BIO II Chapter 15

What were the conditions on early Earth that made the origin of life possible?

• The first atmosphere was probably thick with water vapor and various compounds released by volcanic eruptions, including nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide

• Lightning, volcanic activity, and ultraviolet radiation were much more intense than today.

What is the earliest age when life began on Earth?

• Fossil evidence demonstrates there are 3.5-billion-year-old fossils of stromatolites built by ancient photosynthetic prokaryotes still alive today

• Because these 3.5-billion-year-old prokaryotes used photosynthesis, it suggests that life first evolved earlier, perhaps as much as 3.9 billion years ago.

How did the first living organisms evolve?

• The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nitrogenous bases

• The joining of these small molecules into polymers, such as proteins and nucleic acids

• The packaging of these molecules into “protocells,” droplets with membranes that maintained an internal chemistry different from that of their surroundings.

• The origin of self-replicating molecules that eventually made inheritance possible.

Is there any evidence for these stages?

• -In the 1920s, two scientists, the Russian A. I. Oparin and the British J. B. S. Haldane, independently proposed that organic molecules could have formed on the early Earth

• -In 1953, graduate student Stanley Miller, working under Harold Urey, tested the Oparin-Haldane hypothesis

• Miller set up an airtight apparatus with gases circulating past an electrical discharge, to simulate conditions on the early Earth.

• After a week, Miller’s setup produced abundant amino acids and other organic molecules

Are there other mechanisms that could explain life forming?

• Deep sea environments near submerged volcanoes or hydrothermal vents

• Meteorites as sources of amino acids and other key organic molecules.

How is geologic time organized?

• The Archaean (4 billion years)

• Proterozoic eons lasted (4 billion years)

• The Phanerozoic eon includes the last half billion years.

When did life first evolve?

• Prokaryotes lived alone on Earth for 1.5 billion years, from 3.5 to 2 billion years ago.

• During this time, prokaryotes transformed the atmosphere. Prokaryotic photosynthesis produced oxygen that enriched the water and atmosphere of Earth. Anaerobic and aerobic cellular respiration allowed prokaryotes to flourish.

• Eukaryotes appeared about 2.1 billion years ago.

• The common ancestor of all multicellular eukaryotes lived about 1.5 billion years ago.

When did life first evolve?-continued

• The oldest fossils of multicellular eukaryotes are about 1.2 billion years old.

• The first multicellular plants and fungi began to colonize land about 500 million years ago.

• Humans diverged from other primates about 6 to 7 million years ago.

• Our species, Homo sapiens, originated about 195,000 years ago.

How can fossils be analyzed for their age?

• The age of a fossil can also be inferred from the ages of rock layers above and below the strata in which a fossil is found

• .

Do any eons exhibit major changes in organism diversity?

• The most recent Phanerozoic eon includes the past 542 million years and is divided into three eras

• Paleozoic, Mesozoic, and Cenozoic

• The boundaries between eras are marked by mass extinctions