1. Chapter 26
The Origin of Life

2. Origin of Life
Earth formed 4.5 billion years ago and life began a few hundred million years later
Organisms were only microscopic and unicellular for the first ¾ of evolutionary history
Stromatolites are rocks made of banded domes of sediment in which are formed the most ancient forms of life: prokaryotes dating back as far as 3.5 billion years

3. Major Episodes in the History of Life
the production of oxygen by early photosynthetic prokaryotes created an aerobic atmosphere; aerobic life evolved
oldest eukaryotic fossils are about 1.7 billion years old
eukaryotic cells probably evolved from a symbiotic community of prokaryotes
plants, fungi, and animals came from unicellular eukaryotes during the Precambrian period
plants evolved from green algae

4. Major Episodes in the History of Life
fungi and animals evolved from heterotrophic unicellular organisms
the oldest animal fossils were from about 700 million years ago
life on Earth was aquatic for 90% of its existence
when plants colonized land, they changed the landscape and created more opportunities for life on land

5. Prebiotic Chemical Evolution
the first organisms were products of a chemical evolution in four stages:
abiotic synthesis and accumulation of small organic molecules
the joining of these monomers into polymers
the aggregation of abiotically produced molecules into droplets called protobionts
the origin of heredity

6. Abiotic Synthesis of Organic Monomers
The conditions of early Earth favored the chemical reactions that synthesized organic compounds from inorganic materials.
Primitive atmosphere consisted of reducing (electron adding) gases, instead of oxygen-rich atmosphere, enhancing the creation of complex molecules.
Lightning and UV radiation from the sun probably provided the energy needed to catalyze the reactions.

7. Abiotic Synthesis of Organic Monomers
Stanley Miller and Harold Urey recreated early Earth conditions in a lab and have synthesized several organic compounds including all 20 amino acids, lipids, several sugars, and the bases found in DNA and RNA

8. Production of Organic Polymers
Dilute solutions of organic monomers dripped onto hot sand, clay, or rocks causes polymerization.
Clay was especially important because monomers bind to charged sites on the clay particles
Pyrite, also called fool’s good, could have been the substratum or organic synthesis because it has a charged surface and it yields electrons that support bonding between organic materials

9. Protobionts
Living cells may have been preceded by protobionts, aggregates of abiotically produced molecules.
These protobiont droplets show some characteristics of life: metabolism and excitability.