Origin of Life on Earth

“Beginnings of Cellular Life” Harold Horowitz (1992)

• All life is cellular.• All living things are from 50 to over 90% water, the

source of protons, hydrogen and oxygen in photosynthesis and the solvent of biomolecules.

• The major elements of covalently bound biomolecules are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.

• There is a universal set of small molecules: (i.e. sugars, amino acids, nucleotides, fatty acids, phospholipids, vitamins and coenzymes.)

* The principle macromolecules are proteins, lipids, carbohydrates and nucleic acids.

“Beginnings of Cellular Life” Harold Horowitz (1992)

• There is a universal type of membrane structure (lipid bilayer).

• The flow of energy in living things involves formation and hydrolysis of phosphate bonds, usually ATP.

• The metabolic reactions of any living species is a subset of a universal network of intermediary metabolism (glycolysis, the Krebs cycle, the electron transport chain)

• Every replicating cell has a genome made of DNA that stores the genetic information of the cell which is read out in sequences of RNA and translated into protein.

“Beginnings of Cellular Life” Harold Horowitz (1992)

• All growing cells have ribosomes, which are the sites of protein synthesis.

• All living things translate information from nucleotide language through specific activating enzymes and transfer RNAs.

• All replicating biological systems give rise to altered phenotype due to mutated genotypes.

• Reactions that proceed at appreciable rates in all living cells are catalyzed by enzymes.

Life from inanimate matter?

*Spontaneous interaction of simple molecules (ammonia, phosphates, methane, UV light, heat, electricity, lightning) = primordial organic soup (Haldane & Oparin)

Primary QUESTIONS:(1) Where did the raw materials for life come from?

(2) How did monomers develop?

(3) How did polymers develop?

(4) How did an isolated cell form?

(5) How did reproduction begin?

Where did the raw materials for life come from?

Electric sparks can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia and hydrogen, as was shown in the famous Miller-Urey experiment (1953) = LIGHTNING created building blocks of life on Earth.

- similar amino acids from Murchison meteorite

Community Clay(Alexander Graham Cairns-Smith)

• Polymerization on clays or the evaporation of amino acids containing water near volcanic vents

- Sidney Fox experiment:

heated amino acids = proteinoids• The first molecules of life might have met on clay

• Mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.

Deep Sea Vent Theory

- Suggests that life may have begun at submarine hydrothermal vents, spewing key hydrogen-rich molecules. Their rocky nooks could have concentrated these molecules & provided mineral catalysts for critical reactions.

Chilly Start- Ice might have covered the oceans 3 billion years ago, as the sun was about a third less luminous than it is now.

- This layer of ice, possibly hundreds of feet thick, might have protected fragile organic compounds in the water below from ultraviolet light and destruction from cosmic impacts.

- The cold might have also helped these molecules to survive longer, allowing key reactions to happen.

RNA World

• DNA needs proteins in order to form, and proteins require DNA to form, so how could these have formed without each other?

RNA = store info like DNA, serve as an enzyme, & help create both DNA & proteins.

Question remains: how was RNA created in the

1st place?

Self-Replicating RNA and the Dawn of Natural Selection

RNA molecules called ribozymes have been found to catalyze many different reactions– For example, ribozymes can make complementary copies of

short stretches of their own sequence or other short pieces of RNA

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

• The early genetic material might have formed an “RNA world”

Protobionts

• Replication and metabolism are key properties of life

• Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure

• Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment

Endosymbiosis Hypothesis

• Proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells

• An endosymbiont is a cell that lives within a host cell

Fig. 25-9-4

Ancestral photosyntheticeukaryote

Photosyntheticprokaryote

Mitochondrion

Plastid

Nucleus

Cytoplasm

DNAPlasma membrane

Endoplasmic reticulum

Nuclear envelope

Ancestralprokaryote

Aerobicheterotrophicprokaryote

Mitochondrion

Ancestralheterotrophiceukaryote

Panspermia

• Svante Arrhenius in 1908 • Perhaps life did not begin on Earth at all but was

brought here from elsewhere in space

• Rocks regularly get blasted off Mars by cosmic impacts reaching Earth.

• Martian meteorites brought microbes on Earth thereby evolving into complex systems.

• Other scientists have suggested that life might have hitchhiked on comets/ portals from other star systems

.

Fig. 25-7

Animals

Colonizationof land

Paleozoic

Meso-

zoicHumans

Ceno-zoic

Origin of solarsystem andEarth

ProkaryotesProterozoic Archaean

Billions of years ago

1 4

32

Multicellulareukaryotes

Single-celledeukaryotes

Atmosphericoxygen

Table 25-1a

Table 25-1b

Fig. 25-13

SouthAmerica

Pangaea

Mill

ions

of y

ears

ago

65.5

135

Mes

ozoi

c

251

Pale

ozoi

c

Gondwana

Laurasia

Eurasia

IndiaAfrica

AntarcticaAustralia

North America

Madagascar

Ceno

zoic

Present

Cell Theory

Theodore Schwann, Matthias Schleiden & Rudolf Virchow

Schwann & Schleiden 1839

1. The cell is the basic unit structure, physiology, and organization in living things.

2. The cells retains a dual existence as a distinct identity and a building block in the construction of organism.

3. Cells form by free-cell formation, similar to the formation of crystal (spontaneous generation)

Virchow 1858: All cells came from pre-existing cells (Omnis cellula e cellula).

Modern Cell Theory

1. All known living things are made up of cells

2. The cell is structural and functional unit of all living things

3. All cells comes from pre-existing cells by division.

4. Cells contains hereditary information which is passed from cell to cell by division

5. All cells are basically the same in chemical composition

6. All energy flow (metabolism and biochemistry) of life occurs within the cells

The End!

Primordial Soup Theory

- Russian Chemist A.I. Oparin and English Geneticist J.B.S. Haldane in 1920.

- basic building blocks of life came from simple molecule then energized by lightning and the rain from the atmosphere created the "organic soup”.

- The first organisms would have to be simple heterotrophs. In order to survive, they consume other organisms for energy. They would become autotrophs by mutation. Evidence now suggest the first organisms were autotrophs.

Urey-Miller Experiment (1950)- They mixed gases thought to be present on

primitive earth: ammonia, methane, water, hydrogen

- then electrically sparked the mixture to signify lightning = amino acids

- electricity, UV light, heat, & shock- Sidney Fox experiment:

heated amino acids = proteinoids

Simple Beginnings• Instead of developing from complex molecules such

as RNA, life might have begun with smaller molecules interacting with each other in cycles of reactions.

• These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved

• "metabolism-first" models, as opposed to the "gene-first" model of the "RNA world" hypothesis.