Origin of Life on Earth Chapter 25 Earth originated about 4.6 billion years ago. Cloud dust rocks, water vapor. Settled by 3.9 billion years ago (bya) First atmosphere was reducing. Water vapor, nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide How did life originate? Abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nucleotides Joining of these small molecules into macromolecules including proteins and nucleic acids Packaging of these molecules into “protobionts”, droplets with membranes that maintained an internal chemistry different from that of the surroundings The origin of self replicating molecules that eventually made inheritance possible. Chemistry, geology, physics – evidence of origin of life. 1953 Urey Miller experiment (fig 4.2, 25.2). Abiotic synthesis of organic molecules possible. By dripping solutions of amino acids onto hot sand, clay or rock, researchers have been able to produce amino acid polymers. Protobionts – abiotically produced molecules surrounded by a membrane – like structure – (fig 25.3) Glucose-phosphate 20 mm Glucose-phosphate Phosphorylase Starch Amylase Phosphate Maltose Maltose Simple reproduction Simple metabolism RNA was the first genetic material. Also carry out number of enzyme-like functions, can form variety of shapes and can replicate – Protobiont with RNA (limited genetic information) increased in number. RNA formed base for DNA template. DNA world – diverse life forms. How do we know about the history of life? Fossils accumulated in sedimentary rocks called strata. Bear evidence of macroevolution – major evolutionary events over large span of time, like photosynthesis, mass extinctions etc. Incomplete but substantial record of evolutionary changes. Radiometric dating Ratio of parent isotope to daughter isotope Accumulating “daughter” isotope 1 2 Remaining “parent” isotope 1 1 4 1 2 Time (half-lives) 3 8 1 4 16 Geological record: 3 eons – Archean, Proteozoic, Phanerozoic Phanerozoic has 3 eras marked by mass extinction events – Paleozoic, Mesozoic and Cenozoic. Can also use the analogy of a clock Cenozoic Humans Land plants Animals Key events in evolution of life: First single-celled organisms First eukaryotes Multicellular Origin of eukaryotes multicellularity: Single-celled Colonization of land eukaryotes Origin of solar system and Earth 1 4 Proterozoic Eon Archaean Eon Billions of years ago 2 3 Prokaryotes Atmospheric oxygen First single-celled organisms – 3.5 bya 1 Billions of 4 years ago 2 3 Prokaryotes 4 1 Billions of years ago 2 3 Atmospheric oxygen First single-celled organisms – 3.5 bya. Stromatolites (prokaryotes binding film of sediments) cyanobacteria oxygenated the atmosphere. 2.7 bya O2 accumulated in the water and then in the atmosphere – evidence rusting of rocks Cytoplasm DNA Plasma membrane Ancestral prokaryote Infolding of plasma membrane First eukaryotes: 2.1 bya – complex organization, organelles serial endosymbiosis Endoplasmic reticulum Nuclear envelope Nucleus Engulfing of aerobic heterotrophic prokaryote Cell with nucleus and endomembrane system Mitochondrion Mitochondrion Ancestral heterotrophic eukaryote Engulfing of photosynthetic prokaryote in some cells Plastid Ancestral photosynthetic eukaryote . Evidence of endosymbiosis: In mitochondria and chloroplasts Replication similar to prokaryotes Single circular DNA molecule DNA not associated with histone proteins Ribosomes show similar sensitivity to antibiotics like bacteria Ribosomes similar in size to those of bacteria. 1 4 of years ago Billions Singlecelled eukaryotes 2 3 Origin of multicellularrity: 1.5 bya small algae and other primitive eukaryotes. Several ice ages for 30my Cambrian explosion – 535 – 525 mya – major diversification – bigger organisms, adaptations for hunting and defense 10 mm Origin of multicellularity 1 Multicellular eukaryotes Billions of years ago 2 3 4 Animals 1 4 Billions of years ago 2 3 Colonization of land 500 mya extensive land adaptations tetrapods 365 mya our human species 195,000 thousand years ago. Cenozoic 0 Mesozoic 135 251 Paleozoic Continental drift: Movement of continental plates over time. By the end of the Mesozoic, Laurasia and Gondwana separated into the present-day continents. 65.5 Millions of years ago By about 10 million years ago, Earth’s youngest major mountain range, the Himalayas, formed as a result of India’s collision with Eurasia during the Cenozoic. The continents continue to drift today. By the mid-Mesozoic Pangaea split into northern (Laurasia) and southern (Gondwana) landmasses. At the end of the Paleozoic, all of Earth’s landmasses were joined in the supercontinent Pangaea. Rearrange geography – dramatic effects on life. All land – drained shallow coastal areas; vast interiors Supercontinents break – once connected populations become geographically isolated Mass extinctions ; thriving communities disappear; five events - most famous Cretaceous mass extinction 65.5 million years ago. (6th on the way?) Adaptive radiation – explosion of diversity, species occupying all niches – large number of species; e.g. mammals originated about 180mya but worldwide adaptive radiation 65.5mya.