Geologic Time Scale & Early Earth

• Biology by Slyvia S. Mader and Michael Windelspecht
• Chapter 18 Lecture Outline: Origin and History of Life

Origin of Life

• The last universal common ancestor (LUCA) is common to all organisms that live, and have lived, on Earth since life began
• "Life only comes from life"
• Biomolecules, are organic molecules.
• First cells arose from nonliving chemicals, inorganic substances
• Chemistry, evolutionary biology, paleontology, microbiology help scientists develop hypotheses about life's origins

Origin of Life on Earth

• Life on earth originated about 3.5–4 billion years ago
• Earth's atmosphere likely consisted of water vapor, nitrogen, and carbon dioxide
• Small amounts of hydrogen, methane, ammonia, hydrogen sulfide, and carbon monoxide were also present
• There was little free oxygen
• Earth was originally too hot for liquid water to form
• As the Earth cooled, water vapor condensed to liquid water.

Four Stages of the Origin of Life

• Stage 1: Organic monomers (amino acids, nucleotides, etc.) evolved from inorganic compounds
• Stage 2: Organic monomers joined to form organic polymers (DNA, RNA, proteins, etc).
• Stage 3: Organic polymers became enclosed in membranes to form protocells or protobionts
• Stage 4: Protobionts acquired the ability to self-replicate.

Stage 1: Evolution of Monomers

• Several hypotheses suggest how monomers evolved
• Monomers came from outer space via comets and meteorites, carrying organic chemicals
• Organic molecules could have seeded the chemical origin of life on Earth
• Bacterium-like cells could have been carried to Earth on a meteorite or comet
• Monomers came from reactions in the atmosphere i.e. the Oparin-Haldane hypothesis (early 1900s)
• Organic molecules could be formed with outside energy sources using atmospheric gases
• Monomers came from reactions at hydrothermal vents.

Miller and Urey

• Conducted an experiment in 1953 to test the Oparin-Haldane hypothesis
• Showed that gases like methane, ammonia, hydrogen, and water) can react to produce small organic molecules like amino acids and organic acids
• Strong energy sources are required
• Rainfall washed organic compounds from the atmosphere into the ocean
• Resulting in accumulation in the ocean, making it an organic soup

Stage 2: Evolution of Polymers

• In cells, monomers join to form polymers in the presence of enzymes
• Iron-Sulfur World Hypothesis: Organic molecules reacted with amino acids to form peptides with iron-nickel sulfides
• Protein-First Hypothesis assumes that protein enzymes arose before DNA
• Proteinoids are small polypeptides with catalytic properties
• When placed in water, proteinoids form microspheres (structures made of proteins with many properties of a cell)
• RNA-First Hypothesis suggests only RNA was needed to progress toward formation of first cells
• Some viruses have only RNA genes, and DNA genes would have come afterwards.

Stage 3: Evolution of Protocells

• Before the first true cell arose, there would have been a protocell or protobiont
• Protocells are defined as the hypothesized precursor to the first true cells
• A protocell would have an outer membrane and carry on energy metabolism
• Membrane-First Hypothesis says the first cell had a plasma membrane before its other parts
• If lipids are made available to microspheres, lipids become associated with microspheres, producing a lipid-protein membrane
• Lipids placed in the water form cell-sized double-layered bubbles called liposomes offering a first membranous boundary

Protocell Nutrition

• Protocells needed to carry on nutrition in order to grow
• If organic molecules formed in the atmosphere and were carried into the ocean by rain, simple organic molecules could have served as food
• Under this hypothesis, the protocell was a heterotroph
• Heterotrophs are organisms that consume preformed organic molecules
• If the protocell evolved at hydrothermal events, it could have carried out chemosynthesis
• Chemosynthesis is the synthesis of organic molecules via oxidation of inorganic compounds

Protocell and Natural Selection

• Natural selection would have favored cells that could extract energy from carbohydrates to produce ATP
• Oxygen was not available
• The protocell carried on a form of fermentation
• Glycolysis, which transforms chemical bonds into energy, took millions of years to completely evolve

Stage 4: Evolution of a Self-Replication System

• RNA-first hypothesis: the first cell would have had an RNA gene that directed protein synthesis
• Reverse transcription could have led to DNA genes
• RNA was responsible for both DNA and protein formation
• Protein synthesis was carried out according to the central dogma, with information flowing from DNA to RNA to protein
• Protein-first hypothesis: The protocell would have developed a plasma membrane and enzymes
• DNA and RNA synthesis would have been possible
• After DNA genes evolved, protein synthesis was carried according to the central dogma
• After DNA formed, the genetic code had to evolve

History of Life

• Fossils are the remains and traces of past life
• Paleontology is the study of the fossil record
• Most fossils are traces of organisms embedded in sediment
• Sediment becomes a recognizable stratum (layer) in a stratigraphic sequence
• Strata of the same age tend to contain similar fossil assemblages (index fossils) that can be used for relative dating
• This helps geologists determine relative dates of embedded fossils (relative dating)
• The geologic timescale divides the history of the Earth into eras, periods, and epochs
• Derives from accumulation of data from the age of fossils in strata across the world
• Life arose during the Precambrian time

Precambrian Time

• The Precambrian includes about 87% of the geological timescale
• Characterized by little or no atmospheric oxygen
• The lack of an ozone shield allowed UV radiation to bombard Earth
• First cells came into existence in aquatic environments
• Prokaryotes appeared about 3.5 BYA
• Cyanobacteria fossils were found in ancient stromatolites
• Photosynthetic cyanobacteria added oxygen to the atmosphere
• Aerobic bacteria proliferated in the oxygen-rich atmosphere
• Eukaryotic cells arose about 2.1 BYA and were mostly aerobic
• Contain a nucleus and other membranous organelles
• The Endosymbiotic Theory states that mitochondria were free-living aerobic prokaryotes
• Cholorplasts were also probably free-living photosynthetic prokaryotes
• A nucleated cell probably engulfed these prokaryotes to became various organelles
• Mitochondria and Cholorplasts are similar in size to bacteria
• These organelles have their own DNA and make their own proteins and divide by binary fission
• Surrounded by two membranes
• Multicellularity Arose About 1.4 BYA
• Early multicellular organisms lacked internal organs and could have absorbed nutrients from the sea
• Likely they practiced sexual reproduction
• Ediacaran invertebrates lived from about 630 to 545 MYA

The Paleozoic Era

• Began with the Cambrian period, and lasted over 300 million years
• Includes 3 major mass extinction events
• Extinction is the total disappearance of all members of a species or higher taxonomic group
• Mass extinction has disappearance of a large number of taxa occurred within a relatively short time
• Cambrian period animal evolution
• Abundance of animal fossils of the Cambrian caused by the evolution of outer skeletons
• Ancestry of all modern animals can be traced to the Cambrian period
• Invasion of Land began around 500 MYA
• Seedless vascular plants date back to the Silurian and flourished in Carboniferous period
• Arthropods were the first land animals
• Their outer skeleton and jointed appendages pre-adapted them to live on land
• Fishes first in the Ordovician period
• Amphibians first arose in the Devonian period, diversifying during the Carboniferous
• A mass extinction occurred at the end of the Permian period.

Mesozoic Era

• Triassic Period: Nonflowering seed plants were dominant
• Jurassic Period: Dinosaurs attained enormous size with small and inconspicuous mammals
• Cretaceous Period: Dinosaurs declined at the end due to mass extinction
• Mammals began an adaptive radiation and moved into habitats vacated by dinosaurs

Cenozoic Era

• Mammals continued adaptive radiation and flowering plants were already diverse
• Primate evolution began, some adapted to living in trees for food and protection
• Ancestral apes appeared during the Oligocene epoch
• Megafauna appeared during the Pleistocene epoch

Geological Factors influencing Evolution

• Continental drift positions of continents and oceans are not fixed
• Plate Tectonics Earth's crust consists of slablike tectonic plates
• Tectonic plates float on a lower, hot mantle layer
• Movements of plates result in continental drift
• Modern mammalian diversity results from isolated evolution on separate continents

Mass Extinctions of Species

• Ordovician; 444 MYA; 75% of species disappeared
• Devonian: 360 MYA; 70% of marine invertebrates disappeared -Permian: 251 MYA; 90% of species disappeared
• Triassic: 200 MYA; 60% of species disappeared.
• Cretaceous: 66 MYA; 75% of species disappeared

Explore the geologic timescale, early life, and Earth's atmosphere. Questions cover the hierarchical order of geologic divisions, the emergence of life, epoch characteristics, and the evolution of plants, primates, and hominids. Also includes questions on Earth's early atmosphere.