Transcript The Origin of Life

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
Earth
Just 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
From monomers to 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 Omolecules behind
–UV radiation and iron pyrite catalyze
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 molecules
RNA-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
ribosomes
If 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 molecules
Eventually, 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 environments
Provides stability
and compartmentalization
If one metabolic
process generates
E, a membrane
can keep the E for
itself (nat. sel.)
Protobiont can
evolve as a unit
Earth's History
Projected on a 24-hour Day
Formation
of Earth
First humans
(11:59:40)
First flowers
First Earth
rocks
11 12 1
10
MIDNIGHT
9
8
6 p.m.
5
2
4
3
First eukaryotes
First atmospheric
oxygen
4
5
1 Billions of
years ago
2
NOON
1 12 11
First
prokaryotes
3
4
7
First multicellular
organisms
2
a.m. 6
7
3
8
10
9
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
Aerobic
bacterium
Anaerobic, predatory
prokaryotic cell
engulfs
an 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
Mitochondria-containing
cell engulfs
photosynthetic bacteria
Photosynthetic
bacterium
Descendents of
photosynthetic
bacteria evolve into
chloroplasts
Paramecium sp.
Chlorella sp,
a green
algae
Origins of Diversity
Earth 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 Diversity
Prokaryotes 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 Diversity
Most 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 Diversity
O2 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 Diversity
Multicellular organisms appear ~1.51.2Bya
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 Diversity
Land 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 ~6050Mya
Hominids diverged only ~5Mya
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