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

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 moleculesComplex polymers H20, NH3, CO2nucleotides, amino acids Nucleic acid Cellular life RNA, DNA, protein

RNA: Early Life Forms? Altman and Cech Intron in Tetrahymena genotype phenotype “Pick up the tail”

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 Natural Selection favored shorter RNA sequences over time, as a consequence the bacteriophage became less infectious. faster replication time after a few serial transfers Mills et al., 1967

Test-tube selection and reproduction of RNA Another Experiment Showing Evolution of RNA Beaudry and Joyce (1992)

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 moleculesComplex polymers H20, NH3, CO2nucleotides, amino acids Nucleic acid Cellular life (1) Was the prebiotic environment permissive? (2) How is this achieved in H20 given hydrolysis? (3) How were membranes assembled? RNA, DNA, protein

(1) Was the pre-biotic environment permissive? Miller (1953): Assuming Atmosphere Reducing H2, CH4, and NH3amino acids, sugars, nucleotides Mojzsis et al. (1999): Assuming Atmosphere Oxidizing C02, N2: aldehydes (ribose sugar in RNA) Oro’ (1961): Nucleotides from inorganic molecules HCN, NH3adenine

Polynucleotides 40 nucleotides long have been synthesized using clay as a catalyst. (2) How is this achieved in H20 given hydrolysis? montmorillonite, illite, and hydroxylapatite

Panspermia Hypothesis: Life originated elsewhere and traveled to Earth. Murchison Meteorite (contained amino acids) Martian bacteria?

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 Apatite crystals (20  m) (calcium phosphate minerals magnatite silicate bands 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 Primaevifilum amonenum Primaevifilum conicoterminatum bya (Schopf, 1993)

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?

Fossils allow estimation of the divergence time of eukaryotes BY Australia BY Siberia 0.59 BY China

2 BY Eukaryotic Algae? Michigan Grypania spiralis

Cambrian Explosion Evolutionary Diversification mya

Cambrian Explosion: All major body plans first made an appearance in the fossil record during a 40 my period

Process of Fossilization (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. After remains are buried by sediments:

amber cast Impression fossil permineralized fossils

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 Brachina delicataSpriggina floundersi (sponges, jellyfish, comb jellies) entirely soft-bodied organisms from 565 mya

New Fossil Finds are Pushing Back Estimates of Divergence Times Fossil embryos suggest precambrian diversification of bilateralians Possible flatworm or arthropod zygotes and embryos (Xiao et al. 1998)

Burgess Shale Faunas (trilobites, segmented worms, molluscs, chordates) 520 mya

New Fossil Finds are Pushing Back Estimates Of Animal Divergence Times 530 my Cambrian vertebrate: Haikouichthys eraicunensis (Shu et al. 1999)

most basal earliest fossils Small subunit RNA

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 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 ViewPunctuated Equilibrium (Gould and Eldridge, 1972)

Jackson and Cheetham, 1994

Why Does Stasis Occur? dynamic stasis in pliocene bivalves not for lack of genetic variation