The History of Life on Earth

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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 oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago.

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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).

The Cambrian explosion provides the first evidence of predator-prey interactions.

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

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Cambrian Explosion

Sponges

Late

Proterozoic

eon

Early

Paleozoic era

(Cambrian period)

Cnidarians

Annelids

Brachiopods

Echinoderms

Chordates

Millions of years ago

500

542

Arthropods

Molluscs

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Proterozoic Fossils that may be animal embryos (SEM)

(a) Two-cell stage

150 µm

200 µm

(b) Later stage

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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.

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At three points in time, the land masses of Earth have formed a supercontinent: 1.1 billion, 600 million, and 250 million years ago.

Earth’s continents move slowly over the underlying hot mantle through the process of continental drift.

Oceanic and continental plates can collide, separate, or slide past each other.

Interactions between plates cause the formation of mountains and islands, and earthquakes.

Concept 25.4: The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations

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Earth - Plate Tectonics: Continental Drift

(a) Cutaway view of Earth

(b) Major continental plates

Inner

core

Outer

core

Crust

Mantle

Pacific

Plate

Nazca

Plate

Juan de Fuca