The Tree of Life Chapter 26

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 from these early waters Life may have infected Earth from some other planet -This hypothesis is termed Panspermia

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

Conditions on Early Earth First organisms emerged between 3.8 and 2.5 BYA 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

Evolution of Cells RNA may have been first genetic material Amino acids polymerized into proteins Metabolic pathways emerged Lipid bubbles became living cells with cell membranes Several innovations contributed to diversity of life -Eukaryotic cells -Sexual reproduction -Multicellularity

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 -Homo sapiens

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

The Linnaean Hierarchy Taxa are based on shared characteristics -Domain (most shared) -Kingdom -Phylum -Class -Order -Family -Genus -Species (least shared)

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

Grouping Organisms Carl Woese proposed a six-kingdom system Prokaryotes Eukaryotes

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

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

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

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

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

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 most likely gained entry by endosymbiosis -Mitochondria were derived from purple nonsulfur bacteria -Chloroplasts from cyanobacteria

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

Key Eukaryotic Characteristics Compartmentalization -Allows for increased subcellular specialization Multicellularity -Allows for differentiation of cells into tissues Sexual reproduction -Allows for greater genetic diversity

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

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

A new kingdom, Viridiplantae, has been suggested -Plants + green algae

Origin of Plants Land plants arose from an ancestral green alga, and only once during evolution Green alga consist of 2 monophyletic groups -Chlorophyta -Streptophyta Streptophyta is composed of seven clades, including land plants -Land plants and Charales are sister clades

Some land plants show evidence of horizontal gene transfer The flowering plant Amborella acquired three moss genes Close contact increases the probability of HGT

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

Segmentation is regulated by the Hox gene family -Members were co-opted three times

-Uniramous vs. biramous appendages 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 Continues to emerge based on molecular data The majority 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

The Mammalian Family Tree Afrotheria South American lineage

The Mammalian Family Tree Understanding evolutionary relationships among organisms accomplishes two things -Provides an orderly and logical way to name organisms -Provides insights in understanding the history of major features and functions