Biology Lecture 2: Earth's History and the Origin of Life
Earth's History and Formation
Formation of the Solar System: Began approximately 4.6 billion years ago from a massive, rotating cloud of dust and gas, called a protoplanetary disk.
Composition: Primarily hydrogen and helium (from the Big Bang), with heavier elements from earlier supernovae.
Trigger: A nearby supernova explosion sent shockwaves into the disk, causing parts to collapse under gravity.
Sun's Formation: Concentrated mass in the center heated up, forming the Sun through nuclear fusion of hydrogen into helium.
Protoplanet Formation: Other parts of the disk clumped together due to gravity, forming protoplanets, including Earth.
Earth's Specifics: Located approximately 150 million km from the Sun.
Completion of Formation: Roughly 10-20 million years after the solar system began, rounded to about 4.6 billion years ago.
Early State: Initially a rocky planet but not ready for life; it was a hellish, molten environment.
Geological Eras of Earth
Divisions of Earth's History: Scientists divide time into four major eras, which are uneven in length:
Precambrian
Paleozoic
Mesozoic
Cenozoic
Precambrian Super Eon: Covers the vast majority of Earth's history, over 85\% of its timeline.
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Early Atmosphere: Likely hydrogen and helium, which was blown away by solar wind and heat.
Surface Formation: Surface cooled enough to form a solid crust by about 4.4-4.2 billion years ago.
Atmosphere and Ocean Formation: Between 4.2 and 3.8 billion years ago, heavy asteroid impacts cracked the crust, releasing steam and volcanic gases.
This formed a new atmosphere: methane, ammonia, water vapor, carbon dioxide, and nitrogen (still no free oxygen).
This led to clouds and rain, filling basins and forming oceans (some studies suggest oceans formed as early as 4.2 billion years ago).
Environmental Impact: Lack of an ozone layer meant intense UV radiation, making the surface harsh but chemically reactive—ideal for complex molecules.
Origin of Life: Surprisingly, life began in this extreme environment. Fossil and chemical evidence suggests life was present by 3.8 billion years ago, as soon as conditions allowed.
Characteristics: Earliest life forms were unicellular, and all life remained single-celled for the next 3 billion years.
Archean Eon
Early Life Evidence: Fossils show stromatolites, layered mats formed by cyanobacteria (among the first known life forms, still existing today).
Prokaryotic Domains: Fossils also include both bacteria and archaea, indicating the two major domains of prokaryotic life were already established.
Proterozoic Eon
Multicellular Life Emergence: Around 1 billion years ago, toward the end of the Precambrian, multicellular life appeared.
Updated Timeline: Previously thought to be only in the Cambrian period, but soft-bodied fossils dating back to the late Precambrian have been discovered.
Classification Challenges: These organisms are difficult to classify; some may be early ancestors of modern animals, others extinct lineages.
Evidence of Movement: Trace fossils show burrowing activity, indicating some could move.
Transition: Marked a gradual transition to more complex life.
Paleozoic Era (543-250 million years ago)
Beginning: Marked by the appearance of hard-shelled animals (e.g., trilobites) and an explosion in the fossil record.
Key Groups: Archaeocyathids (reef builders, possibly related to sponges or corals).
Cambrian Explosion: A relatively fast (60-million-year) diversification of animal forms that filled the oceans. New fossil discoveries suggest it was more gradual than previously thought.
Terrestrial Life: Around 450 million years ago, the first land plants (like simple mosses) appeared.
Vertebrates (e.g., fish) evolved legs and moved onto land around 420 million years ago.
Land supported amphibians and then reptiles, leading to more terrestrial ecosystems.
End of Era: The Permian extinction, or "The Great Dying."
Impact: Approximately 96\% of marine life and 70\% of land vertebrates went extinct.
Uniqueness: The only known mass extinction of insects, which are typically resilient.
Severity: The worst extinction event in Earth's history, with current biodiversity loss raising concerns about a potential repeat.
Mesozoic Era (250-65 million years ago) - "Age of Reptiles"
Life's Rebound: Life rebounded after the Permian extinction.
Dominant Species: Dinosaurs appeared about 230 million years ago and dominated the land.
Mammal Emergence: First mammals evolved around 200 million years ago (e.g., Morganucodon, a small, weasel-like creature, an important ancestor of modern mammals).
End of Era: The K-T extinction event, about 65 million years ago.
Impact: Wiped out non-avian dinosaurs.
Evidence: Left a visible K-T boundary in rock layers worldwide.
Cause: Likely a massive asteroid impact and/or volcanic activity, which disrupted ecosystems, blocked sunlight, and collapsed food chains.
Cenozoic Era (Beginning 65 million years ago to present) - "Age of Mammals"
Dominant Species: Mammals rose to prominence.
Human Emergence: Humans arrived only about 2 million years ago, a tiny blip in Earth's grand timeline of life.
The Origin of Life: A Fundamental Question
The Transition: Approximately 3.8 billion years ago, Earth transformed from a lifeless planet of rocks, water, and gases to one inhabited by life.
Common Ancestor: Scientists believe all living things today (from bacteria to humans) descended from a single, simple common ancestor from that time.
Core Question: How did life begin? (Created on Earth or from elsewhere?)
Two Main Ideas for Life's Origin:
Abiogenesis on Earth: Life started on Earth through natural chemical processes.
Panspermia: Life started elsewhere in the universe and arrived on Earth (e.g., via an asteroid).
Note: Panspermia explains where life might have come from, but not how it originated.
Scientific Focus: Most scientists focus on abiogenesis on Earth, as it helps understand the specific steps of life's origin.
Defining Life for its Origin
Broad Definition: Life comes in many forms, so scientists use a very broad definition for the first life forms.
Two Main Characteristics: For something to be considered alive, it must be:
Organized
Able to replicate itself (make copies)
Other Features: Characteristics like having cells, growing, or responding to the environment apply to more complex life, but not necessarily the very first life.
Early Earth Conditions for Life's Origin
Surface: Molten rock, too hot for life on land; early life had to exist in the oceans.
Atmosphere: No oxygen gas.
Contents: Methane, ammonia, water vapor, carbon dioxide, and carbon monoxide.
Nature: A reducing atmosphere, which promotes molecules bonding together.
Contrast: Our modern oxygen-rich atmosphere is oxidizing, tending to break molecules apart.
Energy Input: No ozone layer meant intense exposure to the Sun's harmful UV rays, providing energy that triggered many chemical reactions.
The Four Main Steps for the Origin of Life (Abiogenesis)
Step 1: Abiotic Synthesis of Organic Molecules
Concept: Molecules like amino acids and nucleotides (the building blocks of proteins and genetic material) formed naturally without life.
Miller-Urey Experiment (1953):
Methodology: Stanley Miller and Harold Urey recreated early Earth conditions in a lab.
They combined gases thought to be in the early atmosphere (e.g., methane, ammonia) and added electrical sparks (simulating lightning).
Results: After a week, amino acids and other organic molecules had formed.
Modern Revisions: Later research showed their gas mixture wasn't exactly right (early atmosphere likely more carbon dioxide and nitrogen).
Updated Experiments: Still produce the same kinds of molecules (sugars, lipids, nucleotides, ATP, amino acids), confirming the concept of abiotic synthesis of building blocks.
Step 2: Formation of Polymers
Requirement: Small organic molecules (monomers) needed to join into long chains (polymers) to be useful for life (e.g., amino acids $\rightarrow$ proteins, nucleotides $\rightarrow$ RNA/DNA).
The Proximity Problem: Monomers were scattered in the ocean and needed close contact to bond.
Proposed Solution: Clay particles at the ocean bottom.
Mechanism: Clay contains positively charged ions (e.g., iron, zinc) that attract negatively charged organic molecules, bringing them close together.
Outcome: Allowed polymers to form naturally.
Step 3: Creation of Protocells
Concept: Simple, cell-like structures made from lipids.
Lipid Properties: Lipids (fats) naturally form spherical bubbles in water with hydrophilic heads on the outside and hydrophobic tails on the inside.
Liposome Formation: Enough lipids come together to form a lipid bilayer, creating a hollow sphere called a liposome.
Solving the Proximity Problem: These bubbles could trap molecules inside, enclosing them and concentrating them, rather than being scattered in the ocean.
Characteristics: Protocells were organized (meeting one key life criterion), but could not yet replicate themselves.
Step 4: Origin of Hereditary Material
Initial Genetic Material: Scientists believe the first genetic material was RNA, not DNA.
Why RNA?
Much easier to form naturally (abiotically) than DNA.
Can replicate itself without enzymes in some cases (especially with zinc ions, found in clay).
Can carry genetic information.
Can perform chemical reactions (act as enzymes).
RNA World Hypothesis: The idea that early life was based on RNA.
Evidence from Viroids: Modern molecules that support the RNA world hypothesis.
Description: Tiny pieces of RNA without a protein coat, infecting plants and causing disease.
Size: Extremely small; some contain only 250 nucleotides (compared to 3.3 billion in human DNA).
Replication: Cannot replicate on their own; they hijack the host's replication machinery.
Significance: Not fully alive, but they are organized RNA that almost replicates—very similar to what early life may have looked like.
The Transition to Life
Convergence of Steps: When these four steps (building blocks $\rightarrow$ polymers $\rightarrow$ protocells $\rightarrow$ self-replicating RNA) converge, molecules become both organized and self-replicating.
Defining Moment: At this point, scientists can say that life has begun.
Remaining Mystery: The exact moment where everything came together—the transition from non-living chemistry to living biology—is still unknown.
Challenges: Very difficult to recreate or observe directly.
Timeline: Likely took millions of years of slow chemical changes and reactions.
Scientific Understanding: Despite the unknown final step, the scientific community has a very strong understanding of the process that likely led to the origin of life.