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The Origin of Life. Chapter 26 The History of Life on Earth. Spontaneous Generation Concept stating that life generates from other things unlike itself Ex: rotting meat gives rise to maggots and then to flies. Francesco Redi (1668) - PowerPoint PPT Presentation
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The Origin of Life
Chapter 26
The History of Life on Earth
Spontaneous Generation
–Concept stating that life generates from other things unlike itself
• Ex: rotting meat gives rise to maggots and then to flies
Francesco Redi (1668)
–2 jars with rotting meat; 1 open to the air, the other covered with gauze
Lazzaro Spallanzani (1700’s)
–2 Flasks with gravy, both boiled. One sealed, the other open to the air
Louis Pasteur
Father of Microbiology and its effect on life
In 1862, he too used a broth and boiled the substance
So, if it has been shown that life must come from pre-existing life (biogenesis), then which came first, the chicken or the egg? Where/when/how did the first life appear on Earth?
One credible hypothesis is that chemical and physical processes in Earth’s primordial environment eventually produced simple cells
It’s important to understand that no one knows exactly how life arose on EarthJust like any investigator (Ex: CSI), you must start with one piece of evidence and try to explain it–That means be able to replicate in
the lab how that piece of evidence came to be
Then, when enough experimentally supported pieces of evidence have been gathered, an all-encompassing conclusion can be drawn (theory)
Under one hypothetical scenario, this occurred in four stages:
(1) the abiotic synthesis of small organic molecules;
(2) joining these small molecules into polymers:
(3) origin of self-replicating molecules;
(4) packaging of these molecules into “protobionts”
Abiotic synthesis of small O-molecules
Origin of the universe
–Big Bang – lighter elements (mostly hydrogen)
–Stars (fusion) – up to Carbon
–Super Novae – heavier elements
Origin of Earth
–Crust solidified
–Volcanoes spewed inorganics, creating early atmosphere
Abiotic synthesis of small O-molecules
Earth’s first atmosphere most likely contained: CO, CO2, H2, and H20 mixed with some N2 and possibly other gases such as ammonia (NH3) and methane (CH4)
–All inorganic molecules
Abiotic synthesis of small O-molecules
What is missing?
–Little or no O2. Why not?
• No photosynthetic organisms to produce O2
–O2 binds easily to other compounds – it doesn’t stay O2 for very long
• Ex: CO2, H2O
Abiotic synthesis of small O-molecules
In the 1920’s, A.I. Oparin and J.B.S. Haldane independently proposed idea
Earth’s early atmosphere was much different that today; conditions could have been conducive to the formation of simple organic materials
Abiotic synthesis of small O-molecules
In 1953, American scientist Stanley Miller tested Oparin’s hypothesis by recreating Earth’s early environment with all of the inorganic molecules
He then exposed the environment to electric sparks (simulated lightning)
Abiotic synthesis of small O-molecules
In a few days, organic molecules started to form
Every run of the experiment provided amino acids, ATP, and Adenine
Abiotic synthesis of small O-molecules
Alternate sites proposed for the synthesis of organic molecules include submerged volcanoes and deep-sea vents where hot water and minerals gush directly into the deep, cool ocean
Abiotic synthesis of small O-molecules
Another possible source for organic monomers on Earth is from space, including via meteorites containing organic molecules that crashed to Earth
–Panspermia
From monomers to polymers
With constant energy sources and enough time (millions/billions of years), the newly born Earth’s oceans would have been teeming with simple O-molecules
These monomers just needed a way to combine to become polymers
Clay theory: clay acted as a template from which O-molecules replicated themselves
–Dissolved O-molecules splash on hot sand, clay, or lava (or around deep sea vents)
–Water evaporates, leaving the O-molecules behind
–UV radiation and iron pyrite catalyze
From monomers to polymers
From monomers to polymers
Self-replicating molecules
DNA, RNA, or Protein first?
Combination?
Many believe the first hereditary material was RNA, not DNA–RNA can also function as enzymes
Self-replicating molecules
Short RNA polymers can be synthesized abiotically in the lab–If these polymers are added to a
solution of ribonucleotide monomers, sequences up to 10 bases long are copied
–If zinc is added, the copied sequences may reach 40 nucleotides with less than 1% error
Self-replicating molecules
In the 1980’s Thomas Cech discovered RNA molecules are important catalysts in modern cells
RNA catalysts (ribozymes) remove introns from RNA
Ribozymes also help catalyze the synthesis of new RNA polymers
In the pre-biotic world, RNA molecules may have been fully capable of ribozyme-catalyzed replication
Self-replicating molecules
Because RNA is only single stranded, its conformation can be quite different than DNA, based upon the nucleotide sequence
Varying conformations of RNA strands allows natural selection to favor some strands and “weed out” others
Occasional copying errors lead to mutations – the source of variation
Self-replicating moleculesRNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked– This is one function of rRNA today in
ribosomesIf RNA synthesized a short polypeptide that behaved as an enzyme helping RNA replication, then early chemical dynamics would include molecular cooperation as well as competition
Self-replicating moleculesEventually, an RNA template would have helped synthesize a single strand of DNA, which would have quickly made its complementary strand
DNA is a more stable molecule
– If it was synthesized based upon an RNA code, it could still produce RNA replicas
Road to “Protobionts”
Protobionts are groups of abiotically produced molecules
–Maintain separate internal environment
–“Reproduce”
–May contain required materials for some chemical rxns
• i.e., they exhibit some attributes of living things
Road to “Protobionts”
Amphipathic lipids to form bilayers, which can wrap to form spheres
–Can grow or shrink due to osmosis when placed in different salt concentrations
–Can store E as a membrane potential
–Can “eat” (engulf) smaller spheres
Road to “Protobionts”Membranes separate internal from external environmentsProvides stability and compartmentalizationIf one metabolic process generates E, a membrane can keep the E for itself (nat. sel.)Protobiont can evolve as a unit
Earth's HistoryProjected on a 24-hour Day
Formation of Earth
First Earth rocks
12 12
34
5
89
101112
a.m. 6
7
12
34
5
7
8
910
11MIDNIGHT
NOON
6 p.m.
First prokaryotes
First atmospheric oxygen
First eukaryotes
First multicellular organisms
First flowers
First humans(11:59:40)
First humans(11:59:40)
Billions ofyears ago
4
32
1
Diversity of Life
Simple cells, with genetic info, that could replicate now found on Earth
Mutations driving force behind nat. sel.
Diversity of Life
Geology dictated what life could evolve–Pangea allowed mixing of gene
pools–Breakup of Pangea isolated
populations
Life dictated what life could evolve
–Lack of O2 drove anaerobic resp.
–Photosynthesizers put O2 in air, driving evolution of aerobic resp.
Origins of Organelles
Because of the environment, heterotrophic life could have lived off this organic mix for some time
If one organism engulfed another to eat it, but the prey turned out to benefit the predator, a mutualism could be formed (Endosymbiont Theory)
Heterotrophic eukaryotes could have formed
Anaerobic, predatoryprokaryotic cell
engulfsan aerobic bacterium
Aerobic bacterium
Descendents of engulfed bacterium
evolve into mitochondria
Origins of Organelles
But Natural Selection would have favored organisms that could harness an outside E source to survive
At some point, an ancient form of photosynthesis evolved
The first autotrophs were very successful and spread throughout the environment
Descendents of photosynthetic
bacteria evolve into chloroplasts
Photosynthetic bacterium
Mitochondria-containing cell engulfs
photosynthetic bacteria
ParameciumParamecium sp. sp.
ChlorellaChlorella sp, sp,a green a green algaealgae
Origins of DiversityEarth formed ~4.5 Bya
Earth’s crust didn’t form until ~4Bya
Oldest fossils found formed ~3.5Bya
So, life had to have originated sometime between ~4-3.5Bya
– Crust, cooler temps, liquid water
– The life resembled bacteria
Origins of DiversityProkaryotes dominated from ~3.5-2Bya
Stromatolites are sources of prokaryotic fossils
– Cyanobacteria that lived in huge floating mats
– They’d deposit CaCO3, which left layered effect
– Probably responsible for Earth’s O2
atmosphere
Origins of DiversityMost of the O2 liberated from H2O probably reacted with Fe to form iron oxide
Seen in many “rusted” banded patterns
~2.7Bya, enough O2 was being formed to change the atmospheric compositions
Origins of DiversityO2 oxidizes so much that most of the existing prokaryotic life died off
Others evolved mechanisms to utilize O2
First eukaryotic cells formed ~2.7-2.1Bya – right about the time O2 was becoming dominant
This “coincidence” could help explain how aerobic respiration evolved (environmental factors putting pressures on the organisms)
Origins of DiversityMulticellular organisms appear ~1.5-1.2Bya
Most cnidarians and poriferans were present in late Precambrian
The “Cambrian Explosion” is where the real animal diversity that we see today came from
~550-510Mya
Could be due to a global “thawing” period
Origins of DiversityLand invasion took place ~500Mya
Organisms had to evolve ways to prevent water loss
Plants helped “bring” animals to land by providing food sources
Herbivores “brought” their predators to land
Terrestrial vertebrates, tetrapods, evolved from fishes
Most modern mammals appeared ~60-50Mya
Hominids diverged only ~5Mya
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