Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 26

Origins of Life Cell is the basic unit of life Today all cells come from pre-existing cells 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 2

3 Fundamental Properties of Life Cellular organization Sensitivity Growth Development Reproduction Regulation Homeostasis Heredity

Panspermia –Earth may have been “infected” with life from some other planet –Meteor or cosmic dust may have carried complex organic molecules to earth –Kicked off evolution of life Frozen water found on Mars 4

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 5

In 1953, Miller and Urey did an experiment that 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 6

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Found within a week that methane gas (CH 4 ) 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 –Adenine also produced 8

RNA may have been first genetic material 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 9

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, Carolus Linnaeus instituted the use of two-part names, or binomials –Apis mellifera the European honeybee –Genus name capitalized, all in italics 10

Taxonomy is the science of classifying living things –A classification level is called a taxon Scientific names avoid the confusion caused by common names 11

6-kingdom system –4 eukaryotic kingdoms Plantae Fungi Animalia Protista – did not fit into 3 other kingdoms –2 prokaryotic kingdoms Archaea Bacteria 12 Each fundamentally different

3 domain system –Domain Archaea –Domain Bacteria –Domain Eukarya 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Domain Bacteria (Bacteria) Domain Archaea (Archaebacteria) Domain Eukarya (Eukaryotes) Common Ancestor

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 14

Archaea Shared characteristics –Cell walls lack peptidoglycan (found in bacteria) –Membrane lipids are different from all other organisms Divided into three general categories –Methanogens –Extremophiles –Nonextreme archaea 15

Methanogens –Use H 2 to reduce CO 2 to CH 4 –Strict anaerobes that live in swamps and guts Extremophiles –Thermophiles – High temperatures (60–80ºC) –Halophiles – High salt –Acidophiles – Low pH (pH = 0.7) Nonextreme archaea –Grow in same environments as bacteria –Nanoarchaeum equitens – Smallest cellular genome 16

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 17

Mitochondria and chloroplasts most likely gained entry by endosymbiosis Mitochondria were derived from purple nonsulfur bacteria Chloroplasts from cyanobacteria 18

Key Eukaryotic Characteristics Compartmentalization –Allows for increased subcellular specialization –Nuclear membrane allows for additional levels of control of transcription and translation Multicellularity –Allows for differentiation of cells into tissues Sexual reproduction –Allows for greater genetic diversity 19

Viruses Are literally “parasitic” chemicals –DNA or RNA wrapped in protein Cannot reproduce on their own Not considered alive – cannot be placed in a kingdom 20

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. T4 bacteriophage Ebola virus 100 nm Vaccinia virus (cowpox) Influenza virus Tobacco mosaic virus (TMV) Herpes simplex virus HIV-1 (AIDS) Rhinovirus (common cold) Adenovirus (respiratory virus) Poliovirus (polio)

Making Sense of the Protists Represents tension between traditional classification and use of evolutionary relationships Eukaryotes diverged rapidly as atmosphere shift from anaerobic to aerobic –May never be able to sort out relationships during this time Protist is a catchall for eukaryotes that are not plant, fungus, or animal 22

Land plants arose from an ancestral green alga only once during evolution 23

Sorting Out the Animals Origins of segmentation –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 24

Division based on embryonic development –Protostomes develop the mouth before the anus in embryonic development Annelids and arthropods among others –Deuterostomes develop the anus first Chordates including humans Protostomes divided further into –Lophotrochozoans Flatworms, mollusks, and annelids –Ecdysozoans Roundworms and arthropods 25

26 Segmentation is regulated by the Hox gene family –Hox ancestral genes already present in ancestor to all groups –Members were co-opted at least three times

27 Within the arthropods, insects have traditionally been separated from the crustaceans – Uniramous vs. biramous appendages However, molecular data is questioning this classification – Distal-less, a Hox gene, initiates development of both types of appendages

The Mammalian Family Tree Over 90% of mammals are eutherians or placental mammals –Now divided into four major groups First major split occurred 100 MYA when Africa split from South America 28