The History of Life

1. The History of Life Chapter 14.2

2. Spontaneous Generation • What does it mean for something to be spontaneous? • It used to be thought that living things could arise from nonliving things. • This idea was called spontaneous generation.

3. Experiments disproving spontaneous generation • Redi—1668 • He used a cover on one flask, mesh on another, and no cover on the last one. • He found that maggots didn’t appear on the meat that was covered and free of flies. • Therefore, S.G. was not true.

4. Experiments disproving spontaneous generation • Spallanzani—1700s • Microorganisms were able to be seen with a microscope, were thought to arise from a “vital force” in the air. • He boiled two flasks of broth, one was left open, one was sealed. • The open flask became filled with microorganisms, the sealed one remained clear. • The debate? Broth was exposed to air.

5. Experiments disproving spontaneous generation • Pasteur—mid 1800s • Boiled broth in curved neck flask to allow air to reach broth but not microorganisms • Broth only became cloudy when neck was broken off.

6. New Idea • These experiments proved spontaneous generation incorrect. • The idea that living things come from other living things is called biogenesis.

7. Earth’s History • Evidence from computer models of the sun shows that the early solar system was a swirling cloud of gas and dust about 5 billion years ago. • Most of this material collapsed to become the sun. • Earth formed when large pieces of debris in the early solar system collided over a period of 400 million years.

8. Earth’s History • The estimated age of Earth is 4.6 billion years, which was established using radiometric dating. • The time it takes for half of a sample on an isotope to decay is its half-life. • The age of a material can be determined by measuring the amount of a particular radioactive isotope that it contains.

9. Before life began, two things had to be true: • 1) Simple organic molecules must have formed. • 2) These molecules must have become organized into complex organic molecules (proteins, carbs, etc.)

10. Where did the Organic Compounds come from? • It’s thought that all of the elements found in organic compounds were present on Earth and in the solar system when Earth formed. • How did these organic compounds come to be? • Alexander Oparin developed a hypothesis called the primordial soup hypothesis.

11. Oparin’s Hypothesis • He proposed that the early atmosphere contained ammonia (NH3), hydrogen gas (H2), water vapor, and methane (CH4). • He thought that once the Earth cooled and the organic compounds collected in lakes & seas, energy supplied by lightning and ultraviolet (UV) radiation started complex chemical reactions that led to the formation of proteins and other organic compounds, the precursors to life.

12. Testing Oparin’s Hypothesis • Oparin didn’t test his hypothesis, but scientists Miller & Urey did. • They were able to produce organic compounds including amino acids.

13. From amino acids to… • These amino acids, when bonded to clay, may have formed the first proteins. • It is also thought that RNA was the first system for protein production • In other words, RNA was the first genetic material.

14. How did the first cells possibly form? • Sidney Fox produced protocells from solutions of amino acids by heating them. • These structures have certain life-like properties: take up substances from the surroundings, growth, surrounded by a membrane. • They have properties unlike living things: no hereditary characteristics, can’t respond to natural selection

15. The Evolution of Cells • The first forms of life may have been prokaryotic forms that evolved from a protocell. • There was little or no oxygen gas in existence, so they must have been anaerobic.

16. The Evolution of Cells • They likely took in organic compounds from their environment for food, so they were heterotrophs. • Since these molecules probably became scarce, it was necessary for autotrophs to evolve.

17. The Evolution of Cells • These were probably like present-day archaebacteria (or archaea), which are prokaryotic and live in harsh environments like deep sea vents and hot springs. Since they can’t get energy from light for photosynthesis, where do they get energy from? • Chemosynthesis – energy from inorganic compounds is used to make glucose out of carbon molecules.

18. The Evolution of Cells • Eventually, photosynthesizing prokaryotes evolved. They increased the amount of oxygen in the atmosphere. (Like modern-day cyanobacteria) • This lead to the evolution of organisms that could respire aerobically (with oxygen). • Complex eukaryotic cells were able to evolve due to protection from ultraviolet radiation by the ozone layer.

19. How did eukaryotic cells come about? • Lynn Margulis (1966) has proposed that eukaryotes arose from prokaryotes by way of a process called endosymbiosis, which may have occurred in the following manner: • 1. A small aerobic prokaryote entered a large anaerobic prokaryote (as undigested food or a parasite) and began to live and reproduce inside it. • 2. Eventually this small prokaryote became what is now the mitochondria, which are the sites of aerobic respiration. This may explain why mitochondria have their own DNA.

20. How did eukaryotic cells come about? • 3. Sometime later, a small photosynthetic prokaryote entered the large prokaryote • 4. This photosynthetic prokaryote may have given rise to the chloroplasts, the sites of photosynthesis. Chloroplasts also have their own DNA (circular, like in prokaryotes)

21. What else shows that mitochondria and chloroplasts may have been their own organisms? • They have their own ribosomes. • They reproduce (by fission) independently of the cells that they’re inside. Last point, “The evolution of life is better understood than how the first life appeared. Fossil, geologic, and biochemical evidence supports many of the proposed steps in life’s subsequent evolution.”