Early Earth

Early Earth

• Earth formed 4.6 billion years ago

• oldest fossil organisms - prokaryotes dating back to 3.5 bya

• earliest prokaryotic cells lived in dense mats that resembled stepping stones

Origin of Life?

• Life needed the following to occur:1. Abiotic synthesis of small organic molecules2. Joining of these small molecules into

macromolecules3. Packaging of molecules into protocells,

membrane-bound droplets that maintain a consistent internal chemistry

4. Origin of self-replicating molecules

=primordial soup

Synthesis of Organic Compounds on Early Earth

• Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, and hydrogen)

• As Earth cooled, water vapor condensed into oceans, and most of the hydrogen escaped into space

What was early Earth like?

• Lots of Available Free Energy •  (What Form)

• Absence of Oxygen•  (reducing atmosphere)

Miller Urey experiment

• Tested Oparin and Haldane's hypothesis

Too many assumptions?

• evidence is not yet convincing that the early atmosphere was in fact reducing

• Instead of forming in the atm, the first organic compounds may have been synthesized near volcanoes or deep-sea vents

• Organic molecules have also been found in meteorites

Abiotic Synthesis of Macromolecules

• RNA monomers have been produced spontaneously from simple molecules

• Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock

Protocells• Replication and metabolism are key properties

of life and may have appeared together• Protocells may have been fluid-filled vesicles

with a membrane-like structure• In water, lipids and other organic molecules

can spontaneously form vesicles with a lipid bilayer

Adding clay can increase the rate of vesicle formation

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

Figure 24.4a

Precursormolecules only

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Time (minutes)

0.4

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00 20 40 60

Precursor molecules plusmontmorillonite clay

(a) Self-assembly

Figure 24.4b

Figure 24.4b

20 m(b) Reproduction

What is so cool about RNA?• The first genetic material was probably RNA,

not DNA: “RNA World” hypothesis• RNA molecules called ribozymes have been found to catalyze

many different reactions– For example, ribozymes can make complementary copies

of short stretches of RNA

• Vesicles with RNA capable of replication would have been protocells

• RNA could have provided the template for DNA, a more stable genetic material

Vesicleboundary

1 m

(c) Absorption of RNA

Fossil Evidence of Early Life

• Many of the oldest fossils are stromatolites, layered rocks that formed from the activities of prokaryotes up to 3.5 billion years ago

Figure 24.5

Time (billions of years ago)

10 m

30

m

5 cm

Nonphotosynthetic bacteria

Cyanobacteria

Stromatolites

Possibleearliestappearancein fossil record

4 3 2 1 0

• What came first the aerobe or the anaerobe? Why?

• What came first a Eukaryotic or Prokaryotic cell? Why?

• What came first the autotroph or the heterotroph? Why?

History of Life• 4.6 bya - Earth formed• 3.5 bya - Earliest prokaryotes

diverged into bacteria and archaea• 2.7bya - Cyanobacteria 02 revolution• 2.1 bya – eukaryotic evolution, protists• 1.2 bya – multicellular eukaryotes• 540 mya – earliest animals• 500 mya – plants & fungi colonize land,

animals follow (amphibians descended from air-breathing fish with fleshy fins that could support their weight), then reptiles and mammals

http://www.pbs.org/wgbh/evolution/library/03/4/quicktime/l_034_03.html

http://www.youtube.com/watch?v=H2_6cqa2cP4 History of Life in 24 hrs

Endosymbiotic Theory

Review of Hox Genes

• http://www.pbs.org/wgbh/evolution/library/03/4/quicktime/l_034_04.html