Download lecture 20

Chapter 14
Origins of Life and
Precambrian Evolution
Origins of Life

Questions



What was the earth’s environment life
when life arose?
What is life and how did it arise from
non-life?
What were the first cells and what
characteristics did they have?
Origins of Life



Earth formed between 4.5 and 4.6
Bya
First few million years earth too hot
to support life
Sometime around 3.8 Bya life began
Origins of Life





Which molecule came first?
Proteins or DNA
Proteins do complicated biological
tasks but cannot reproduce
themselves
DNA cannot do biological functions
but replicates itself perfectly
Discovery of catalytic RNA =
ribozymes
Origins of Life



Ribozymes can do biological work
and reproduce themselves
It is now thought that ribozymes
were the first life
Ribozymes can simultaneously
possess a genotype and a phenotype

Has particular traits and a coding
sequence
Defining Life


All living organisms possess a phenotype
and a genotype
Other characteristics defining life are
controversial



Growth
Reproduction
Evolution requires ability to record and
make alteration in heritable information
with some way of distinguishing valuable
changes from detrimental ones

Genotype and phenotype
Defining Life


Ribozymes have all of these
characteristics and were probably
the first life forms
RNA World hypothesis

Catalytic RNA-based life eventually
evolved into system we see today
 DNA
is a more stable molecule for
holding genetic information

Avoids problem of having proteins
before there was DNA to encode them
RNA World

Experimental Evolution of Ribozymes



If ribozymes evolve, they must have a
way of changing and recording the
change in its genome
RNA evolved faster replication and a
shorter genome
In all the experiments, investigators had
to add enzymes for ribozymes to
replicate themselves

Still self-replication has not been
demonstrated
RNA World


By recognizing an RNA World,
problem of self-replication is pushed
back before DNA
RNA World was probably not first
self-replicating system


Likelihood of making RNA abiotically is
too minute
How did the earliest self-replicating
system work?
First Self-Replicating
System




Information-containing molecules
need to be made from simple
inorganic compounds
Chemical reactions that constructed
larger molecules needed energy
source
Building blocks had to self-assemble
Larger biomolecules had to be
protected from harsh environments
Panspermia Hypothesis



Some argue that it is just as likely
that life came from outer space as it
is that life was spontaneously
created on the earth
Biomolecules have been found on
meteroites, asteroids, and comets
Other planets may have more
appropriate chemical composition
than earth
Panspermia Hypothesis

Three versions of panspermia theory

Life arose on another planet in our solar
system
Microbes could have been dislodged by
meteorite impact and traveled to earth
 Mars and Europa may have had water
and life
 Trip to earth would kill most life unless it
was sheltered in carbonate rock
 Earth bacteria survived space walk on
Apollo 12

Panspermia Hypothesis

Three versions of panspermia theory

Life originated in another solar system
and traveled to earth


Would need to escape solar system’s
gravity and be shielded from radiation
Intelligent extraterrestrials intentionally
brought life to earth to seed the galaxy
with life

Directed panspermia
Where did Stuff of Life
Come From?

Meteroites from Australia contained
five amino acids

Contained equal proportions of both
stereoisomers


All living things on earth have only Lform
If these formed spontaneously in space,
could the same thing happen on earth?
Debate about early atmosphere
 Reducing or oxidizing

Where did Stuff of Life
Come From?

Up until 4 Bya earth was bombarded
by impacts

If life formed it was probably continually
destroyed
Where did Stuff of Life
Come From?

Oparin-Haldane Model

Miller attempted to re-create the
primordial soup and simulate lightning



Biomolecules were spontaneously
created in reducing environment
It is now believed that CO2 and N2
instead of methane and ammonia were
the dominant gases
Oparin-Haldane model still used as a
null model
Cellular Life

Search for the most recent common
ancestor of all life = cenancestor


Except viruses
All extant life is made up of cells


Descended from cenancestor population
of interbreeding individuals
Cenancestors lived at least 2 Bya
Cellular Life


Cells allow compartmentalization and
organization
Allows genotype and phenotype to be
linked

Genotype codes for a product and that
product stays within the organism


Does not diffuse to other members of
population with other genotypes
Lab mixtures of polyamino acids
spontaneously organize themselves into
cell-like structures in solution
Cellular Life

Examine fossil record for earliest
cells





3.465 Bya first known cells from Apex
Chert in Australia
Cells growing in filaments
Probably cyanobacteria
Identification made by extant
comparison
Must have been cells before these
because cyanobacteria are relatively
advanced
Cellular Life


Fossil record spotty before 2.5 Bya to
have continuous fossil record
Apex Chert may represent extant or
extinct branch of life


No way to tell if those cells were really
our ancestors
Because fossil record is inadequate,
try to reconstruct a phylogeny of all
life for answers on cenancestor
Phylogeny of All Living
Things

First attempts at a phylogeny of
everything were morphological


Doesn’t work for prokaryotes because they
don’t have sufficient structural diversity
Reconstructed a phylogeny using
molecular characters

Needed to find a gene that all living
organisms have that has strong stabilizing
selection

Otherwise too much drift would accumulate
Phylogeny of All Living
Things

Woese selected small-subunit
ribosomal RNA (ssu-rRNA)



Is present in all organisms
Strong stabilizing selection because of
important function of translating
proteins
Surprising results from tree that
changed view of life’s phylogeny
Phylogeny of All Living
Things

Now there are three domains





Bacteria split into Bacteria and Archaea
Archaea more closely related to Eukarya
than Bacteria
Protista should be split into many
kingdoms
Fungi should be split into two kingdoms
Eukaryotes are minor twigs on tree of
life
Phylogeny of All Living
Things

Where is exact root on tree?



There are no outgroups
Root tree by genetic distance
Most parsimonious solution to root is
that there was one most recent common
ancestor population that gave rise to all
life

Was relatively advanced, similar to
modern bacteria
Phylogeny of All Living
Things

Estimates of universal phylogeny
would be expected to be congruent
among genes



This has not been found to be the case
Phylogeny using HMGCoA reductase
gene shows very different phylogeny
Because of horizontal (or lateral) gene
transfer

Bacterial gene introgressed into Archaea
genome
Phylogeny of All Living
Things


We now know that horizontal gene
transfer was rampant in early life and
many genes may not belong to the
species that have them
Most recent common ancestor could
have been a pool of species that
exchanged genes
Date for Root of Tree of Life

Attempts to use molecular clock to date
the root have been unsuccessful


Oldest definite eukaryotic fossil is 950 My
old



There is not reason to believe that clock
was constant over billions of years
Probable eukaryotic fossil 1.5 By old
Fossil cyanobacteria at least 2 Bya
If Apex Chert is correct, root must be at
least 3.5 By old
Date for Root of Tree of Life


Earliest possible date 4.4 to 3.85 Bya
Youngest possible date 3.5 to 2 Bya
Origin of Organelles

Bacteria and Archaea are similar to
fossil common ancestor


Gradual refinement of form
Eukarya are fundamentally different




Complex cells
Organelles
Complex genomes with introns
Some are multicellular
Origin of Organelles





Mitochondria and chloroplasts are
derived from prokaryotes
Giardia, a protist, lacks complex
organelles
Mitochondria and chloroplasts have
double membranes and
chromosomes
Mitochondria are purple bacteria
Chloroplasts are cyanobacteria
Origin of Organelles


Endosymbiont Theory of Lynn
Margulis
Both organelles arose by symbiotic
relationship with other larger
bacteria


Their bacterial relatives are
endoparasites
Phylogeny of life including organelles
shows that they are bacteria
Evolution of All Life


From the root of the tree of life,
Bacteria and Archaea changed little
Eukarya evolution involved major
innovations

Acquiring mitochondria and chloroplasts
is part of what changed them from
prokaryotes to eukaryotes