Transcript EO4 tsw4
THE ORIGIN OF LIFE By Abby Wilson TSW 4 In the beginning, there was methane, and there was ammonia, and there was no free oxygen … WHAT DO WE KNOW? Living organisms are incredibly diverse * ~1.5 million species identified so far * Many more remain unidentified 1.Primordial soup • Life on Earth began more than 3 billion years ago, evolving from the most basic of microbes into a dazzling array of complexity over time. But how did the first organisms on the only known home to life in the universe develop from the primordial soup? 2. Electric Spark • Electric sparks can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia and hydrogen, as was shown in the famous Miller-Urey experiment reported in 1953, suggesting that lightning might have helped create the key building blocks of life on Earth in its early days. Over millions of years, larger and more complex molecules could form. Although research since then has revealed the early atmosphere of Earth was actually hydrogen-poor, scientists have suggested thatvolcanic clouds in the early atmosphere might have held methane, ammonia and hydrogen and been filled with lightning as well. 3. Community Clay • The first molecules of life might have met on clay, according to an idea elaborated by organic chemist Alexander Graham Cairns-Smith at the University of Glasgow in Scotland. These surfaces might not only have concentrated these organic compounds together, but also helped organize them into patterns much like our genes do now. WHAT DO WE KNOW? • All living organisms share common ancestry – Populations of organisms change through time – Change (evolution) may be slow or relatively rapid – Given life, evolution is inevitable 4. Deep-Sea Vents • The deep-sea vent theory suggests that life may have begun at submarine hydrothermal vents, spewing key hydrogen-rich molecules. Their rocky nooks could then have concentrated these molecules together and provided mineral catalysts for critical reactions. Even now, these vents, rich in chemical and thermal energy, sustain vibrant ecosystems. 5. Chilly Start • Ice might have covered the oceans 3 billion years ago, as the sun was about a third less luminous than it is now. This layer of ice, possibly hundreds of feet thick, might have protected fragile organic compounds in the water below from ultraviolet light and destruction from cosmic impacts. The cold might have also helped these molecules to survive longer, allowing key reactions to happen. 6. RNA World • Nowadays DNA needs proteins in order to form, and proteins require DNA to form, so how could these have formed without each other? The answer may be RNA, which can store information like DNA, serve as an enzyme like proteins, and help create both DNA and proteins. Later DNA and proteins succeeded this "RNA world," because they are more efficient. RNA still exists and performs several functions in organisms, including acting as an on-off switch for some genes. The question still remains how RNA got here in the first place. And while some scientists think the molecule could have spontaneously arisen on Earth, others say that was very unlikely to have happened. 7. Simple Beginnings • Instead of developing from complex molecules such as RNA, life might have begun with smaller molecules interacting with each other in cycles of reactions. These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved, scenarios dubbed "metabolism-first" models, as opposed to the "gene-first" model of the "RNA world" hypothesis. HISTORY OF THE UNIVERSE • 12 – 15 billion years ago – “Time zero” – Everything compressed into volume of sun • Incredibly dense, incredibly hot – Big bang • Matter and energy very rapidly distributed throughout • • • universe Temperatures dropped Fusion reactions created light elements Resulting background radiation is still detectable HISTORY OF THE UNIVERSE • First billion post-big bang years – – – – – – – Gaseous particles collide, condense under force of gravity First stars are formed As stars grew, nuclear reactions ignited Heat and light liberated Heavier elements formed Explosive deaths of stars released these heavy elements Released elements incorporated into newly forming stars and orbiting planets – Still heavier elements formed • New star formation currently visible in dust clouds of Orion, etc. ORIGIN OF THE EARTH • Contracting cloud formed our solar system – H2, H2O, Fe, Silicates, HCN, NH3, CH4, H2CO, and other small inorganic and organic molecules present • Planets formed ~ 4.6 – 4.5 billion years ago • Earth was hot – Asteroid impacts, internal compression, radioactive decay of minerals – Much of rocky interior melted – Many heavier elements moved toward interior – Lighter elements floated toward surface EARTH • Crust – Surface zone – Basalt, granite, and other low-density rocks • Mantle – Interior to crust – Intermediate-density rocks • Core – High-density, partially molten nickel and iron EARTH • Earth 4 billion years ago – Thin-crusted inferno • Earth ~3.8 billion years ago – Life arose, but how did this happen? EARLY ATMOSPHERE • 1950s: Stanley Miller & Harold Urey recreated the assumed early atmosphere – Contained H2O, H2, CH4, NH3 – Lacked free O2 – Energy input in forms of heat and electrical sparks • Mimic geothermal heat and lightning ORIGIN OF LIFE • Can we identify the physical and chemical conditions that prevailed on the Earth when life originated? • Do the known principles of physics, chemistry, and evolution support or disprove the hypothesis that organic molecules formed spontaneously and evolved into molecular systems with the fundamental properties of life? • Can we design experiments to test the hypothesis that living systems emerged through chemical evolution? ORIGIN OF LIFE • Unfortunately, our understanding of the origin of life is incomplete • Many laboratory experiments lend support to an abiotic origin of life through chemical evolution EARLIEST LIFE • Life arose ~3.8 billion years ago • The earliest cells were prokaryotic – Lack a membrane-bound nucleus • Early in the history of life, populations diverged into two major lineages – bacteria – archaea & eukaryotes EARLIEST LIFE • How do we know that domain Eukarya is more closely related to domain Archaea than to domain Bacteria? – Analysis of rRNAs and other highly conserved genes and proteins provide the strongest evidence TIMELINE