Chapter 25 The History of Life on Earth Antarctica many millions of years ago Antarctica now… WOW!! • Past organisms were very different from today’s. • The fossil record shows macroevolutionary changes over large time scales including – The origin of photosynthesis – The emergence of terrestrial vertebrates – Long-term impacts of mass extinctions Prebiotic Chemical Evolution & the Origin of Life - Hypothesis: First cells originated by chemical evolution non living materials became organized into molecules; molecules were able to replicate & metabolize. - possible because atmosphere was really different; no O2, volcanoes, UV, lightning, etc. Four Main Stages of Cell Emergence: 1. small organic molecules are made abiotically 2. monomers polymers (macromolecules) 3. protocells (droplets of aggregated molecules) 4. Origin of self replicating molecules/ beginning to heredity Stage 1: Synthesis of Organic Compounds on Early Earth • Earth formed about 4.6 bya • Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide) TED Talk: The Line Between Life and Nonlife • A. I. Oparin & J. B. S. Haldane hypothesized that the early atmosphere was a reducing environment (no oxygen) • Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible = Primeval Soup Hypothesis OR… Organic compounds were created near hydrothermal vents OR… They rained down from outer space Video: Hydrothermal Vent Stage 2: Abiotic Synthesis of Macromolecules • What came first, the amino acid or the enzyme? – How would macromolecules form without enzymes/dehydration synthesis? • Dilute solutions containing monomers dripped onto hot sand, clay, or rock vaporizes water – “Proteinoids” (proteins formed abiotically) were made this way • Maybe waves splashed monomers onto hot lava? Stage 3: Protocells • Replication & metabolism are key properties of life • Protocellss are aggregates of abiotically produced molecules surrounded by a membrane or membranelike structure • Exhibit – simple reproduction – metabolism – maintain an internal chemical environment Protocells can behave similarly to a cell (osmotic swelling, membrane potential like nerve cell) 20 µm Glucose-phosphate Glucose-phosphate Phosphatase Starch Phosphate (a) Simple reproduction by liposomes (aggregates of lipids) Amylase Maltose Maltose (b) Simple metabolism Possible to contain enzyme within; catalyze RXNs, give off product Stage 4: Self-Replicating RNA and the Dawn of Natural Selection • RNA = probably the first genetic material, then DNA • Ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA • Base sequences provide blueprints for amino acid sequence (polypeptides) • Early protocells with self-replicating, catalytic RNA would have been more effective at using resources (“fitness”) & would have increased in # due to natural selection. • RNA could have provided template for DNA (more stable, better at replicating) The stage has now been set for life! Fig. 25-7 Humans Colonization of land Animals Origin of solar system and Earth 4 1 Proterozoic 2 Archaean 3 Multicellular eukaryotes Single-celled eukaryotes Atmospheric oxygen Prokaryotes Fig. 25-4 Present Rhomaleosaurus victor, a plesiosaur Dimetrodon Casts of ammonites Hallucigenia Coccosteus cuspidatus Dickinsonia costata Stromatolites Tappania, a unicellular eukaryote Fossilized stromatolite Table 25-1 Table 25-1a Table 25-1b Animation: The Geologic Record Fig 25-UN2 1 4 2 3 Prokaryotes The First Single-Celled Organisms • Oldest known fossils are stromatolites – rock-like structures composed of many layers of bacteria and sediment – Dated 3.5 billion years ago • Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago Fig. 25-4i Stromatolites 3.5 BYA Fossilized stromatolite Fig 25-UN3 1 4 2 3 Atmospheric oxygen Photosynthesis & the Oxygen Revolution • By about 2.7 bya, O2 began accumulating in the atmosphere rusting iron-rich terrestrial rocks – O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations • “Oxygen revolution” = rapid increase in O2 around 2.2 bya – Posed a challenge for life; some microbes hid out in anaerobic environments – Provided opportunity to gain energy from light – Allowed organisms to exploit new ecosystems as old ones died, opening up new niches • Source of O2 was likely bacteria similar to modern cyanobacteria – Later rapid increase attributed to evolution of eukaryotes Fig. 25-8 Fig 25-UN4 1 4 2 Singlecelled eukaryotes 3 The First Eukaryotes Plasma membrane Cytoplasm Ancestral prokaryote Endoplasmic reticulum DNA • Oldest fossils of eukaryotes go back 2.1 bya • Endosymbiosis Nucleus Nuclear envelope Aerobic heterotrophic prokaryote Photosynthetic prokaryote Mitochondrion Ancestral heterotrophic eukaryote Mitochondrion Plastid – mitochondria & plastids (chloroplasts & related organelles) were formerly small prokaryotes living within larger host cells – At first, undigested prey or internal parasites? – 2 became interdependent; host + endosymbionts became a single organism Ancestral photosynthetic eukaryote Evidence supporting endosymbiosis: – Similarities in inner membrane structures and functions between chloroplasts/mitochondr ia and prokaryotes – Organelle division is similar to prokaryotes – Organelles transcribe & translate their own DNA – Organelle ribosomes are more similar to prokaryotic ribosomes than eukaryotic ribosomes Fig. 25-4h 1.5 BYA Tappania, a unicellular eukaryote The Origin of Multicellularity • eukaryotic cells allowed for a greater range of unicellular forms • Once multicellularity evolved then… algae, plants, fungi, and animals • Ancestor appeared rougly 1.5 bya, though oldest fossil is algae dated to 1.2 bya Multicellular eukaryotes 1 4 2 3 Ediacaran biota (Proterozoic Eon) – large & more diverse soft-bodied organisms that lived from 565 to 535 mya after snowball Earth – Thaw opened up niches that allowed for speciation Fig. 25-4g 565 MYA Dickinsonia costata 2.5 cm Fig 25-UN6 Animals 1 4 2 3 The Cambrian Explosion • sudden appearance of fossils resembling modern phyla in the Cambrian period (Phanerozoic Eon, 535 to 525 mya) • first evidence of predator-prey interactions; claws, hard-shells, spikes, etc. Burgess Shale Fig. 25-4f 525 MYA Hallucigenia Fig. 25-4e Coccosteus cuspidatus 400 MYA Early Paleozoic era (Cambrian period) 542 Late Proterozoic eon Sponges 500 Arthropods Molluscs Annelids Brachiopods Chordates Echinoderms Cnidarians Millions of years ago Fig. 25-10 Fig. 25-11 (a) Two-cell stage 150 µm (b) Later stage 200 µm Fig 25-UN7 Colonization of land 1 4 2 3 The Colonization of Land • Fungi, plants, and animals began to move to land 500 mya • Plants & fungi 420 mya: adaptations to reproduce on land • Arthropods & tetrapods are the most widespread and diverse land animals – Tetrapods evolved from lobe-finned fishes around 365 million years ago – Amphibians, reptiles, then birds and mammals Fig 25-UN8 1.2 bya: First multicellular eukaryotes 2.1 bya: First eukaryotes (single-celled) 535–525 mya: Cambrian explosion (great increase in diversity of animal forms) 3.5 billion years ago (bya): First prokaryotes (single-celled) Millions of years ago (mya) 500 mya: Colonization of land by fungi, plants and animals Major Influences on Life on Earth • Continental Drift: 3 occasions of formation, then separation of supercontinents; next one will occur in roughly 250 million years. – Collision and separation of oceanic and terrestrial plates shape mountains, cause earthquakes – Pangaea (250 mya) caused drastic changes in habitats = evolution! • Mass extinctions: 5 major ones in Earth’s history – Opens up niches for future species – Usually takes 5-10 million years to return diversity to its preextinction levels • Adaptive Radiation: Periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different niches (with little competition) Adaptive Radiation – Occur after mass extinctions • Rise of mammals after Cretaceous extinction – Colonized regions (i.e. new islands) • Hawaiian Islands How can evolutionary novelties/major changes in form come about? • Evolutionary developmental biology, or evodevo, is the study of the evolution of developmental processes in multicellular organisms • Genomic information shows that minor differences in gene sequence or regulation can result in major differences in form …think fruit flies with legs instead of antennae •allometric growth Newborn 2 Evo-devo 5 Age (years) 15 Adult (a) Differential growth rates in a human Chimpanzee fetus Chimpanzee adult • Changes in rate and timing (regulation) of developmental genes is called heterochrony – Accelerated growth in bone structures (finger bones to wings in bats) or slowed growth (reduction in leg bones in whale ancestors) – Paedomorphosis: fast development of reproductive system compared to other development; leads to maintenance of juvenile features though sexually mature (phenotypic variation) Gill s Human fetus Human adult (b) Comparison of chimpanzee and human skull growth More Evo-devo Fig. 21-17 • Changes in spatial pattern of developmental genes (homeotic genes = master regulatory genes) – determine where, when, and how body segments develop – Small changes in regulatory sequences of certain genes lead to major changes in body form Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse Hox genes of the fruit fly and mouse show the same linear sequence on the chromosomes • Homeobox/Hox genes code for transcription factors that turn on developmental genes in embryos The expression of 2 Hox genes in snakes suppresses the development of legs…the same genes are expressed in chickens in the area between their limbs Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster First Hox duplication Hypothetical early vertebrates (jawless) with two Hox clusters Second Hox duplication Vertebrates (with jaws) with four Hox clusters • Change in location of two Hox genes in Crustaceans led to the conversion of swimming appendage to feeding appendage • Duplications of Hox genes in vertebrates may have influenced the evolution of vertebrates from invertebrates Even more Evo-devo Fig. 25-22 • Changes in genes and where they are expressed – Differing patterns of Hox gene expression = variation in segmentation – Suppression of leg formation in insects vs. crustaceans – Change in expression, not gene, can cause differences in form Genital Fig. 21-18 Thorax Thorax segments Abdomen Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya insect crustaceans Artemia Drosophila Ubx gene expressed in Abdomen – supressing leg formation Ubx gene expressed In main trunk – doesn’t supress legs Abdomen Brine shrimp Artemia in comparison to grasshopper Hox expression Fig. 25-23 RESULTS Test of Hypothesis A: Differences in the coding sequence of the Pitx1 gene? Result: No Test of Hypothesis B: Differences in the regulation of expression of Pitx1 ? Result: Yes Marine stickleback embryo Close-up of mouth Close-up of ventral surface The 283 amino acids of the Pitx1 protein are identical. Pitx1 is expressed in the ventral spine and mouth regions of developing marine sticklebacks but only in the mouth region of developing lake stickbacks. Lake stickleback embryo Evolutionary “Novelties” are actually just new forms arising by slight modifications of existing forms Fig. 25-24 Pigmented cells Pigmented cells (photoreceptors) Epithelium Nerve fibers (a) Patch of pigmented cells Fluid-filled cavity Epithelium Optic nerve Nerve fibers (b) Eyecup Cellular mass (lens) Pigmented layer (retina) (c) Pinhole camera-type eye Cornea Optic nerve (d) Eye with primitive lens Cornea Lens Retina Optic nerve Eye Evolution Video (e) Complex camera-type eye