Origin Of Life• Earth formed 4.6 billion years ago

• Life ~ 3.5 billion years ago

• What was the primitive environment of Earth like?– Reducing (electron adding) atmosphere– No electron hogging oxygen gas was present– “Primordial Soup” just waiting for lightning…

Life….

• Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages:1. Abiotic synthesis of small organic molecules

2. Joining of these small molecules into polymers

3. Packaging of molecules into “protobionts”

4. Origin of self-replicating molecules

Step One: Chemical Evolution• Oparin and Haldane:

– Separate theories about early Earth– Reducing atmosphere + high energy levels

contributed to spontaneous chemical evolution

• Miller & Urey (1953, U of C) tested hypothesis

• First organic compounds may have been synthesized near submerged volcanoes and deep-sea vents

Other theories

• Some organic compounds may have come from space– Carbon compounds have been found in some

meteorites that landed on Earth

• Small organic molecules polymerize when concentrated on hot sand, clay, or rock (Sidney Fox – Protenoids, microspheres)

Protobionts

• Aggregates of abiotically produced molecules surrounded by membrane/membrane-like structure

• Experiments demonstrate that protobionts could have formed spontaneously

• Liposomes– small membrane-bounded droplets that can

form when lipids/other org molec added to water– Grow and appear to split “give birth”

Protobionts

• Maintain different internal environment

• Microspheres – selectively permeable protein membrane– Display osmotic swelling– Stores energy– Has a membrane potential

• Polypeptide, nucleic acid, and polysaccharide aggregates– Hydrophobic macromolecules surrounded by

shell of water molecules– Can absorb substrates– Release products– May have enzymes associated with them

Coacervates

What’s Missing?

• In chemical evolution the environment selects for best suited molecules to survive

• What ingredient necessary for life is missing in all protobionts?

• No hereditary material that would ensure continuity!

Which Came First RNA or DNA?

• RNA!

• RNA has the following properties:– Less complex than DNA– Less stable than DNA– Short strands of RNA can replicate– Ribozymes (RNA molecules) - show

enzymes can be non-protein molecules

• Experiments have proven that RNA can evolve

• The RNA world was slowly able to direct protein synthesis and preserve and copy the genetic information

• As evolution continued different early cells showed varying success with life

• One trend resulted in DNA as the ultimate hereditary material

• RNA world DNA world!

Early Life

• Prokaryotes – only life (3.5 ~ 2 bya)

• Electron transport systems essential to early life

• Earliest types of photosynthesis did NOT produce O2

• Oxygenic photosynthesis ~ 3.5 bya in cyanobacteria

Land plants

Animals

Paleozoic

Meso-

zoic

Ceno-zoic

Origin of solarsystem andEarth

Multicellulareukaryotes

Single-celledeukaryotes

Prokaryotes

Atmosphericoxygen

ProterozoicEon

ArchaeanEon

Humans

Billions of years ago

41

2 3

Endosymbiotic Theory • How did eukaryotes evolve from prokaryotes?• Oldest eukaryotic cell fossils ~ 2.1 bya• Endosymbiosis theory

– mitochondria & plastids were formerly small prokaryotes living within larger host cells

• Prok ancestors of mitochondria/plastids gained entry to host cell as undigested prey/parasites

• In the process of becoming more interdependent, the host and endosymbionts would have become a single organism

Plasmamembrane

Cytoplasm

DNA

Ancestralprokaryote

Endoplasmic reticulum

Nuclear envelope

Infolding ofplasma membrane

Engulfing of aerobicheterotrophicprokaryote

Nucleus

Cell with nucleusand endomembranesystem

Engulfing ofphotosyntheticprokaryote insome cells

Mitochondrion

Ancestralheterotrophiceukaryote

Ancestralphotosynthetic eukaryote

Mitochondrion

Plastid

Earliest Multicellular Eukaryotes• Molec clocks – earliest common ancestor of multi

euk ~ 1.5 bya (oldest fossils – 1.2 bya)• The first multicellular organisms were colonies,

collections of autonomously replicating cells

“Cambrian Explosion”• Most of the major phyla of animals appear in

the fossil record of the first 20 million years of the Cambrian period

In this format the RNA would be shared by all competing RNA molecules

Shows molecules smaller than RNA can self-replicate!