Origins of Life

Chapter 26 p. 510

• Current Atmosphere
• Primarily oxygen (21%) and nitrogen (78%);
• an oxidizing atmosphere (removal of hydrogen) i.e. iron is oxidized to form rust (iron oxide);
• Amino acids and sugars react with (are broken down by) oxygen to form CO₂ and water
• if oxygen had been in the “primordial” atmosphere self-assembling molecules would have been impossible (oxygen would have destroyed these molecules as they were forming)…the usual evolutionary theorizing where UNIFORMISTICS view is held (i.e. where processes remain constant over vast stretches of time)
• Early Atmosphere
• A “reducing” atmosphere ( contains free hydrogens);
• Consisting of CO₂, methane (CH₄), carbon monoxide (CO), ammonia (NH₃), free hydrogen and water vapor…newer theories exclude ammonia and methane.
• Currently evidence (geological) has the existence of an oxidizing atmosphere as far back as it can be determined; limestone (calcium carbonate) in great quantities; and the oxidation of ferrous iron in early rocks.
• Ozone problem (O₃): protects earth from harmful UV rays;
• No layer, no protection; organic molecules would be broken down and eliminated.
• Need oxygen for ozone to form.

 

• Atmosphere with oxygenàno amino acidsà no life
• Atmosphere withoutà no ozoneà no life

 

 

Origins of the Universe

• Big Bang approx. 10-18 billion years ago
• Formation of carbon and higher elements in the first generations of stars
• Hydrogen and helium: main elements in the early universe.

Formation of the Earth and Solar Systems

• Earth approx. 4.7 billion years old
• Earth’s crust becomes stable 3.9 million years ago
• Life appears around 3.6-3.7 billion years ago

Reducing versus oxidizing atmosphere

• Current atmosphere is oxygen rich (oxidizing)
• Breaks down organic compounds

Early Earth’s atmosphere was slightly reducing

• Organic molecules are much more stable
• Little free O₂

 

 

First Organisms

• Referred to as the Universal or Common Ancestor;
• Would have had the following characteristics (based on environment in which it evolved);
• anaerobic; hyperthermophilic and halophilic;
• could have been a chemolithoautotroph;  obtaining both energy and carbon from inorganic sources;
• Using H₂ or H₂S (reduced sulfur compounds) as electron donors (hydrogen source);
• CO₂ or oxidized sulfur as electron acceptors to provide energy;
• fixing CO₂ as a carbon source
• Modern chemolithoautotrophs, thought to be similar to universal ancestor have found in extreme environments like hot sulfur springs and hydrothermal vents on the ocean floor. Conditions in these environments are thought to be like those of the early Earth. Modern chemolithoautotrophs grow optimally under anaerobic conditions, in high salt concentrations, at 80-110^oC, independent of sunlight…they could grow on another planet if water was available  See page 521 Figure 26.14

 

 

The appearance of Life

Overview

• ~3.5 billion years ago (BYA): appearance of life
• 2.5 BYA oxygen-forming photosynthesis
• ~2.2 BYA: aerobic respiration
• ~1.5 BYA: first evidence of fossil eukaryotes See page 514 Figure 26.6

The appearance of Life: anaerobic heterotrophs

• ~3.5 BYA: appearance of life See page 512 Figure 26.3
• Most likely cells were anaerobic, heterotrophic bacteria
• Anaerobic- does not require free oxygen
• Heterotrophic- does not make own food
• Stromatolites are the oldest known fossils, dating back more than 3 BY See page 513 Figure 26.4
• Colonial structures formed by photosynthesizing and other microbes.
• Lack a cellular nucleus
• Thrive in warm water and built reefs much the same way as coral does today
• Cyanobacteria most likely responsible for the creation of Earth’s oxygen atmosphere
• They were the dominant life form for ~2BY

Next; anaerobic autotrophs

• Were able to fix CO₂
• Turning CO₂ +H into organic molecules
• Hydrogen donors initially were H₂ and H₂S

 

Energy for sources for autotrophs

• First used chemical energy from elements in surrounding medium
• Chemoautotrophs (deep-sea vents)
• As this energy ran low, evolved ability to capture energy from light
• Photoautotrophs

Life’s first major crisis

• Easy hydrogen donors (H₂, H₂S) used up quickly
• Key innovation around 2.5 BYA
• Oxygen-forming photosynthesis (cyanobacteria)
• Use of H₂O as a hydrogen donor

Life’s second major crisis

• Huge amounts of toxic O₂ released
• Most of the initial O₂ was locked up by iron in the ocean and soil (banded iron formations...rust)
• More O₂ from water kept coming, creating O₂ rich atmosphere

Life’s next major innovation

• Aerobic respiration
• Much more efficient than anaerobic respiration
• Allowed larger cells and the future potential of multicellular organisms

 

 

Experimental Studies

• Early thinking of the origin of life (the first cells)
• Spontaneous generation
• Pasteur’s experiments See page 517 Figure 26.9
• Oparin, Haldane
• Primeval soup
• Key steps in the origin of life
• Formation of complex organic molecules
• Self-replicating systems
• Protein synthesis
• DNA is the genetic material, but it requires proteins to replicate
• Compartmentalization: the first cells
• Origins of complex organic molecules
• Nucleosynthesis in stars to form complex molecules
• Molecular clouds
• Significant fraction of the Earth’s carbon came from extensive cometary bombardment of the primitive Earth
• Model systems for prebiotic evolution
• Miller-Urey experiment See page 518 Figure 26.10
• Fox’s microspheres
• Cech’s Catalytic RNA
• The Miller-Urey experiment (1953)
• Showed that complex organic molecules (amino acids) can be built up from very simple organic molecules (such as methane)

 

• Compartmentalization: Fox’s microspheres (1970’s)
• Showed that by heating certain proteins, microspheres form spontaneously
• Catalytic RNAs
• Self-cleaving rRNA
• RNA can both cleave itself as well as polymerase itself
• The solution to the chicken vs egg problem

Don’t need proteins as RNA can act as an enzyme

• The first cells most likely to have had RNA genomes Read page 519-520
• RNA is the only known macromolecule than can both encode genetic information and also act as biocatalyst (ribozymes)
• This is the basis for the “RNA world” model… the genetic and enzymatic components of early cells were RNA molecules.
• FYI: Currently, known natural ribozymes seem limited and not capable of performing the required enzymatic function for early life. Recent research has produced ribozymes, though slow,  that can perform most of the required functions. Interestingly, in the process of this discover these ribozymes had a cloverleaf conformation….similar to tRNA.
• Proteins are much quicker and catalyze nearly all chemical reactions in today’s organisms (probably taking over quickly after the formation of ribozymes). However, proteins cannot carry a cell’s genetic information.
• DNA synthesis requires RNA primer
• RNA, not DNA used in protein synthesis
• Reverse transcriptase takes RNA àDNA

 

 

Hypothesis on Origin of Macromolecules

• RNA-first hypothesis (The RNA World)
• RNA could carry out processes associated with Life; RNA can act as a substrate and/or an enzyme (Cech 1989 (Nobel Prize)
• Protein-first hypothesis (Fox experiment)
• Proteinoids form from many acids at 180^o
• Proteinoids can form microspheres
• Clay catalyzed RNA and Protein synthesis (combines RNA and protein hypothesis)
• Clay helps in polymerizing proteins and nucleic acids
• Attracts small organic compounds
• Contains zinc and iron (metal catalyst)
• Collects energy from radioactive decay and releases it when temperature and/or humidity change.

 

 

 

 

 

 

Macromolecules to Living Cells

• Protobionts Read p. 520 and Figure 26.12
• Nonliving structures that evolved into the first living things
• Coacervates
• Organic molecules surrounded by a film of water molecules
• Selectively absorb material from surrounding water; incorporates them into their structure
• Microspheres
• Organic molecules surrounded by a double membrane
• Can be formed from proteinoids, when placed in boiling water and cooled
• Shrink and swell depending on the osmolarity of the water
• Can absorb material from the environment, grow and buds
• Have been shown to form nucleic acids and polypeptides (when ATP is present)
• Microspheres = protocells 

 

Recent Evidence for Earliest Multicellular Life

Researchers recently found evidence of worm-like animals in rocks that are over 1 BY old; multicellular animals are thought to have begin with a sudden explosion during Cambrian period (540 MYA)

 

 

 

 

Extraterrestrial Organic Compounds

• In 1969, the Murchison meteorite, hit Murchison, Australia;
• the meteorite had an abundance of amino acids (92 in all), only 19 are found on Earth. The remaining amino acids have no apparent terrestrial (earth) source.
• Possible ideas include that the chemistry of life was available in the makeup of the primitive solar nebula and did not need any special circumstances for their appearance on Earth.
• Tholins are hard, red-brownish substances made of complex organic compounds.
• Earth’s present oxidizing atmosphere does not allow tholin synthesis. However, tholins can be produced in the lab (with methane, ammonia, water vapor and simulated lightning discharges).
• Comets and icy moons may have reservoirs of tholin and other prebiotic organic compounds.
• Additional experimental data with simulations using “abiotic chemistry” and the discovery of extraterrestrial organic compounds have produced: sugars, purines and pyrimidine bases and other biologically significant substances.
• Some scientists think that there has not been enough time on Earth for life to evolve from inorganic chemicals so Earth must have been seeded with organic molecules from an extraterrestrial source.

 

 

Evidence for a common origin for Life on Earth

• DNA and RNA are apparently the universal basis for all life on Earth
• Only 20 known amino acids are used in all living things
• ATP universal energy used in all living cells
• Glycolysis is the first step in all metabolism

 

 

 

Earth Timeline (See page 511 Figure 26.1)

Precambrian Period All time before life existed, volatile environment; origin of life came existence during this period

Cambrian Period Development of multicellular organisms: sponges, primitive animals, and various plants See page 515 Figure 26.8

Permian Period (Ordovician) More specialized organisms; transition from sea to land; increasing genetic diversity via sex

Triassic/Jurassic Periods Arrival and dominance of dinosaurs

Cretaceous Period Dinosaur extinction; emergence of mammals and other more complex organisms

Tertiary Period Evolution of man, transition from Neanderthal to modern man. Diversification of all species

Quaternary Period Continued evolution of man and the diversification of all other species.