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Geological events that alter environments

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change the course of biological evolution; conversely life changes the planet that it inhabits

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Most biologists now think that it is at least a credible hypothesis

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that chemical and physical processes on early Earth produced very simple cells through a sequence of stages

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There were four main stages in the evolution of life (hypothesis)

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Abiotic synthesis of small organic molecules Joining of these molecules into macromolecules Packaging of molecules into protocells Origin of self replicating molecules

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Earth formed about 4.6 billion years ago

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along with the rest of the solar system; the first life evolved as early as billion years ago.

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Earth's early atmosphere

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contained water vapor and many chemicals releases by volcanic eruptions Oparin and Haldane hypothesis earths early atmosphere was reducing (didn't have any oxygen, only nitrogen, hydrogen, sulfuric acid etc) and oceans were a solution of organic molecules

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Laboratory experiments simulating an early Earth atmosphere- Miller and Urey

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It showed that organic molecules (like amino acids ) could be produced from inorganic molecules but the existence of such an atmosphere on early Earth is unlikely

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Some of the organic compounds from which the first life on earth arose

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was from outer space carbon compounds found on meteorites

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The possibility that life

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is not restricted to earth is becoming more accessible to scientific testing

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Small organic molecules

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polymerize when they are concentrated on hot sand, clay or rock

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Protocells

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Replication and metabolism are key properties of life an may have appeared together in protocells Protocells may have formed from vesicles with a membrane like structure In water lipids and other organic molecules cam spontaneously form vesicles with a lipid bilayer Adding clay can increase the rate of vesicle formation Vesicles exhibit simple reproduction and metabolism and maintain internal chemical environment

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The first genetic material

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was probably RNA not DNA

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WHhy RNA and not DNA?

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called ribozymes have been found to catalyze many differenct reactions including self splicing making complementary copies of short streteches of their own sequences or other short pieces of RNA

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How RNA developed

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ONce

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the main explanation for the lack of continuing abiotic origin of life on Earth today is that

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our oxidizing atmosphere is not conducive to the spontaneous formation of complex molecules

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One can view the chronology of major episodes that shaped life as a

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phylogenetic tree

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The key events

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1-1st cell 2- photosynthesis 3- aerobic respiration 4- eukaryotes 5- particularity 6- Cambrian explosion (535-525 million years ago) 7- Invasion of land

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The analogy of a clock

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can be used to place major events in the earth's history in the context of the geological record

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Eons

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Hadean (origin of earth) Archaean (Oldest fossils of cells (prokaryotes) Appear) Proterozoic (Oldest fossils of eukaryotes appear) Phanerozoic:

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Eras

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Paleozoic- The Cambrian Explosion; first tetrapods and insects appear, Mesozoic- origins of mammals; dinosaurs evolved and radiate. (Amphibians, reptiles), Cenozoic- Humans appear; major radiation of pollution insects (Mammals and angiosperms)

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The oldest known fossils are stromatolites,

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rocks formed by the accumulation of sedimentary layers on bacterial mats

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Stromatolites -date back

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3.5 billions years ago

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Prokaryotes were

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earth's sole inhabitants for more than 1.5 billion years

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Electron transport system of a variety of types

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were essential to early life Have some aspects that possibly precede life itself

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The earliest types of photosynthesis

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did not produce oxygen

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Oxygenic photosynthesis

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probably evolved around 3.5 billion years ago in cyanobacteria (oxygen revolution)

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When oxygen began to accumulate in the atmospheres about 2.7 billion years ago

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it posed a challenge for life

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The oldest fossils of eukaryotes cells

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date back 2.1 billion years

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The theory of endosymbiosis

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Proposed that mitochondria and plastids were formerly small prokayotrs living within larger host cells probably gained entry to the host cells as undigested prey or internal parasites

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Key evidence supporting an endosymbiotic origin of mitochondria and plastids

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Inner membranes are similar to plasma membranes of prokaryotes Division and DNA structure is similar in these organelles and some proykaryotes Theses organelles transcribe and translate their own DNA Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes

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Molecular clocks

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date the common ancestor of multicellular eukaryotes to 1.5 billion years

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The oldest known fossils of eukaryotes

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are of relatively small algae that lived about 1.2 billion years ago

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Larger organisms do not appear in the fossil record

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until several hundred million years later Chinese paleontologists recently described 570 million year old fossils that are probably animal embryos

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Most of the major phyla of animals

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appear suddenly in the fossil record that was laid down the first 20 million years it the Cambrian period

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The fossil record

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provides a glimpse into he evolution of life on Earth. It shows that there have been remarkable changes in the kinds of organisms that have been abundant at different times

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How do body characteristics affect the likelihood of fossilization? `

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If an organism has a soft body, it is less likely to be fossilized; a hardshell is not crushed easily and does not decompose quickly

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How does habitat location affect the likelihood of fossilisation

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Organisms that live in an area with a low oxygen content are more likely to fossilize, organisms that live near a beach are more likely to fossilize; organisms that live in an area where sediments are actively being deposited are more likely to fossilize

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The oldest fossils usually

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are found in the deepest strata

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Radiometric dating

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allows us to indirectly date fossils up to billion of years old based on minerals in surrounding volcanic strata

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The geologic record

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evidence in the fossil record of large scaled extinction and or enormous radiation of many diverse new life forms defines the main boundaries of the geologic record

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Precambrian Time

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Began 4600 million years ago; the first prokaryotic cells and cyanobacteria appeared as well as the first animals

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Paleozoic Era (Cambrian Period)

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Began 542 million years ago; the first plants, jawless fish, bony fish. Amphibians became dominant vertebrate life form and the first reptiles and seed plants appeared . By the end many marine and terrestrial organisms became extinct.

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Mesozoic Era

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251-65 million years ago ; reptiles radited out as dinosaurs to dominate the landscape, the first birds and mammals originated. flowering plants appeared and many organisms including most dinosaurs became extinct.

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Cenozoic Era

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Followed the mesozoic and continues through modern times. Angiosperms evolved to dominate the landscape; primate groups evolved leading to the origin of apelike human ancestors and finally humans.

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The atmosphere of early earth probably contained no oxygen until the emergence of organisms that

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used water as an electron source from photosynthesis; oxygen began to accumulate in the atmosphere due to the action of photosynthetic cyanobacteria

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Adaptive radiations involved

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the splitting of a single lineage into multiple species each adapted to a specific set of ecological demands

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Convergent evolution

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occurs when selective pressures produced by similar conditions (such as habitats) favor the evolution of similar solution

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Hox genes are thought to play an important role int he development of different morphologies because

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they provide positional information in the embryo

Chapter 25: The History of life on Earth – Flashcards

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