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

dense mats

Laboratory versions of protobionts, aggregates of abiotically produced molecules

Liposomecarrying out metabolic functions

Ribozymes – RNA Catalysts that make complementarystrands of RNA

The Miller-Urey Experiment (1953)

Amino acidscreated

Stromatolites (bacteria and sediment) in Northern Canada (3.5 Billion Yrs Old)

Oldest are in Shark Bay, Australia

Evolutionary clock: Prokaryotes

3.5 billion yrsago

Humans

Cyanobacteria

The First Eukaryotes• The oldest fossils of eukaryotic cells date

back 2.1 billion years.• The hypothesis of endosymbiosis

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.

• The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites.

• In the process of becoming more interdependent, the host and endosymbionts would have become a single organism.

• Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events.

Endosymbiotic Sequence:

Ancestral photosyntheticeukaryote

Photosyntheticprokaryote

Mitochondrion

Plastid

Nucleus

Cytoplasm

DNA

Endoplasmic reticulum

Nuclear envelope

Ancestral Prokaryote Invagination of Plasma Membrane

Serial Endosymbiosis: Aerobic heterotrophic prokaryote

Mitochondrion

Ancestralheterotrophiceukaryote

• Key evidence supporting an endosymbiotic origin of mitochondria and plastids:

–Similarities in inner membrane structures and functions.

–These organelles transcribe and translate their own DNA.

–Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes.

Evolutionary clock: Atmospheric oxygen

•While oxygen accumulation was gradual between 2.7 and 2.2 billion years ago, it shot up to 10% of current values shortly

afterward.

• This “corrosive” O2 had an enormous impact on life, dooming many prokaryote groups.

• Other species evolved mechanisms to use O2 in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Evolutionary clock: Eukaryotes

Evolution occurredbetween2.0-2.5 bya

The Origin of Multicellularity• The evolution of eukaryotic cells

allowed for a greater range of unicellular forms.

• A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals.

• Comparisons of DNA sequences date the common ancestor of multicellular eukaryotes to 1.5 billion years ago.

• The “snowball Earth” hypothesis suggests that periods of extreme glaciation confined life to the equatorial region or deep-sea vents from 750 to 580 million years ago.

• The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period (535 to 525 million years ago).

• Fossils in China provide evidence of modern animal phyla tens of millions of years before the Cambrian explosion.

Fossilized Animal Embryos from Chinese Sediments 570 Million Years Old

The Cambrian Radiation of Animals

The Colonization of Land• Fungi, plants, and animals began to

colonize land about 500 million years ago.

• Plants and fungi likely colonized land together by 420 million years ago.

• Arthropods and tetrapods are the most widespread and diverse land animals.

• Tetrapods evolved from lobe-finned fishes around 365 million years ago.

Some Major Episodes in the History of Life

Table 26.1

p. 519

90% of marine creatures extinct half of

all marinecreatures and manyplant familiesextinct

Mass Extinctions• Occurred at the end of

the following periods:

• Ordovician– 438 mya

– 75% of species disappeared

• Devonian– 360 mya

– 70% of marine invertebrates disappeared

• Permian– 245 mya

– 90% of ocean species disappeared

• Triassic– 208 mya

– 60% of species disappeared

• Cretaceous– 66 mya

– 75% of species disappeared

Clock Analogy for Some Key Events in Evolutionary History

4-5 mya475 mya

550-600 mya

1.5 bya

2.0 bya 3.5 bya

What gases did Miller and Ureyuse in their experiment backin 1953? What did they create?

Of what materials arestromatolites composed?

The first land plants appeared…

EK 1D1: There are several hypotheses about the natural origin of life on earth, each with supporting scientific evidence. Students should understand EACH of the following:

1)Primitive Earth provided inorganic precursors from which organic molecules could have been synthesized due to a presence of free available energy and the absenceof a significant quantity of oxygen.

2. In turn, these molecules served as monomers or building blocks for the formation of more complex molecules, including amino acids and nucleotides.3. The joining of monomers produced polymers with the ability to replicate, store, and transfer information.4. These complex reaction sets could haveoccurred in solution (organic soup model)or as reactions on solid reactive surfaces.5. The RNA World hypothesis proposesthat RNA could have been the earliest genetic material.