Chapter 26

CHAPTER 26
EARLY EARTH AND THE ORIGIN OF LIFE

Lecture Outline
• The history of life
• Life originated 3.5 – 4 bya
• Changes in the atmosphere and early organisms
• Origins of major groups
• The origin of life
• Chemical evolution
• Genetic information
• Locations
• Major lineages of life

A painting of early Earth showing volcanic activity and photosynthetic prokaryotes in dense mats

Recent activities:
island of Surtsey near Iceland in 1963-1967.

Geological time scale

• clock analogy for some key events in evolutionary history:

History of life
• planet Earth formed
•
• life on Earth originated
• first organisms
• oldest known fossils
- simple filamentous
prokaryotes

Prokaryotes

Prokaryotes for 1.5 billion years
• prokaryotes from
• bacteria and archaea
• mat-like fossil formations:

Early (left) and modern (right) prokaryotes

• major episodes that shaped life:

Oxygen ~ 2.7 billion years ago
• photosynthesis first by
• O2 production

Fossilized filamentous cyanobacteria

Eukaryotes

Eukaryotic life ~ 2.1 billion years ago
• larger and more complex
• presence of descendents of “endosymbiotic prokaryotes”

• fossils from China:
• ~570 million years ago
•

The Precambrian era (origin-543mya)
• 2-1.2 bya unicellular explosion, many forms
• multicellular eukaryotes evolved by 1.5 bya
• multicellular algae common by 1.2 bya
• plants
• fungi and animals
• oldest animal fossils
• geologic evidence for severe ice age 750 to 570 mya (“snowball Earth” hypothesis)

Geological time scale

The Cambrian explosion (543-510 mya)
Multicellular Organisms diversify during early Paleozoic era
• most major groups of animals
• terrestrial invasion ~ 500 mya

… more recently
• fishes, amphibians, reptiles, flowering plants
- Paleozoic & Mesozoic eras
--

• modern mammals
- Cenozoic era
• hominids

CHAPTER 26
EARLY EARTH AND THE ORIGIN OF LIFE
Lecture Outline
• The history of life
• Life originated 3.5 – 4 bya
• Changes in the atmosphere and early organisms
• Origins of major groups
• The origin of life
• Chemical evolution
• Genetic information
• Locations
• Major lineages of life

Origin of life on Earth
• 4.0 -3.5 bya
--

• Early Earth:
• reducing atmosphere
• little O2
• chemical building blocks
• intense energy:
- lightning
- volcanic activity
- ultraviolet sunlight
- meteorite strikes

First cells by ‘chemical evolution’?
• life on Earth may have developed from nonliving materials that became ordered into aggregates
--
• possible scenario:
--

Four-stage hypothesis for the origin of life
(1) the abiotic synthesis of small organic molecules
(2) joining these small molecules into polymers
(3) origin of self-replicating molecules
(4) packaging of these molecules into “protobionts”

-- hypothesis leads to predictions that can be
tested in the laboratory

1. Abiotic synthesis of organic molecules
• Oparin & Haldane (1920’s):
conditions on early Earth favored synthesis of organic compounds from inorganic precursors
• reducing atmosphere, lightning, UV
• Miller & Urey (1953):
tested Oparin-Haldane
hypothesis in the lab
• gas mixture, water vapor
• electric discharge
--

2. Synthesis of organic polymers
• abiotic origin hypothesis:
monomers should link to form polymers without enzymes and other cellular equipment
• polymers may be synthesized by dehydration & concentration increase by evaporation
• Experimental: drip solution of organic molecules on hot rock or sand
--
• Early Earth: dilute solutions of monomers splashed onto fresh lava or reacted at deep-sea vents

3. Origin of self-replication molecules
• first hereditary material
• short polymers of ribonucleotides can be synthesized abiotically (improved by adding zinc)
• polymers can self-replicate
• RNA can also act as an enzyme
• polymers can ‘evolve’ & ‘mutate’

• later switch to

4. Protobionts can form by self-assembly
• protobionts: aggregates of abiotically produced molecules
--
• Properties of protobionts:
• maintain internal chemical environment
• show simple metabolic reactions
• excitable ? store energy as a membrane potential

• laboratory versions of protobionts:

5. Protobionts containing hereditary information
• RNA genes and their polypeptide products may have been packaged within a membrane
RNA

primitive metabolic enzymes

energy source for other reactions

Abiotic origin of life
• Laboratory simulations describe steps that could have happened
• Laboratory simulations cannot prove that these kinds of chemical processes actually created life on the primitive Earth
• Alternative views:
• organic monomers may have reached Earth on comets and meteorites

Locations for the origin of life

Life restricted to Earth?
• Mars may have had a warm
CO2-rich atmosphere bya
• NASA explorations
• Mars meteorites

Traditionally only two kingdoms
• Traditional classification of life forms:
• started with Linnaeus
• organisms were divided into two kingdoms
-- animals
(including some photosynthetic mobile microbes)
-- plants
(including bacteria, fungi, photosynthetic microbes)

• R.H Whittaker (1969) five-kingdom system:
-- Monera, Protista, Plantae, Fungi, and Animalia.

• two fundamentally different types of cells:
--
--

-multicellular eukaryotes separated by nutrition:
-- plants:
-- fungi:
-- animals:

Recent reorganization of 5-kingdom system
• problems with the five-kingdom system:
• two distinct lineages of prokaryotes: bacteria & archaea

-- three-domain system (domain = superkingdom):
- - -

• Further subdivisions based on cladistic analysis of molecular data:

Key terms 26
stromatolites
snowball Earth
Cambrian explosion
spontaneous generation
chemical evolution
abiotic synthesis
biogenesis
protobionts
three-domain system