The History of Life on Earth

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4

the real world

Slide 23

About 2.7 billion years ago, O2 began accumulating in the atmosphere and rusting iron-rich terrestrial rocks.

Slide 24

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.

Slide 25

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.

Slide 26

Invagination of Plasma Membrane

Nucleus

Cytoplasm

DNA

Plasma membrane

Endoplasmic reticulum

Nuclear envelope

Ancestral

prokaryote

Slide 27

Serial Endosymbiosis

Aerobic

heterotrophic

prokaryote

Mitochondrion

Ancestral

heterotrophic

eukaryote

Slide 28

Serial Endosymbiosis

Ancestral photosynthetic

eukaryote

Photosynthetic

prokaryote

Mitochondrion

Plastid

Slide 29

Endosymbiotic Sequence:

Ancestral photosynthetic

eukaryote

Photosynthetic

prokaryote

Mitochondrion

Plastid

Nucleus

Cytoplasm

DNA

Endoplasmic reticulum

Nuclear envelope

Ancestral Prokaryote

Invagination of Plasma Membrane

Serial Endosymbiosis:

Aerobic heterotrophic

prokaryote

Mitochondrion

Ancestral

heterotrophic

eukaryote

Slide 30

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.

Slide 31

The Origin of Multicellularity

The evolution of eukaryotic cells allowed for a greater range of unicellular forms.