Transcript Nov30_Dec4_09

The Origin of Life
Science has not provided a step by step recipe for
making life.
Science has provided data to support some of
the possible or necessary steps.
What defines life?
1. Has a genotype (genetic blueprint that stores
and transmits information).
2. Has form and function (i.e. phenotype: expression
of genotype).
3. Life evolves.
Oparin-Haldane Model for the Origin of Life
Simple molecules
Complex polymers
H20, NH3, CO2
nucleotides, amino acids
Nucleic acid
RNA, DNA,
protein
Cellular life
RNA: Early Life Forms?
Intron in
Tetrahymena
phenotype
genotype
“Pick up the tail”
Altman and Cech
Evidence for RNA as an Early Life Form
1. Stores information and is catalytic
2. Basic component of:
a. ribosomes and tRNA
b. energy carrier molecules (ATP, GTP)
c. electron-transfer cofactors (NAD, SAM)
RNA Evolves
faster
replication
time
after a
few serial
transfers
Natural Selection
favored shorter
RNA sequences
over time, as a
consequence the
bacteriophage
became less
infectious.
Mills et al., 1967
Another Experiment Showing Evolution of RNA
Beaudry and Joyce (1992)
Test-tube selection and reproduction of RNA
Contains promotor
region for RNA polymerase
Sprinkle mutations
throughout the
Tetrahymena
ribozyme
Select for RNAs that can
cleave a DNA substrate
Some mutations
increased in
frequency
Many were selected out
Four mutations increased
by > 50%
Test Tube Experiments Show:
(1) RNA can evolve (via artificial selection)
(2) Ribozymes have been selected to perform
a number of protein-like tasks:
phosphorylation, aminoacyl transfer,
peptide bond formation, carbon-carbon
bond formation, ribonucleotide synthesis
However, can RNA self-replicate?
(i.e. can an RNA dependent replicase be found?)
But Where Did RNA Come From?
Seems unlikely that RNA can be made in one
step from inorganic molecules.
Did a self-replicating system predate RNA?
So, before RNA….
Where did simple organic molecules originate?
Did Earth Have All of the Ingredients?
Oparin-Haldane Model
Simple molecules
H20, NH3, CO2
Complex polymers
(1) Was the prebiotic
environment permissive?
nucleotides, amino acids
(2) How is this achieved in H20
given hydrolysis?
Nucleic acid
RNA, DNA,
protein
(3) How were membranes
assembled?
Cellular life
(1) Was the pre-biotic environment permissive?
Miller (1953): Assuming Atmosphere Reducing
H2, CH4, and NH3
amino acids, sugars, nucleotides
Mojzsis et al. (1999): Assuming Atmosphere Oxidizing
C02, N2:
aldehydes (ribose sugar in RNA)
Oro’ (1961): Nucleotides from inorganic molecules
HCN, NH3
adenine
(2) How is this achieved in H20
given hydrolysis?
Polynucleotides 40 nucleotides
long have been synthesized
using clay as a catalyst.
montmorillonite, illite,
and hydroxylapatite
Panspermia Hypothesis: Life originated
elsewhere and traveled to Earth.
Martian bacteria?
Murchison Meteorite
(contained amino acids)
The History of Large Impacts on Earth and It’s Moon
Moon (red)
Earth (blue)
Did meteors bring
molecules
necessary for life
to earth?
Yes, but what about
friction……
When was earth hospitable enough
for life to evolve?
Banded iron formation
Greenland 3.85 bya
magnatite
silicate
bands
Apatite crystals (20 mm)
(calcium phosphate minerals
carbonaceous
material
carbonaceous
speck with high
C12 to C13 ratio
What was the oldest common
ancestor like? (cenancestor)
a. Used DNA, mRNA, and amino acids to make
proteins.
b. Cellular
c. Structurally similar to filamentous
cyanobacteria.
Oldest known fossils
of living organisms
3.465 bya
Primaevifilum amonenum
(Schopf, 1993)
Primaevifilum conicoterminatum
Phylogeny of all living organisms
(small-subunit rRNA)
Woese (1996)
Evidence for Horizontal
Gene Transfer
Will it be possible
to reconstruct
the branching
sequence at the
root of the tree
of life?
0.85-0.9 BY
Siberia
0.59 BY China
Fossils allow estimation of
the divergence time of
eukaryotes.
1.4-1.5 BY
Australia
2 BY Eukaryotic Algae?
Michigan
Grypania spiralis
Cambrian Explosion
Evolutionary Diversification 543-506 mya
Cambrian
Explosion:
All major
body plans
first made
an appearance
in the fossil
record during
a 40 my period
Process of Fossilization
After remains are buried by sediments:
(1) Compression/impression/casts/molds:impressions or casts
made before decomposition (like footprints).
(2) Permineralized fossils: precipitation of minerals in
cells before decomposition.
Or Occasionally:
(1) Unaltered remains: Frozen , amber embedded, peat bogs.
permineralized fossils
amber
cast
Impression fossil
Fossil Record is Biased
Fossilization is higher for organisms that are:
Durable and likely to be buried in an
anoxic environment (low land or marine habitats)
Also, there is temporal and geographic bias:
Probability that an organism will be fossilized depends
on the geographical area and historical time.
Ediacaran Fuanas
entirely soft-bodied organisms from 565 mya
Brachina delicata
Spriggina floundersi
(sponges, jellyfish, comb jellies)
New Fossil Finds are Pushing Back
Estimates of Divergence Times
Fossil embryos
suggest precambrian
diversification of
bilateralians
(Xiao et al. 1998)
Possible flatworm or arthropod zygotes and embryos
Burgess Shale Faunas
520 mya
(trilobites, segmented worms, molluscs, chordates)
New Fossil Finds are Pushing Back Estimates
Of Animal Divergence Times
(Shu et al.
1999)
530 my Cambrian vertebrate: Haikouichthys eraicunensis
Small subunit RNA
most basal
earliest fossils
Cambrian: Diversification of Animal
Body Plans
Symmetry
a. Radial or asymmetrical:
Diploblast (endoderm and ectoderm)
b. Bilateral:
Triploblast (endo, ecto, and mesoderm)
Coelomate
i. Protostomes
ii.Deuterostomes
Also: segmented body plans, shells,
exoskeletons, appendages, notochords
Was the Cambrian Explosion Explosive?
Molecular clock estimates suggest 900-1200 my divergence
times for the major animal groups (Wray et al., 1996).
i.e.
Major animal lineages were established pre - Cambrian.
if so
There should be fossil evidence!
What Caused the
Cambrian Explosion?
Environmental change: Higher oxygen
may have allowed for larger, energetically
costly morphologies?
Diversification of phytoplankton may have
spurred the evolution of herbivores and
Predators?
Genetic changes?
Mass Extinction of proterozoic fauna?
Cloudina
Stasis Is Evolution Too!
Darwin’s View
Punctuated Equilibrium
(Gould and Eldridge, 1972)
Jackson and Cheetham, 1994
Why Does Stasis Occur?
not for lack of
genetic variation
dynamic stasis
in pliocene bivalves