The Tree of Life

Chapter 26

2

Origins of LifeThe Earth formed as a hot mass of molten

rock about 4.5 billion years ago (BYA)

-As it cooled, chemically-rich oceans were formed from water condensation

Life arose spontaneously from these early waters

Life may have infected Earth from some other planet

-This hypothesis is termed Panspermia

3

Fundamental Properties of Life

Cellular organization

Sensitivity

Growth

Development

Reproduction

Regulation

Homeostasis

Heredity

Earthforms

Oldestfossils

Photo-synthesisevolves

Eukaryoticcellsevolve

Multi-cellularorganisms

Abundantlife

Origin of Life

Figure 1.3 Life’s Calendar

Earthforms

Oldestfossils

Photo-synthesisevolves

Eukary-oticcells

Multi-cellular

Abundantlife

Aquatic life

Abundant fossils

First land plants

First land animals

ForestsInsectsFirst mammalsDinosaurs dominant

BirdsFlowering plantsRise of Mammals

Origin of Life

Figure 1.3 Life’s Calendar

First hominids

Homo sapiens

Modern humansappeared in thelast 10 minutes

of day 30.

Earthforms

Oldestfossils

Photo-synthesisevolves

Multi-cellular

Abundantlife

Origin of Life

Eukary-oticcells

Recorded historyfills the

last 5 secondsof day 30.

Aquatic life

Abundant fossils

Land plants

Land animals

ForestsInsectsFirst mammalsDinosaurs dominant

BirdsFlowering plantsRise of Mammals

Figure 1.3 Life’s Calendar

7

Conditions on Early Earth

First organisms emerged about 3.8 BYA (some suggest even earlier)

Early atmosphere had CO2, N2, H2O and H2

-Reducing atmosphere

In 1953, Miller and Urey did an experiment that reproduced this early atmosphere

-Used electrodes to simulate lightning

-Small organic molecules were generated in their apparatus

8

12

Evolution of CellsRNA may have been first genetic materialAmino acids polymerized into proteinsMetabolic pathways emergedLipid bubbles became living cells with cell

membranesSeveral innovations contributed to diversity

of life-Eukaryotic cells-Sexual reproduction-Multicellularity

14

15

Classification of Organisms

More than 2000 years ago, Aristotle divided living things into animals and plants

Later, basic units were called genera

-Felis (cats) and Equus (horses)

In the 1750s, Karl von Linne (aka. Carolus Linnaeus) instituted the use of two-part names, or binomials

-Homo sapiens

16

Classification of Organisms

Taxonomy is the science of classifying living things

-A classification level is called a taxon

Scientific names avoid the confusion caused by common names

17

The Linnaean Hierarchy Taxa are based on shared characteristics

-Domain (most shared)

-Kingdom

-Phylum

-Class

-Order

-Family

-Genus

-Species (least shared)

18

19

20

Limitations of the Hierarchy

Many hierarchies are being re-examined based on the results of molecular analysis

-Linnaean taxonomy does not take into account evolutionary relationships

-Linnaean ranks are not equivalent

-Legume family (16,000 species)

-Cat family (36 species)

The phylogenetic and systematic revolution is underway

21

Grouping OrganismsCarl Woese proposed a six-kingdom system

Prokaryotes Eukaryotes

22

Grouping Organisms

Biologists are increasingly adopting a three-domain phylogeny based on rRNA studies

-Domain Archaea

-Domain Bacteria

-Domain Eukarya

Each of these domains forms a clade

Archaea and Eukarya are more closely related to each other than to bacteria

23

During evolution, microbes swapped genetic information via horizontal gene transfer (HGT)

24

Bacteria

Most abundant organisms on Earth

Extract nitrogen from the air, and recycle carbon and sulfur

Perform much of the world’s photosynthesis

Responsible for many forms of disease

Highly diverse

Most taxonomists recognize 12-15 different groups

25

Archaea

Prokaryotes that are more closely related to eukaryotes

Characteristics

-Cell walls lack peptidoglycan

-Membrane lipids are branched

-Distinct rRNA sequences

Divided into three main groups

26

Methanogens

-Use H2 to reduce CO2 to CH4

-Strict anaerobes that live in swamps

Extremophiles

-Thermophiles – High temperatures

-Halophiles – High salt

-Acidophiles – Low pH

Nonextreme archaea

-Grow in same environments as bacteria

-Nanoarchaeum equitans – Smallest cellular genome

27

28

Eukarya

Prokaryotes ruled the earth for at least one billion years

Eukaryotes appeared about 2.5 BYA

Their structure and function allowed multicellular life to evolve

Eukaryotes have a complex cell organization

-Extensive endomembrane system divides the cell into functional compartments

29

Mitochondria and chloroplasts most likely gained entry by endosymbiosis

-Mitochondria were derived from purple nonsulfur bacteria

-Chloroplasts from cyanobacteria

30

The Four Eukaryotic Kingdoms

Protista

-Unicellular with few multicellular organisms

-Not monophyletic

Fungi

Plantae

Animalia

-Largely multicellular organisms

-Each is a distinct evolutionary line derived from a unicellular protist

31

32

Key Eukaryotic Characteristics

Compartmentalization

-Allows for increased subcellular specialization

Multicellularity

-Allows for differentiation of cells into tissues

Sexual reproduction

-Allows for greater genetic diversity

33

34

35

Viruses

Are not organisms and so cannot be placed in any of the kingdoms

Are literally “parasitic” chemicals

-DNA or RNA wrapped in protein

Can only reproduce within living cells

Vary greatly in appearance and size

36

Making Sense of the Protists

Protists are a paraphyletic group

-Catchall for eukaryotes that are not plant, fungus or animal

Divided into six groups

-However, at least 60 protists do not fit into any of these groups

37

A new kingdom, Viridiplantae, has been suggested

-Plants + green algae

38

Sorting Out the Animals

Molecular systematics is leading to a revision of evolutionary relationships among animals

Segmentation has been used in the past to group arthropods and annelids close together

-rRNA sequences now suggest that these two groups are distantly related

Segmentation likely evolved independently in these two groups, as well as in chordates

39

Segmentation is regulated by the Hox gene family

-Members were co-opted three times