1

The Beginning of Life

I. Conditions necessary to allow life to begin

Fig. 20-4b, p.321

Earth was a very hostile environment 4 billion years ago

2

First Atmosphere

• Hydrogen gas

• Nitrogen

• Carbon monoxide

• Carbon dioxide

• No gaseous oxygen

I. Conditions necessary to allow life to begin

to vacuum pump

CH4NH3H2OH2

gases

electrodes

spark discharge

water out

condenser

water in

water droplets

water containingorganic compounds

liquid water in trap

boiling water

Fig. 20-4c, p.321

3

I. Conditions necessary to allow life to begin

four formaldehyde molecules with four pyrrole rings

porphyrin ring system

6H

chlorophyll a

Stepped ArtFig. 20-5, p.322

4

Origin of Organic Compounds

• Amino acids, other organic compounds can form spontaneously under conditions like those on early Earth

• Clay may have served as template for complex compounds

• Compounds may have formed near hydrothermal vents

RNA ‘Ribozyme’ produced in the laboratory

Store biological information and also act as catalyst (enzyme)

Could something like this have been the first source of genetic material?

RNA World

• DNA is genetic material now• DNA-to-RNA-to-protein system is complicated• RNA may have been first genetic material• RNA can assemble spontaneously • How switch from RNA to DNA might have

occurred is not known

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II. Origin of cells – precursor to living cells

Proto-Cells

• Microscopic spheres of proteins or lipids can self assemble

• Tiny sacs like cell membranes can form under laboratory conditions that simulate conditions in evaporating tidepools

• Nanobes may resemble proto-cells

Amino acids heated, then moistened.Formed protein membranes.

6

II. Origin of cells – precursor to living cells

Fig. 20-7b, p.323

Membrane of fatty acids and alcohol surrounding RNA-coated clay .

II. Origin of cells – precursor to living cells

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Scientists have managed to create "primitive cells" in an experiment which may indicate that life began in space and was delivered to Earth.

Researchers working with the American space agency NASA say their "proto-cells" mimic the membranous structures found in all living things. They were produced in a laboratory experiment that duplicated the harsh conditions of cold interstellar space.

BBC NewsMonday, 29 January, 2001

Could precursors and /or proto-cells have arrived from outer space?

This new work suggests that the early chemical steps believed to be important for the origin of life do not require an already-formed planet.

Instead, they seem to take place in deep space long before planet formation occurs.

This implies that the vastness of space is filled with chemical compounds which, if they land in a hospitable environment like our Earth, can readily jump-start life.

spontaneous formation of lipids,carbohydrates, amino acids, proteins,nucleotides under abiotic conditions

livingcells

self-replicating system enclosed in aselectively permeable, protective lipid sphere

DNA RNA enzymes andother proteins

formation ofprotein-RNA systems,

evolution of DNAformation oflipid spheres

membrane-bound proto-cells

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III. The first true cells

Fig. 20-8a, p.324

3.5 billion y –oldprokaryote

Anaerobic or aerobic?

The First Cells

• Originated in Archeon Eon

• Were prokaryotic heterotrophs

• Secured energy through anaerobic pathways– No oxygen present

– Relied on glycolysis and fermentation

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Stromatolites – fossils of cyanobacteria; 3by-old

Proterozoic Eon

• Origin of photosynthetic Eubacteria

– Cyclic pathway first

– Noncyclic pathway next (cyanobacteria)

• Oxygen accumulates in atmosphere

• Origin of aerobic respiration

Fig. 20-8b, p.324

Eukaryotes2.1 bya

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IV. Where did eukaryotic organelles come from? The evolution of the eukaryotic cell

Where did all these organelles come from?

IV. Where did eukaryotic organelles come from? The evolution of the eukaryotic cell

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DNA

infolding of plasma membraneFig. 20-10a, p.326

ER ?

Possible origin of nucleus

IV. Where did eukaryotic organelles come from? The evolution of the eukaryotic cell

Origin of mitochondria and chloroplasts

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Endosymbiosis example in modern time

Amoeboid organism with green alga within

Evidence

Occurred by phagocytosis

nucleusmitochondrion

photosynthetic organelle thatresembles a cyanobacterium

Evidence

Theory of Endosymbiosis• Lynn Margulis• Mitochondria and chloroplasts are the

descendents of free-living prokaryotic organisms

• Prokaryotes were engulfed by early eukaryotes and became permanent internal symbionts

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V. The timeline of development

hydrogen-rich anaerobic atmosphere atmospheric oxygen, 10%

archaeanlineage

dancestors ofeukaryotes

hendomembrane

system and nucleus

b

origin ofprokaryotes

cyclic pathwayof photosynthesis

e

noncyclic pathwayof photosynthesisf

g aerobic respiration

3.8 billion years ago

3.2 billion years ago

2.5 billion years ago

a

Fig. 20-12b, p.328

atmospheric oxygen, 20%; the ozone layer slowly develops

iendosymbiotic origin

of mitochondira

jorigin of eukaryotes,

the first protists

k origin of animals

jendosymbiotic origin

of chloroplasts

Aerobic species becomes endosymbiot of anaerobic forerunner of eukaryotes.

k origin of fungi

k origin of lineage leading to plants

1.2 billion years ago

900 million years ago

435 million years ago