Chapter 25 ~ The History of Life on Earth “…sparked by just the right combination of physical events & chemical processes…”

The Origin of Life is Hypothesis Extra-terrestrial Origin – Was the original source of organic (carbon) materials comets & meteorites striking early Earth? – testable Spontaneous Abiotic Origin – Did life evolve spontaneously from inorganic molecules? – testable

Conditions on early Earth Reducing atmosphere – water vapor (H 2 O), CO 2, N 2, NO x, H 2, NH 3, CH 4, H 2 S – lots of available H & its electron – no free oxygen Energy source – lightning, UV radiation, volcanic low O 2 = organic molecules do not breakdown as quickly What’s missing from that atmosphere?

Water vapor Condensed liquid with complex, organic molecules Condenser Mixture of gases ("primitive atmosphere") Heated water ("ocean") Electrodes discharge sparks (lightning simulation) Water Origin of Organic Molecules Abiotic synthesis – 1920 Oparin & Haldane propose reducing atmosphere hypothesis – 1953 Miller & Urey test hypothesis formed organic compounds – amino acids – adenine CH 4 NH 3 H2H2

Stanley Miller University of Chicago produced -amino acids -hydrocarbons -nitrogen bases -other organics Why was this experiment important??!

The Origin of Life The 4-stage Origin of life Hypothesis: 1- Abiotic synthesis of organic monomers 2- Polymer formation 3- Origin of Self-replicating molecules 4- Molecule packaging (“protobionts”)

Key Events in Origin of Life Origin of Cells (Protobionts) – lipid bubbles  separate inside from outside  metabolism & reproduction Origin of Genetics – RNA is likely first genetic material – multiple functions: encodes information (self-replicating), enzyme, regulatory molecule, transport molecule (tRNA, mRNA) makes inheritance possible makes natural selection & evolution possible Origin of Eukaryotes – endosymbiosis

The fossil record Sedimentary rock: rock formed from sand and mud that once settled on the bottom of seas, lakes, and marshes Dating: 1- Relative~ geologic time scale; sequence of species 2- Absolute~ radiometric dating; age using half-lives of radioactive isotopes

Timeline Key events in evolutionary history of life on Earth – 3.5–4.0 bya: life originated – 2.7 bya: free O 2 = photosynthetic bacteria – 2 bya: first eukaryotes

First Eukaryotes Development of internal membranes – create internal micro-environments – advantage: specialization = increase efficiency natural selection! infolding of the plasma membrane DNA cell wall plasma membrane Prokaryotic cell Prokaryotic ancestor of eukaryotic cells Eukaryotic cell endoplasmic reticulum (ER) nuclear envelope nucleus plasma membrane ~2 bya

mitochondrion chloroplast Eukaryotic cell with chloroplast & mitochondrion Endosymbiosis photosynthetic bacterium 2 nd Endosymbiosis Evolution of eukaryotes – origin of chloroplasts – engulfed photosynthetic bacteria, but did not digest them – mutually beneficial relationship natural selection! Eukaryotic cell with mitochondrion

Evidence – structural mitochondria & chloroplasts resemble bacterial structure – genetic mitochondria & chloroplasts have their own circular DNA, like bacteria – functional mitochondria & chloroplasts move freely within the cell mitochondria & chloroplasts reproduce independently from the cell Theory of Endosymbiosis Lynn Margulis

Cambrian explosion Diversification of Animals – within 10–20 million years most of the major phyla of animals appear in fossil record 543 mya

Biogeography: the study of the past and present distribution of species Pangaea-250 mya √ Permian extinction Geographic isolation-180 mya√ African/South American reptile fossil similarities √ Australian marsupials

Mass extinction Permian (250 million years ago): 90% of marine animals; Pangea merge Cretaceous (65 million years ago): death of dinosaurs, 50% of marine species; low angle comet

5 Major Extinctions in the past 500 million years: are humans causing the 6 th ?

The Origin of New Groups of Organisms Mammals belong to the group of animals called tetrapods The evolution of unique mammalian features can be traced through gradual changes over time Tetrapods include, reptiles, birds, amphibians, mammals and other now extinct groups (dinosaurs)

Major Adaptive Radiations Evolve novel characteristic Novel characteristic opens an adaptive zone Extinction of dinosaurs opened ecological niches for mammals

Major changes in body form can result from changes in the sequences and regulation of developmental genes Genetic Mechanisms explain large changes Evo-Devo is explained by genetic changes

Molecular Evidence for Evolution Homeotic Genes (HOX genes) Determine an organisms body plan Changes in HOX genes correlate with changes in body plan Vertebrate vs. invertebrate