
The RNA World Model
RNAs have catalytic properties (ribozymes):
- splice introns
- copy 2nd RNA strand
- peptide bond formation in ribosome
- from RNA world to the machinery in the model cell
Ribozyme > Ribonucleoprotein enzyme > Protein enzyme with nucleotide coenzyme
Diversification of life
3 domains of life:
- Bacteria
- Archaea
- Eucaryota
