Chapter 20-Origin and History of Life

20.1 Origin of Life

• chemical evolution-an increase in the complexity of chemicals
• the solar system probably formed over a 10-billion-year period from aggregates of dust particles and debris, settling around 4.6 billion years ago
• the primitive atmosphere consisted mostly of water vapor (H₂O), nitrogen (N₂), carbon dioxide (CO₂), with a small amount of hydrogen (H₂) and carbon monoxide (CO)
• at first the earth was so hot that the water vapor formed thick, dense clouds that rained heavily, forming the oceans
• or, the oceans were fed by celestial comets that entered Earth's gravitational field; the ice from the icy comets became water vapor that later came down as enough rain to raise the oceans
• other comets may have carried the first organic molecules to Earth
• Aleksandr Oparin suggested that the first organic molecules could have been produced on Earth from primitive atmospheric gases in the presence of strong energy sources (heat from volcanoes and meteroties, radioactivity from isotopes in the earth's crust, powerful electric discharges in lightning, ultraviolet/solar radiation)
• ocean ridges-where molten magma wells up and adds material to the ocean floor
• amino acids polymerize abiotically when exposed to dry heat
• proteinoids-small polypeptides that have some catalytic properties
• microspheres-structures composed only of protein that have many properties of a cell
• protein-first hypothesis-DNA genes came after protein enzymes arose (Sidney Fox)
• clay was the source of energy for polymerization, and both polypeptides and RNA arose at the same time (Graham Cairns-Smith)
• RNA-first hypothesis-only RNA was needed to form cells
• protocell-a structure that has a lipid-protein membrane and carries on energy metabolism
• if lipids are made available to microspheres, they tend to become associated with microspheres producing a lipid-protein membrane
• coavercate droplets-under appropriate conditions of temperature, ionic composition and pH give rise to complex units that have a tendency to absorb and incorporate various substances from the surrounding solution
• liposomes-droplets formed by phospholipid molecules in a liquid environment
• DNA directs protein synthesis, and there is a flow of information from DNA => RNA => protein
• general view of origin of the first cells
1. there was an abiotic synthesis of small organic molecules such as amino acids and nucleotides in either the atmosphere or hydrothermal vents
2. these monomers joined together to form polymers either on land or at the vents; they could have been rna, proteins, or evolved together
3. the aggregation of polymers inside a membrane produced a protocell, which had some enzymatic properties so that it could grow; if it developed in the ocean, it was a heterotroph, and if it developed at hydrothermal vents, it was a chemoautotroph
4. once the protocell contained DNA it became a true cell; the first genes may have been RNA but later DNA became the information storage molecule of heredity

20.2 History of Life

• fossils-remains and traces of past life or any other direct evidence of past life (trails, footprints, burrows, worm casts, or even preserved droppings)
• sedimentation-the process of embedding fossils in sedimentary rock
• stratum-a recognizable layer in the stratigraphic sequence
• paleontology-science of discovering and studying the fossil record and, from it, making decisions about the history of life
• relative dating-stratum of the same age tended to contain the same fossil, so the sequence of fossils helped determine relative strata dates
• absolute dating-radioactive techniques that assign an actual date to a fossil
• geological timescale-the division of the history of the earth into eras, then periods and epochs
• the Precambrian encompasses the first two eras: Archean and Proterozoic
• during the Precambrian, the first cells (probably prokaryotes that do not have a nucleus or any membrane-bounded organelles) came into existence
• today the archea live in hot springs, very salty lakes, and airless swamps
• stromatolites-strange looking boulders that littered beaches and shallow waters whose outer surfaces are live with cyanobacteria
• cyanobactera in ancient stromatolites added oxygen into the atmosphere
• the presence of oxygen caused photosynthetic cyanobactera and aerobic bacteria to proliferate and anaerobic prokaryotes declined
• ozone shield-oxygen in the upper atmosphere that filters out UV rays of the sun
• eukayotic cells are nearly always aerobic and contain nuclei as well as membranous organelles
• endosymbiotic hypothesis-a nucleated cell engulfed certain prokaryotes, which became organelles
• flagella/cilia may have arisen by endosymbiosis
• the first multicellular forms were most likely microscopic, and later fossils of soft-bodied invertebrates were found
• extinction-total disappearance of all memebrs of a species or higher taxonomic group
• the Paleozoic era lasted over 300 million years
• mass extinctions-the disappearance of a large number of species or higher taxonomic group within an interval of just a few million years
• molecular clock-the principle that DNA differences in certain parts of the genome occur at a fixed rate and are not tied to natural selection
• the number of DNA base pairs tells how long two species have been evolving separately
• no fossil evidence occurs until the Cambrian period, as skeletons are capable of surviving forced that are apt to destroy fossils
• skeletons may have evolved during the Cambrian period to protect animals from predators
• during the Paleozoic era, algae began to grow in fresh water and then invade damp land
• mycorrhizae allowed plants to live on bare rocks, absorbing minerals and passing them to the plant, who passes carbohydrates (photosynthesis) back
• in the Silurian period, fossils of vascular plants with tissue for water transport were found
• in the Carboniferous period, club mosses, horsetails, and seed ferns flourished
• various arthropods (spiders, centipedes, mites, millipedes) proceeded insects on land, who entered during the Carboniferous period
• invertebrates have an outer skeleton and jointed appendages
• vertebrates are animals with a vertebral column that began in the early Ordovician period with jawless fishes and then fishes with jaws
• cartilaginous and ray-finned fishes appeared in the Devonian period (Age of Fishes)
• amphibians are thin-skinned vertebrates that are not fully adapted to life on land and developed during the Carboniferous period (Age of the Amphibians)
• at the end of the Carboniferous period, cold and dry weather ended the Age of Amphibians and turned Carboniferous forests into coal/fossil fuel
• plant and animal evolution continued into the Triassic period (first of the Mesozoic era)
• nonflowering seed plants became dominant in the Permian period (conifers, cycads)
• cycads are short and stout with palmlike leaves, and developed during the Jurassic period (Age of the Cycads)
• reptiles can be traced back to the Permian period
• during the Jurassic period, pterosaurs ruled the air, marine reptiles with paddlelike limbs ruled the sea, and dinosaurs ruled the land
• since dinosaurs were ectothermic, they were so large as to maintain a favorable volume-to-surface ratio for retaining heat
• the Cenozoic era consists of the Paleogene period and the Neogene period, our current time
• at the end of the Mesozoic era, other mammals began to adapt to environments vacated by the dinosaurs
• mammals are endothermic, have hair, and use mammary glands to produce milk to feed their young; they were still small at the end of the Paleocene epoch, but began to diversify during the Eocene epoch
• angiosperms (flowering plants) evolved at the end of the Mesozoic and began their adaptive radiation during the Cenozoic era
• primates are mammals that live in angiosperms; apes diversified during the Miocene epoch and gave rise to the first hominids
• the latter two epochs of the Neogene period are known as the Ice Age (humans, giant ground sloths, beavers, wolves, bison, woolly rhinoceroses, mastodons, and ammoths)

20.3 Factors that Influence Evolution

• continental drift-the continents' positions and positions of the oceans change over time (Alfred Wegener)
• the continents used to be part of Pangaea, then Gondwanaland and Laurasia, and then the continents of today
• they are still drifting
• explains why some coastlines are mirror images of each other and unique distribuation patterns of some fossils
• plate tectonics-branch of geology that studies movements of the earth's crust
• ocean ridges-seafloor spreading that occurs as molten mantle rock rises and material is added to the ocean floor
• at subduction zones, the forward edge of a moving plate sinks into the mantle and is destroyed
• two plates meet along a transform boundary where they scrape past each other
• there have been mass extinctions at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods
• Walter and Luis Alvarez proposed in 1977 that the Cretaceous extinction was due to a bolide (asteroid), as Cretaceous clay contains an abnormally high level of iridium, an element common in asteroids and meteorites
• in 1984, David Raup and John Sepkoski suggested mass extinctions have occurred every 26 million years, since then the solar system will appraoch other members of the Milky Way that could lead to a bollide
• continental drift contributed to the Ordovician extinction
• the Permian extinction could have been caused by excess carbon dioxide, which would have led to global warming and altered the pattern of vegetation
• the Triassic period extinction is attributed to a meteorite collision, as Quebec has a huge crater