TEXTBOOK

• Earth was very different billions of years ago
• Most scientists agree that Earth is billions of years old
• The conditions of the early planet and its atmosphere were very different from those of today
• The solar system was formed by a condensing nebula, a cloud of gas and dust in space, about 4.6 billion years ago
• Over time, most of the material in the nebula pulled together because of gravity, forming the Sun and planets of our solar system
• Earth was violent and very hot for its first 700 million years, known as the Hadean eon
• During the Hadean eon, many asteroids, meteorites, and comets struck the planet, releasing enormous amounts of heat
• The early atmosphere contained compounds such as ammonia, water vapor, methane, and carbon dioxide
• Free oxygen was not abundant until about 2 billion years ago, after the first forms of life had begun to evolve
• Impacts on Earth became less frequent towards the end of the Hadean eon, allowing the planet to cool down
• Once liquid water was present, organic compounds could be formed from inorganic materials
• Living matter is organic, including sugars and amino acids
• The leap that resulted in life on Earth required conditions other than just the presence of water
• Several hypotheses propose how life began on Earth
• Scientists have proposed hypotheses about early organic molecules, the evolution of cell structures, and early genetic material since the 1950s
• Two general hypotheses about the appearance of life supporting molecules on early Earth are the Miller-Urey experiment and the meteorite hypothesis
• The Miller-Urey experiment demonstrated that organic compounds could be made by passing an electrical current through a mixture of gases
• Analysis of a meteorite revealed that organic molecules can be found in space, suggesting they could have been present when Earth formed or arrived through impacts
• There are hypotheses about the formation of the first cells, including the iron-sulfide bubbles hypothesis and the lipid membrane hypothesis
• The iron-sulfide bubbles hypothesis proposes that biological molecules combined in compartments of chimney-like structures, acting as the first cell membranes
• The lipid membrane hypothesis suggests that lipid molecules spontaneously formed membrane-enclosed spheres, which may have given rise to the first true cells
• A hypothesis gaining support proposes that RNA, rather than DNA, was the genetic material that stored information in living things on early Earth
• Ribozymes are RNA molecules that can catalyze specific chemical reactions, suggesting RNA's role in early genetic material
• RNA can replicate itself without additional enzymes, unlike DNA which needs enzymes for replication
• RNA can catalyze reactions and copy itself, while DNA requires enzymes for replication
• Single-celled organisms existed 3.8 billion years ago
• Microbes have changed the physical and chemical composition of Earth
• Eukaryotic cells may have evolved through endosymbiosis
• The evolution of sexual reproduction led to increased diversity
• Early single-celled organisms changed Earth's surface by depositing minerals and changed the atmosphere by giving off oxygen as a byproduct of photosynthesis
• Stromatolites are evidence of Earth's early life, made by cyanobacteria which were among the first organisms on early Earth
• Endosymbiosis is a relationship in which one organism lives within the body of another, benefiting both
• Mitochondria and chloroplasts found in eukaryotic cells descended from ancestors of infection-causing bacteria and cyanobacteria, respectively
• Sexual reproduction increases genetic variation, masking harmful mutations and bringing beneficial mutations together
• Sexual reproduction may have resulted in an increase in the rate of evolution by natural selection, allowing populations to adapt quickly to new conditions
• Recombination is an important source of genetic variation, resulting in many different phenotypes through the process of producing gametes and during crossing over in meiosis
• Humans appeared late in Earth's history
• Humans share a common ancestor with other primates
• There are many fossils of extinct hominids
• Modern humans arose nearly 200,000 years ago