The Origin and Diversity of Life

Biodiversity

• ~1.5 Million species identified• Many more remain to be identified

Possible origin(s) of life on earth?• Extraterrestrial aliens brought it• Came with meteors from other planets• Divine creator• Chemicals from primordial soup combined • None of the above

Review of what have we learned

• Cell theory–All living organisms are made of cells, and all living

cells come from other living cells.• Molecular basis of inheritance

–DNA encodes genes which make-up and control living organisms.

• Evolutionary change–Life-forms have evolved varying characteristics to

adapt to varied environments.• Evolutionary conservation

–Some characteristics of earlier organisms are preserved and passed on to future generations.

Fundamental Properties of Life

• Cellular organization• Sensitivity• Growth• Development • Reproduction• Regulation• Homeostasis• Heredity• Evolution

Where did the first cell come from?

• Or….Can life arise from non-life?• Can we test this scientifically?

–Conditions on early earth–Formation of organic molecules–Chemical evolution–Primitive cell–Prokaryotes–Eukaryotes–Multicellular organisms

Origins of Life

• The 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

–Ocean’s edge, hydrothermal deep-sea vents, or elsewhere

Conditions on Early Earth

• Seems likely that Earth’s first organisms emerged and lived at very high temperatures

• First organisms emerged between 3.8 and 2.5 BYA• Early atmosphere composition not agreed on

–May have been a reducing atmosphere –Would have made it easier to form carbon-rich

molecules

• Reproduced early atmosphere–Assembled reducing atmosphere rich in hydrogen

with no oxygen gas–Atmosphere placed over liquid water–Temperature below 100ºC–Simulate lightning with sparks

Miller and Urey Experiment

• Found within a week that methane gas (CH4) converted into other simple carbon compounds –Compounds combined to form simple molecules and

then more complex molecules• Later experiments produced more than 30 carbon

compounds including amino acids and nucleotides

What Was Found?

Chemical Evolution

• If life originally arose from non-life, how might this have happened?

• Consider the following scenario• Synthesis and accumulation of small organic

molecules• Joining of these monomers into polymers• Aggregation of these molecules into droplets to

form localized microenvironments• Origin of heredity

• Sidney Fox (University of Miami) demonstrated the abiotic polymerization of organic monomers

• Polymers were formed when dilute solutions of organic molecules were dripped onto hot sand, clay, or rock• “Proteinoids”

• Clay can serve to concentrate these molecules• Monomers bind to charged sites on clay particles

• Metal ions in clay have catalytic function

Polymer Formation

A. DNA

B. RNA

C. Protein

DNA RNA PROTEIN

(Information) (Information and Catalytic) (Catalytic and Structural)

Which came first?

Most Likely

• RNA may have been first genetic material–Ribozyme activity

• Amino acids polymerized into proteins• Metabolic pathways emerged

–Primitive organisms may have been autotrophic – built what they needed

• Lipid bubbles could increase the probability of metabolic reactions–Leads to cell membranes

• Other innovations contributed to diversity of life

Probable Scenario

Classification of OrganismsClassification 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, Carolus Linnaeus instituted the use of two-part names, or binomials–Apis mellifera the European honeybee–Genus name capitalized, all in italics

• Taxonomy is the science of classifying living things–A classification level is called a taxon

• Scientific names avoid the confusion caused by common names

Taxonomy

The Linnaean Hierarchy

• Taxa are based on shared characteristics–Domain → → → Species

• Early taxonomists not aware of distinction between derived and ancestral traits–Many hierarchies now being re-examined

• Categories at the different levels may include many, a few, or only one taxon

• Limitations–Many higher ranks are not monophyletic–Linnaean ranks not equivalent in any meaningful

way

• 3 domain system–Domain Archaea–Domain Bacteria –Domain Eukarya

DomainBacteria

(Bacteria)

DomainArchaea

(Archaebacteria)

DomainEukarya

(Eukaryotes)

Common Ancestor

Grouping Organisms OrganismsCarl Woese proposed a 6-kingdom system• 4 eukaryotic kingdoms

• Plantae• Fungi• Animalia• Protista – did not fit into 3 other kingdoms

–2 prokaryotic kingdoms• Archaea• Bacteria

• Tree based on rRNA analysis• Archaea and Eukarya are more closely related to

each other than to bacteria

Each fundamentally different

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Prokaryotes Eukaryotes

Bacteria

• Most abundant organisms on Earth• Key roles in biosphere

–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

Archaea

• Shared characteristics–Cell walls lack peptidoglycan (found in bacteria)–Membrane lipids are different from all other

organisms–Distinct rRNA sequences

• Divided into three general categories–Methanogens–Extremophiles–Nonextreme archaea

• Methanogens

–Use H2 to reduce CO2 to CH4

–Strict anaerobes that live in swamps and guts• Extremophiles

–Thermophiles – High temperatures (60–80ºC)–Halophiles – High salt–Acidophiles – Low pH (pH = 0.7)

Archaea

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

Mitochondria and Chloroplasts

• Mitochondria and chloroplasts most likely gained entry by endosymbiosis

• Mitochondria were derived from purple nonsulfur bacteria

• Chloroplasts from cyanobacteria

Archaebacteria Animalia Fungi Protista Plantae Bacteria

Brownalgae

Photosyntheticprotists

Thermophiles

Halophiles

Methanogens

Ancestraleukaryotic cell

Purplebacteria

Photosyntheticbacteria

Otherbacteria

Chloroplasts

Mitochondria

Redalgae

Greenalgae

Nonphotosyntheticprotists

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VirusesViruses

• Are literally “parasitic” chemicals–DNA or RNA wrapped in protein

• Cannot reproduce on their own• Not considered alive – cannot be placed in a kingdom• Viewed as detached fragments of a genome• Tobacco mosaic virus (TMV) first discovered in 1933