The Origins Of Life

The Origins of Life

A History of Geologic Time

22.1

A History of Geologic Time

22.1

The Origin of Life ~ A Four Step Hypothesis

• The early atmosphere – Made of N2 and CO2 – Near volcanic vents there was methane and ammonia (reducing conditions)

22.2

• Laboratory experiments simulating the atmosphere around volcanic vents have produced organic molecules from inorganic precursors Miller and Urey set up a closed system in their EXPERIMENT laboratory to simulate conditions thought to have existed on early Earth. A warmed flask of water simulated the primeval sea. The strongly reducing “atmosphere” in the system consisted of H2, methane (CH4), ammonia (NH3), and water vapor. Sparks were discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds into the miniature sea.

NH

3

H2

Cold water

As material circulated through the apparatus, Miller and Urey periodically collected samples for analysis. They identified a variety of organic molecules, including amino acids such as alanine and glutamic acid that are common in the proteins of organisms. They also found many other amino acids and complex, oily hydrocarbons.

Organic molecules, a first step in the origin of life, can form in a strongly reducing atmosphere.

Electrode

Condenser

RESULTS

CONCLUSION

CH4

Water vapor

H2O

Cooled water containing organic molecules Sample for chemical analysis

Extraterrestrial Sources of Organic Compounds • Some of the organic compounds from which the first life on Earth arose – May have come from space

• Carbon compounds – Have been found in some of the meteorites that have landed on Earth

Step #1 - Small organic molecules (monomers) were produced (somehow)

22.2

Step #2 - Synthesis of Polymers • Small organic molecules – Polymerize when they are concentrated on hot sand, clay, or rock – This resulted in polymers such as peptides and RNA

22.2

Step #3 – Synthesis of Protobionts • Laboratory experiments demonstrate that protobionts – Could have formed spontaneously from abiotically produced organic compounds

• For example, small membrane-bounded droplets called liposomes – Can form when lipids or other organic molecules are added to water 22.2

Liposome Formation Glucose-phosphate 20 µm

Glucose-phosphate Phosphorylase

Starch Amylase

Phosphate

Maltose

Maltose (a) Simple reproduction. This liposome is “giving birth” to smaller liposomes (LM).

22.2

(b) Simple metabolism. If enzymes—in this case, phosphorylase and amylase—are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products.

Step # 4 – Synthesis of RNA (heritable material) • RNA molecules called ribozymes have been found to catalyze many different reactions, including – –

Self-splicing Making complementary copies of short stretches of their own sequence or other short pieces of RNA

3′ Template

Nucleotides

Complementary RNA copy

22.3

Ribozyme (RNA molecule)

5′

5′

How do you prove it?

Mass Extinctions • The fossil record chronicles a number of occasions

400

500

300

200

100

0 2,500

Number of taxonomic Permian mass families

80 Extinction rate ) 60

1,500

40

1,000 Cretaceous mass extinction 500

20

0

22.4

Paleozoic

Mesozoic

Paleogene

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Proterozoic eon

0

Cenozoic

Neogene

Extinction rate (

2,000

extinction

)

When global environmental changes were so rapid and disruptive that a majority of species were swept away

600 100

Number of families (

–

Millions of years ago

Permian and the Cretaceous Mass Extinctions • The Permian extinction – Claimed about 96% of marine animal species and 8 out of 27 orders of insects – Is thought to have been caused by enormous volcanic eruptions

22.4

• The Cretaceous extinction – Doomed many marine and terrestrial organisms, most notably the dinosaurs – Is thought to have been caused by the impact of a large meteor NORTH AMERICA

Yucatán Peninsula

22.4

Chicxulub crater

Prokaryotes Emerged 3.5 BYA Lynn Margulis (top right), of the University of Massachussetts, and Kenneth Nealson, of the University of Southern California, are shown collecting bacterial mats in a Baja California lagoon. The mats are produced by colonies of bacteria that live in environments inhospitable to most other life. A section through a mat (inset) shows layers of sediment that adhere to the sticky bacteria.

Some bacterial mats form rocklike structures called stromatolites, such as these in Shark Bay, Western Australia. The Shark Bay stromatolites began forming about 3,000 years ago. The inset shows a section through a fossilized stromatolite that is about 3.5 billion years old.

22.5

Oxygenic photosynthesis – Probably evolved about 3.5 billion years ago in cyanobacteria

22.5

Iron oxide sediments in 3 bya rock

• When oxygen began to accumulate in the atmosphere about 2.7 billion years ago – It provided an opportunity to gain abundant energy from light – It provided organisms an opportunity to exploit new ecosystems

22.5

The First Eukaryotes • The oldest fossils of eukaryotic cells – Date back 2.1 billion years

22.6

Cytoplasm

DNA

Plasma membrane Ancestral prokaryote of 1. plasma Infolding membrane

The Evolution of Eukaryotes – Endosymbiosis

Endoplasmic reticulum Nuclear envelope Engulfing of aerobic heterotrophic prokaryote

Mitochondrion

3. Ancestral heterotrophic eukaryote

22.6

Nucleus

2. Cell with nucleus and endomembrane system Mitochondrion

Engulfing of photosynthetic prokaryote in some cells Plastid

3.

Ancestral Photosynthetic eukaryote

Eukaryotes became multicellular 1.5 bya • Formed colonies collections of autonomously replicating cells • Some cells in the colonies became specialized for different functions 22.6

10 µm

The “Cambrian Explosion” • Most of the major phyla of animals – Appear suddenly in the fossil record that was laid down during the first 20 million years of the Cambrian period (540 mya)

Domain Archaea

Domain Bacteria Universal ancestor Domain Eukarya

Charophyceans

Chlorophytes

Red algae

Cercozoans, radiolarians

Stramenopiles (water molds, diatoms, golden/brown algae)

Chapter 27

Alveolates (dinoflagellates, apicomplexans, ciliates)

Euglenozoans

Diplomonads, parabasalids

Euryarchaeotes, crenarchaeotes, nanoarchaeotes

Korarchaeotes

Gram-positive bacteria

Cyanobacteria

Spirochetes

Chlamydias

Proteobacteria

One current view of biological diversity Chapter 28

Plants

Fungi Animals

Bilaterally symmetrical animals (annelids, arthropods, molluscs, echinoderms, vertebrates)

Cnidarians (jellies, coral)

Sponges

Choanoflagellates

Chapter 31 Club fungi

Sac fungi

Arbuscular mycorrhizal fungi

Chapter 30 Chapter 28 Zygote fungi

Chytrids

Amoebozoans (amoebas, slime molds)

Angiosperms

Chapter 29

Gymnosperms

Seedless vascular plants (ferns)

Bryophytes (mosses, liverworts, hornworts)

Continued Chapter 32 Chapters 33, 34