12-2 Complex Organisms Developed

I. Prokaryotes Are the Oldest Group of Organisms

A. Fossils show that the earliest prokaryotes had evolved by 3.5 billion years ago

B. Prokaryotes are divided into two major groups based on cellular characteristics

• Archaebacteria are thought to be related to the earliest life on Earth

     1. lack peptidoglycan in their cell walls
     2. have unique lipids in the cell membranes
     3. DNA contains introns
     4. ribosomes resemble eukaryotic ribosomes
     5. 1st eukaryotes are thought to have evolved from archaebacteria 

• Eubacteria include a huge variety, including consumers, like Escherichia coli, and photosynthesizers, like cyanobacteria

     1. contain peptidoglycan in their cell walls
     2. have lipid membranes like eukaryotes
     3. DNA lacks introns
     4. ribosomes differ from eukaryotes

II. The first eukaryotes appear in the fossil record by 1.5 billion years ago

A. eukaryotes differ from prokaryotes in three key ways

• DNA is inside of a nucleus
• organelles sepearate the cytoplasm into compartments
• mitochondria provide ATP

B. evidence shows that mitochondria and chloroplasts evolved by endosymbiosis from eubacteria

• mitochondria are about the same size and struture as most eubacteria
• mitochondria and chloroplasts have circular DNA and lack introns, similar to eubacteria
• mitochondrial and chloroplast ribosomes resemble those of eubacteria
• mitochondria and chloroplasts divide by binary fission, as do eubacteria

III. Multicellularity evolved many times

A. eukaryotic organisms are divided into 4 kingdoms: protists, fungi, plants and animals

• unicellular bacteria, protists and fungi make up 1/2 of the biomass of life
• unicellular organisms are limited to simpler lifestyles

B. multicellularity allows for cellular specialization, division of labor, and thus greater complexity and size

• the major groups of complex multicellular animals evolved during the Cambrian Period, 540 and 505 million years ago
• during the Ordovician Period, 505 to 438 million years ago, animals continued to diversify, including trilobites

IV. Mass extinctions have had a major impact on the history of life on Earth

A. five major (and many minor) mass extinctions are recorded in the fossil record

• 438 million years ago, at the end of the Ordovician Period
• 360 million years ago, at the end of the Devonian Period
• 245 million years ago, at the end of the Permian Period, killing 96% of all species
• 205 million years ago, at the end of the Triassic Period
• 65 million years ago, at the end of the Cretaceous Period, killing of 65% of terrestrial animals, including dinosaurs

B. a sixth mass extinction is happening now, as humans destroy habitat, especially tropical rainforests

12.3 Lecture Notes