Science has not provided a step by step recipe for making 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.

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 molecules Complex polymersH20, NH3, CO2 nucleotides, 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 tRNAb. energy carrier molecules (ATP, GTP)c. electron-transfer cofactors (NAD, SAM)

RNA Evolves

Natural Selectionfavored shorterRNA sequencesover time, as a

consequence the bacteriophage

became lessinfectious.

faster replication

timeafter a

few serialtransfers

Mills et al., 1967

Test-tube selection and reproduction of RNA

Another Experiment Showing Evolution of RNABeaudry and Joyce (1992)

Contains promotor region for RNA polymerase

Sprinkle mutationsthroughout theTetrahymena

ribozyme

Select for RNAs that can cleave a DNA substrate

Some mutationsincreased infrequency

Many were selected out

Four mutations increasedby > 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 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

(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

Polynucleotides 40 nucleotideslong have been synthesized

using clay as a catalyst.

(2) How is this achieved in H20given 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 bringmoleculesnecessary for life to earth?

Yes, but what aboutfriction……

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

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.

Oldest known fossils of living organisms

Primaevifilum amonenumPrimaevifilum conicoterminatum

3.465 bya

(Schopf, 1993)

Phylogeny of all living organisms(small-subunit rRNA)

Woese (1996)

Evidence for Horizontal Gene Transfer

Will it be possibleto reconstructthe branching sequence at theroot of the tree

of life?

Fossils allow estimation of the divergence time of eukaryotes.

1.4-1.5 BYAustralia

0.85-0.9 BYSiberia0.59 BY China

2 BY Eukaryotic Algae?Michigan

Grypania spiralis

Cambrian Explosion

Evolutionary Diversification 543-506 mya

Cambrian Explosion:

All major body plansfirst made

an appearancein the fossil

record duringa 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 incells 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 dependson the geographical area and historical time.

Ediacaran Fuanas

Brachina delicata Spriggina floundersi

(sponges, jellyfish, comb jellies)

entirely soft-bodied organisms from 565 mya

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)

Burgess Shale Faunas

(trilobites, segmented worms, molluscs, chordates)

520 mya

New Fossil Finds are Pushing Back EstimatesOf 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. Protostomesii.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 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

Stasis Is Evolution Too!

Darwin’s View Punctuated Equilibrium(Gould and Eldridge, 1972)

Jackson and Cheetham, 1994

Why Does Stasis Occur?

dynamic stasisin pliocene bivalves

not for lack ofgenetic variation