Fig. 25-1

25 History of Life on Earth25 History of Life on Earth

WHAT IS THE EVIDENCE?WHAT IS THE EVIDENCE?WHAT ARE THE HYPOTHESES?WHAT ARE THE HYPOTHESES?

500 mya this relative of T. rex roamed Antarctica. Today the largest fully terrestrial animal found there is a 5 mm long fly!

MACROEVOLUTION

• Broad pattern of evolution over time

– Origin of groups of animals or plants

– Origin of photosynthesis

– Impact of mass extinctions on diversity of life

• Microevolution is the change in allele (gene) frequency in a population

A TIMELINE

• ~13.8 Billion Years Ago – Origin Of The Universe

• ~ 4.6 BYA – Formation of the Solar System/Earth

• ~ 3.5 BYA – Evidence of Life (Prokaryotes)

• ~ 2.7 BYA – Photosynthesis – cyanobacteria – oxygen begins to accumulate in atmosphere

• ~ 2 BYA – Eukaryotic Cells

• ~ 1.5 BYA – Multicellular Organisms

• ~ 500 Million Years Ago – Colonization of Land

• ~200,000 Years Ago – Modern Humans

25.1: Conditions on early Earth made origin of life possible

• Chemical/physical processes on early Earth and natural selection may have produced very simple cells

• 4 STAGES

1. Abiotic synthesis of small organic molecules

2. Joining of these small molecules into macromolecules

3. Packaging of molecules into protobionts

4. Origin of self-replicating molecules

STAGE 1: Abiotic Synthesis of Small Organic Molecules

• Can we test this hypothesis?

• Oparin & Haldane 1920s

• Miller and Urey 1950s

• What are alternative ideas that are testable?

STAGE 1: Abiotic Synthesis of Small Organic MoleculesOparin-Haldane Hypothesis – 1920s

•Organic molecules formed in ocean from simple molecules

•Reducing Atmosphere – electron donating

•Oxygen free (oxygen takes electrons - oxidizing)

•Energy provided by lightning and UV radiation

•Produced coacervates – carbohydrate and protein spheres which enclosed molecules

STAGE 1: Abiotic Synthesis of Small Organic MoleculesMiller-Urey Experiment - 1953

•Tested Oparin and Haldane ideas

•Placed H2, CH4, NH3 in closed apparatus

•Heated water to create water vapor and to collect chemical which formed

•Electric spark provided energy (lightning)

•Molecules produced: some amino acids, hydrocarbons, formaldehyde CH2O, cyanide HCN

Page 59

Results published May 1953 in Science

*Watson and Crick published the structure of DNA April 1953

STAGE 1: Abiotic Synthesis of Small Organic MoleculesMiller-Urey Experiment - 1953

•Supports Oparin and Haldane ideas

•Experiment has been repeated many times by other scientists; many variations; similar results

•Conclusion: Organic molecules can be produced abiotically

•Questions: What gases were actually in the early atmosphere? Are there other testable explanations?

STAGE 1: Abiotic Synthesis of Small Organic Molecules• There is some evidence early atmosphere was

primarily N2 and CO2; neither oxidizing nor reducing; Miller-Urey-type experiments have produced organic compounds

• Atmosphere may have been reducing near the mouth of volcanoes and deep-sea thermal vents

• Deep-sea vents and submerged volcanoes - rich in iron and sulfur compounds - are also likely candidates for the first organic compounds

• Iron and sulfur are important components of some enzymes use in ATP production

STAGE 1: Abiotic Synthesis of Small Organic Molecules• Extraterrestrial origin?

• Type of meteorite called a carbonaceous chondrite are 1-2% carbon compounds

• 4.5 billion year old meteorite from Australia had more that 80 amino acids; some similar to those produced in Miller-Urey experiment

• Earth Contamination? No. Amino acids were 50:50 mix of L and D isomers

• Organisms make/use L isomers

COULD ABIOTIC SYNTHESIS HAPPEN?

• YES. Miller-Urey and related evidence

• YES. Volcanoes and deep-sea vents provide ideal environment

• ORIGIN could be EXTRATERRESTRIAL – meteorite evidence

STAGE 2: Abiotic Synthesis of Macromolecules

• Could polymers form spontaneously without the help of enzymes? Can we test this?

• Polypeptides and RNA have been assembled abiotically

• Drip solutions over hot sand, clay or rock

• Provides a support which concentrates and holds the monomer.

STAGE 3: Protobionts – Membrane-bound Structures

• Replication first? Metabolism first?

• The next stage may have been enclosing abiotically produced molecules inside a cell-like structure – protobionts

• Can we test this hypothesis?

• We have produced very simple cell-like structures in the laboratory

Protobionts

• Liposomes – form when lipids and other organic compounds are added to water

– A bilayer forms – do you know why?

– They reproduce, swell and shrink in response to the concentration of solutions they are place in, perform simple metabolic reactions

Fig. 25-3

(a) Simple reproduction by liposomes (b) Simple metabolism

Phosphate

Maltose

Phosphatase

MaltoseAmylase

Starch

Glucose-phosphate

Glucose-phosphate

20 µm

STAGE 4: Self-Replicating Molecules - Inheritance

• Can we test this hypothesis?

• It is likely the first genetic material was RNA – “RNA World Hypothesis”

• Cell contain ribozymes – RNA molecules that function as enzymes – given RNA nucleotides they will build short complementary pieces of RNA

• RNA is single stranded – exposed bases serve as pattern or template for forming a new strand

• RNAs have a wide variety of shapes – different shaped molecules could carry out different function

STAGE 4: Self-Replicating Molecules - Inheritance

• Natural selection would favor RNAs whose sequence was more stable and could replicate faster in existing environmental conditions

• Copy errors occur – some would produce a more stable shape with even better self-replication

• Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection

STAGE 4: Self-Replicating Molecules - Inheritance

• RNA is the “Swiss Army knife” of life

• Functions as a catalyst and comes in many shapes

STAGE 4: Self-Replicating Molecules - Inheritance

• Eventually DNA replaces RNA as the information molecule of the cell

– DNA is a more stable molecule – double stranded, bases are attached to each other not exposed to the surroundings as in RNA

– DNA makes fewer errors when replicating

– RNA video on website

– http://www.laurel.kyschools.us/olc/page.aspx?id=13786&s=480

LOOK OUT WORLD – HERE COMES LIFE• We’ve got self-

replicating molecules, metabolism, and a way to maintain and environment different for surroundings

“It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present.— But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity etc. present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter would be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.

Charles Darwin, Letter to J. D. Hooker, Feb1871]

JUST FOR FUN: American Chef Julia Child

• The National Air and Space Museum in its The Universe gallery used to show a video of Julia Child recreating Stanley Miller's famous experiment, cooking up Primordial Soup.

https://massasoit.instructure.com/media_objects/0_44kfay76