Where Did Life Come From?

Early thoughts about where life came from: For a long time, it seemed as if life just appeared. As far back as Aristotle (4 B.C.) people believed in spontaneous generation, the idea that non-living objects could give life to living organisms.

Spontaneous Generation Beetles in cow dungMaggots found on meat Mice in grain Recipe for Bees (Roman poet, around 20 A.D.) 1. Kill a bull during the first thaw of winter. 2. Build a shed. 3. Place the dead bull on branches and herbs inside the shed. 4. Wait for summer. The decaying body of the bull will produce bees. Recipe for Bees (Roman poet, around 20 A.D.) 1. Kill a bull during the first thaw of winter. 2. Build a shed. 3. Place the dead bull on branches and herbs inside the shed. 4. Wait for summer. The decaying body of the bull will produce bees.

Francesco Redi, 1668 Maggots often seen on rotting meat Logical conclusion at the time: maggots must come from meat Redi set up an experiment: Conclusion: Maggots came from flies, not meat. Spontaneous generation did not occur.

Invention of Microscope Anton van Leeuwenhoek, 1674 Allowed scientists to see living things that they couldn’t see before

Lazzaro Spallanzani, 1765 Boiled 2 flasks of broth – One open to the air: microorganisms grew – One sealed: no microorganisms grew Conclusion: living things came from the air. Supported Redi’s hypothesis, however, many thought there was a “life force” in the air.

Louis Pasteur, 1864 Designed a flask with a curved neck – Allows air in – Does not allow microorganisms in Boiled broth Stayed clear! Conclusion: Living things only come from other living things. Theory of Biogenesis.

How did the first life form arise? We don’t know, but we know the conditions of the early earth Formation of Earth Swirling gas and dust Some dust collapsed and formed the sun and planets Meteorites hit earth and created liquid lava As liquid settled, heavy elements sunk toward core, lighter ones floated on top. Light gases formed the atmosphere. As the earth cooled, water condensed to form ocean 3.8 billion years ago. Earliest rocks with fossil formed 3.5 billion years ago. ircamera.as.arizona.edu

Conditions of the early Earth Anaerobic Contained ammonia, methane, hydrogen gas, and water High temperatures, above 100 degrees C Amino acids formed and collected in the water as the Earth cooled mwsu-bio101.ning.com

Miller and Urey’s Experiment We can make predictions about how the first organic molecules arose. Stanley Miller and Harold Urey conducted experiments in the 1950s that suggest how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on the early earth. Carl Sagan describes Miller and Urey’s experiment in-the-cosmic-fugue in-the-cosmic-fugue (47:30-51:35)

Proteinoid Microspheres Large organic molecules can sometimes form these tiny bubbles Not cells, but have some characteristics of cells, like membranes, and means of storing energy. Some hypothesis suggest that structures like these may have been precursors to living cells (image courtesy of Cornell University)

Early life forms Fossils show that single- celled prokaryotes lived 3.5 billion years ago, in the absence of oxygen Photosynthetic bacteria lived 2.2 billion years ago What does photosynthesis produce?

Oxygen! How did the living organisms respond to the rise of oxygen in the atmosphere? Some went extinct Some found new places to live Some developed ways to use oxygen and protect themselves from oxygen’s reactive nature…

Life as we know it: Eukaryotes Cells with a nucleus and other organelles which enable them to perform more complex functions. How did they get here?

The Endosymbiotic Theory Eukaryotic cells arose from associations formed between prokaryotic organisms. Certain prokaryotic cells became ingested by another cell and formed a symbiotic relationship which is not harmful to either organism. The interior prokaryotes were mitochondria and chloroplasts. This discovery was made about 100 years ago when scientists observed that the membranes of these organelles looked like bacterial membranes.