Transcript notes
Tree of Life 9 September 2016 Other Worlds Earth Extremes: possible analogies to other planets • Hotsprings • The deep sea • Hypersaline environments • Evaporites • The atmosphere • Ice, permafrost, snow • Subsurface environments Other Worlds Extremes on other planets • If we’ve seen life thrive in extreme circumstances on Earth, why not on other planets? • Mars holds most promise • What about moons in our solar system: – Europa – Titan – Enceladus Other Worlds Europa • Life exists w/o photosynthesis in the deep ocean • Europa has a subsurface ocean • Life may exist beneath the surface Other Worlds Titan • Airborne micro-organisms? • Extremes to withstand: – Dessication – Radiation Other Worlds Mars • Host to several extreme environments – Deserts – Ice, permafrost, snow – Subsurface Other Worlds Mars: ice, permafrost, snow • Microbes and algae exist in frozen environments on Earth • Maybe not thriving, but microbial survivors could exist Other Worlds Mars: subsurface environments • Best chance of withstanding Martian extremes – – – – No liquid water at surface Low pressure CO2-rich atmosphere Only 43% solar radiation at Earth • Subsurface provides – Protection – Possible liquid water – Energy source for chemolithoautotrophs Summary • Earth life arose more than 3.5 billion years ago. Our oldest ancestors may have been extremophiles • Life has become progressively larger and more complicated, but large organisms are recent: 650 million years • Extremophiles live at high and low T, P, pH; high salinity and radiation • Earth’s extreme environments The 3 Domains of Life Origins of Earth life • Early Earth had abiotic environment with non-living organic chemicals • First life known had DNA, RNA, enzymes and proteins • All uses same gentic code Key events in life history • Microbial evolution: first cells were anerobic and chemotrophic • Natural selection leads to rapid differentiation: mutations cause the tree of life to branch out • Photosynthesis uses solar energy and produces oxygen • O2 increases: crisis! Some evolved, others went extinct, some went to anerobic niches, eukarya develop Eukarya • Cell structures develop and specialize • Diverse forms, but all life was microbial until 1.2 billion years ago • Multicellular organisms lead to the Cambrian explosion Tree of Life: All life is related • Evolutionary branches determined by DNA comparisons. • Bacteria seem most ancient; archaea closer to eukarya. The Cambrian Explosion • Macroscopic life diversity “exploded” ~540 Myr ago: – Oxygen levels had risen: A good energy source! – But O2 attacks organic bonds: many microbes went extinct! – Soon after several Snowball Earths: coincidence or trigger? Cambrian sea Trilobite fossil Colonization of the Land • For macroscopic life, protective ozone (O3) layer was vital step to inhabiting land: – Shields the surface from dangerous ultraviolet sunlight. • Life generated O2 (through photosynthesis), which forms O3, which protects life! Impacts and extinctions • Played a key role • Ended species like the dinosaurs • A giant impact leads to a mass extinction • We can see craters on the Moon (no erosion) and Chicxulub crater in Yucatan • Pruned branches of the tree of life and allowed new species Origin of Life: Summary 1. 2. 3. 4. 5. “Organic soup” vs. dilute solution. Complex organics developed (mineral templates?). “Pre-cells” enclosed complex organics. Natural selection increased RNA complexity. DNA developed within some successful cell(s). A reasonable scenario, though many details are missing! Summary • Earth life arose more than 3.5 billion years ago. Our oldest ancestors may have been extremophiles • Life has become progressively larger and more complicated, but large organisms are recent: 650 million years • 3 domains of life: Bacteria, Archaea, Eukarya • All Earth life has a common ancestor