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A BRIEF HISTORY OF LIFE

ON EARTH

Objectives

• Describe how Earth’s environment has changed over

the past 4 billion years.

• Identify the minimum requirements for life.

• Describe the difference between prokaryotic and

eukaryotic organisms.

• Define the theory of natural selection and how

evolution works.

• Describe several ways in which fossils form.

• Describe the dramatic change in Earth’s biota during

the Cambrian Period.

The Ever-

Changing Earth• Changes in the

atmosphere and

hydrosphere

– Early atm. had no O2

– Oldest bacteria: 3.5 by

– Blue-green algae-2.5by,

– Photosynthesis

– Banded iron formation

2.5-1.8 by

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The Ever-Changing Earth• Photosynthesis

– A chemical reaction

whereby plants use light

energy to induce carbon

dioxide to react with water,

producing carbohydrates

and oxygen

• Oxygen content increases

in the atmosphere

Banded iron formations

formed during the transition to

more oxygen rich atm.

Cyanobacteria (algae)

produced the oxygen initially.

They formed the first fossils,

which were mounds of

calcium carbonate

(stromatolites)

The Ever-Changing Earth

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Oxygen content in the atmosphere increased starting

around 200 my. The deposition of large amounts of

organic matter on shallow marine shelves due to plate

tectonics may have allowed this increase.

Early Life

• Archean and proterozoic

life

– Prokaryote• A single celled organism with

no distinct nucleus… or, no

membrane separates its DNA

from the rest of the cell- all

bacteria are prokaryotes

Early Life

• Eukaryote

– An organism composed

of eukaryotic cells-

– Cells have a well defined

nucleus

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Evolution and the Fossil Record

• Evolution

– The theory that life on

earth has developed

gradually, from one or a

few simple organisms

to more complex

organisms

– Charles Darwin

• “On the Origin of

Species by Means of

Natural Selection”

• Natural selection

– Individuals that are well adapted to their environment have a

survival advantage

– They then pass on their favorable characteristics to their

offspring

• Species

– A population of genetically and/or morphologically similar

individuals that can interbreed and produce fertile offspring

Evolution and the Fossil Record

• How fossils form

– Fossil

• Remains of an organism from a

past age

• Embedded/ preserved in rock;

– Trace fossil

• Fossilized evidence of an

organism’s life processes

– Includes tracks, footprints,

and burrows

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Life in the Phanerozoic Eon

• The Paleozoic era

– Began 542 million years

ago with the Cambrian

period, a time of

incredible diversification

of life; known as the

Cambrian explosion

– Development of hard

skeletons allowed much

more widespread

preservation as fossils.

trilobite

Life in the Phanerozoic Eon

• From sea to land (any

organism)

– Must have structural

support

– Must have internal

aquatic environment

– Must be able to exchange

gases with air instead of

water

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• Plants (first to move from ocean to

land)

– Land plants evolved from algae

600 million years ago

– Vascular plants evolved in the

Silurian period (444-416 my);

stems and limbs

– Stomata (openings in leaves for

gas exchange)

– Earliest plants were seedless:

mosses and ferns

– Gymnosperm (415-359 my)

• A naked-seed plant

• Sexual reproduction

• Ginkos and conifers

• Requires spreading of pollen

for reproduction

Life in the Phanerozoic Eon

Flowering plants and trees;

much later; Cretaceous time

(145-65 my); angiosperms-

seed enclosed

Fossil and modern ginko;

naked seed plant.

Life in the Phanerozoic Eon

• Arthropods

– First creatures to make

the transition from sea

to land

– Small, light and covered

in a hard shell called

chitin

– Modern arthropods

include crabs, spiders,

centipedes and insects

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Life in the Phanerozoic Eon

• Fishes and amphibians

– Chordates

• Must have at least a primitive

version of a spinal cord

• First fish to venture onto land

may have been a member of

an obscure order-

Crossopterygii

• First terrestrial chordates,

amphibians, have never

become fully independent of

aquatic environment

(originated in Devonian).

lungfish

coelacanth

Life in the Phanerozoic Eon• The Mesozoic Era

– Angiosperm

• A flowering , or seed-enclosed,

plant

– Reptiles, birds and Mammals

• One branch of amphibians

evolved into reptiles, the first

fully terrestrial animal

– Amniotic eggs

• Dinosaurs and birds appeared in

the Jurassic period

– 96% of all species became

extinct at the end of the

Paleozoic (the great dying).

Archaeopteryx-early

bird.

Life in the Phanerozoic Eon

First mammals; cretaceous.

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Life in the Phanerozoic Eon

• Mass extinction

– A catastrophic episode in

which a large fraction of

living species become

extinct within a

geologically short time

– Most famous mass

extinction occurred 65

million years ago.

• An estimated 70% of all

species died out.

• The Cenozoic Era

– The departure of the dinosaurs gave

mammals a chance to grow and

diversify

– Mammals also benefitted from

high oxygen levels in the

atmosphere, larger brain sizes

continued to evolve

– The human family evolved

• Australopithecus (walked upright)

3.9-3.0 my

• Homo Erectus 1.8 my-300,00 y

• Homo Neanderthalensis 230,000-

30,000

• Homo sapiens

Life in the Phanerozoic Eon

Life in the Phanerozoic Eon

• Mass extinction

– A catastrophic episode in

which a large fraction of

living species become

extinct within a

geologically short time

– Most famous mass

extinction occurred 65

million years ago.

• An estimated 70% of all

species died out.

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Life in the Phanerozoic Eon

Life in the Phanerozoic Eon