The History of Life on Earth

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Slide 16

Evolution of Mammals

Very late cynodont (195 mya)

Later cynodont (220 mya)

Early cynodont (260 mya)

Therapsid (280 mya)

Synapsid (300 mya)

Temporal

fenestra

Temporal

fenestra

Temporal

fenestra

EARLY

TETRAPODS

Articular

Key

Quadrate

Dentary

Squamosal

Reptiles

(including

dinosaurs and birds)

Dimetrodon

Very late cynodonts

Mammals

Synapsids

Therapsids

Earlier cynodonts

Slide 17

The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons.

The Phanerozoic encompasses multicellular eukaryotic life.

The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.

Major boundaries between geological divisions correspond to extinction events in the fossil record.

Concept 25.3: Key events in life’s history include the origins of single-celled and multicelled organisms and the colonization of land

Slide 18

Geologic Record

Slide 19

Geologic Time Table

Animals

Colonization

of land

Paleozoic

Meso-

zoic

Humans

Ceno-

zoic

Origin of solar system

and Earth

Prokaryotes

Proterozoic

Archaean

Billions of

years ago

1

4

3

2

Multicellular

eukaryotes

Single-celled

eukaryotes

Atmospheric

oxygen

Slide 20

The First Single-Celled Organisms = Prokaryotes

The oldest known fossils are stromatolites, rock-like structures composed of many layers of bacteria and sediment.

Stromatolites date back 3.5 billion years ago

Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago.

Slide 21

Photosynthesis and the Oxygen Revolution

Most atmospheric oxygen (O2) is of biological origin.

O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations.

The source of O2 was likely bacteria similar to modern cyanobacteria.

Slide 22

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

This “oxygen revolution” from 2.7 to 2.2 billion years ago

Posed a challenge for life

Provided opportunity to gain energy from light

Allowed organisms to exploit new ecosystems.