Science has not provided a step by step recipe for making life.
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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.
The Origin of Life
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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.
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Oparin-Haldane Model for the Origin of Life
Simple molecules Complex polymersH20, NH3, CO2 nucleotides, amino acids
Nucleic acid
Cellular life
RNA, DNA, protein
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RNA: Early Life Forms?
Altman and Cech
Intron in Tetrahymena
genotype
phenotype
“Pick up the tail”
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Evidence for RNA as an Early Life Form
1. Stores information and is catalytic
2. Basic component of:a. ribosomes and tRNAb. energy carrier molecules (ATP, GTP)c. electron-transfer cofactors (NAD, SAM)
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RNA Evolves
Natural Selectionfavored shorterRNA sequencesover time, as a
consequence the bacteriophage
became lessinfectious.
faster replication
timeafter a
few serialtransfers
Mills et al., 1967
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Test-tube selection and reproduction of RNA
Another Experiment Showing Evolution of RNABeaudry and Joyce (1992)
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Contains promotor region for RNA polymerase
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Sprinkle mutationsthroughout theTetrahymena
ribozyme
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Select for RNAs that can cleave a DNA substrate
Some mutationsincreased infrequency
Many were selected out
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Four mutations increasedby > 50%
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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?)
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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?
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Did Earth Have All of the Ingredients? Oparin-Haldane Model
Simple molecules Complex polymersH20, NH3, CO2 nucleotides, amino acids
Nucleic acid
Cellular life
(1) Was the prebiotic environment permissive?
(2) How is this achieved in H20given hydrolysis?
(3) How were membranes assembled?
RNA, DNA, protein
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(1) Was the pre-biotic environment permissive?
Miller (1953): Assuming Atmosphere ReducingH2, CH4, and NH3 amino acids, sugars, nucleotides
Mojzsis et al. (1999): Assuming Atmosphere OxidizingC02, N2: aldehydes (ribose sugar in RNA)
Oro’ (1961): Nucleotides from inorganic moleculesHCN, NH3 adenine
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Polynucleotides 40 nucleotideslong have been synthesized
using clay as a catalyst.
(2) How is this achieved in H20given hydrolysis?
montmorillonite, illite,and hydroxylapatite
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Panspermia Hypothesis: Life originated elsewhere and traveled to Earth.
Murchison Meteorite(contained amino acids)Martian bacteria?
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The History of Large Impacts on Earth and It’s Moon
Moon (red) Earth (blue)
Did meteors bringmoleculesnecessary for life to earth?
Yes, but what aboutfriction……
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When was earth hospitable enough for life to evolve?
Banded iron formationGreenland 3.85 bya
Apatite crystals (20 m)(calcium phosphate minerals
magnatitesilicatebands
carbonaceousmaterial
carbonaceousspeck with highC12 to C13 ratio
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What was the oldest common ancestor like? (cenancestor)
a. Used DNA, mRNA, and amino acids to make proteins.
b. Cellular
c. Structurally similar to filamentouscyanobacteria.
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Oldest known fossils of living organisms
Primaevifilum amonenumPrimaevifilum conicoterminatum
3.465 bya
(Schopf, 1993)
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Phylogeny of all living organisms(small-subunit rRNA)
Woese (1996)
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Evidence for Horizontal Gene Transfer
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Will it be possibleto reconstructthe branching sequence at theroot of the tree
of life?
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Fossils allow estimation of the divergence time of eukaryotes.
1.4-1.5 BYAustralia
0.85-0.9 BYSiberia0.59 BY China
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2 BY Eukaryotic Algae?Michigan
Grypania spiralis
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Cambrian Explosion
Evolutionary Diversification 543-506 mya
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Cambrian Explosion:
All major body plansfirst made
an appearancein the fossil
record duringa 40 my period
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Process of Fossilization
(1) Compression/impression/casts/molds:impressions or casts made before decomposition (like footprints).
(2) Permineralized fossils: precipitation of minerals incells before decomposition.
Or Occasionally:
(1) Unaltered remains: Frozen , amber embedded, peat bogs.
After remains are buried by sediments:
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amber
cast
Impression fossil
permineralized fossils
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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 dependson the geographical area and historical time.
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Ediacaran Fuanas
Brachina delicata Spriggina floundersi
(sponges, jellyfish, comb jellies)
entirely soft-bodied organisms from 565 mya
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New Fossil Finds are Pushing Back Estimates of Divergence Times
Fossil embryossuggest precambrian
diversification of bilateralians
Possible flatworm or arthropod zygotes and embryos
(Xiao et al. 1998)
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Burgess Shale Faunas
(trilobites, segmented worms, molluscs, chordates)
520 mya
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New Fossil Finds are Pushing Back EstimatesOf Animal Divergence Times
530 my Cambrian vertebrate: Haikouichthys eraicunensis
(Shu et al.1999)
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most basal
earliest fossils
Small subunit RNA
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Cambrian: Diversification of Animal Body Plans
Symmetry
a. Radial or asymmetrical: Diploblast (endoderm and ectoderm)
b. Bilateral:Triploblast (endo, ecto, and mesoderm)Coelomate
i. Protostomesii.Deuterostomes
Also: segmented body plans, shells, exoskeletons, appendages, notochords
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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!
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What Caused the Cambrian Explosion?
Environmental change: Higher oxygenmay have allowed for larger, energeticallycostly morphologies?
Diversification of phytoplankton may have spurred the evolution of herbivores andPredators?
Genetic changes?
Mass Extinction of proterozoic fauna?
Cloudina
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Stasis Is Evolution Too!
Darwin’s View Punctuated Equilibrium(Gould and Eldridge, 1972)
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Jackson and Cheetham, 1994
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Why Does Stasis Occur?
dynamic stasisin pliocene bivalves
not for lack ofgenetic variation