Lesson 24: Earth's History and the Diversification of Life

Life on Earth displays immense diversity while sharing fundamental characteristics: all living organisms are comprised of at least one cell, transfer energy for function and growth, and encode hereditary information in their DNA. Despite these similarities, varying environments have driven the evolution of unique features, leading to a vast array of species.

1. Geological Timescale and Dating Methods

To understand early life, scientists use the geological timescale, which marks major intervals of Earth's history. This scale is divided hierarchically:

  • Eons: The largest spans of time.

  • Eras: Divisions within eons.

  • Periods: Further divisions within eras.

  • Epochs and Ages: Smaller units, often used for more recent events or specific spans of "Deep Time."

Time is typically discussed in billions or millions of years ago (bya/mya). Deep Time refers to the immense scale of geological time, with common reference points being the first appearance of organisms in the fossil record, continental configurations, and climate changes.

1.1 Dating Techniques

  • Relative Dating: Historically, scientists used the position of fossils in strata (rock layers). Layers closer to the surface are more recent. This provided insight into the order of evolution but not absolute numerical dates.

  • Isotope Dating (Absolute Dating): This technique relies on the constant decay rates of specific atoms.

    • Carbon-14: Used for materials less than 50 to 60 thousand years old.

    • Potassium: Suitable for rocks and fossils older than 50 thousand years due to its slower decay rate.

    • Half-Life: The time it takes for half of the original parent isotopes to transform into daughter isotopes. After each successive half-life, half of the remaining parent isotopes decay.

2. History of Life on Earth
2.1 The Precambrian

The Hadean, Archean, and Proterozoic eons are collectively known as the Precambrian, preceding the major diversification of multicellular life.

  • First Evidence of Life: Prokaryote microfossils dating back to 3.5 billion years ago (Archean Eon). It's important to state that life existed at least 3.5 bya, as earlier evidence may yet be undiscovered.

  • Eukaryotes: Reliably identified eukaryote fossils appear around 1.5 billion years ago (Proterozoic Eon).

  • Multicellular Organisms: Emerge very late in the Proterozoic.

2.2 The Phanerozoic Eon

This eon encompasses all time after the Precambrian and is divided into smaller units due to the rapid diversification of life and global changes.

  • Cambrian Period: Marks the start of the Phanerozoic and is characterized by the Cambrian Explosion, an extraordinary diversification of multicellular organisms confined to the oceans.

2.3 Contextualizing Deep Time

Visualizing deep time can be challenging:

  • Earth formed: approx. 4.6 billion years ago.

  • Oldest prokaryote fossils: 3.5 billion years ago.

  • Multicellular organisms: existed for only about 12\% of Earth's history.

  • Birds and mammals: existed for about 4\% of Earth's history.

  • Humans: existed for about 0.2\% of Earth's history.

3. Origin of Life and Early Evolution

The exact origin of early organic molecules and organisms remains unknown, with several hypotheses:

3.1 Hypotheses for the Origin of Life

  • Terrestrial Origin: Life originated on Earth from simple compounds exposed to lightning, forming complex molecules like amino acids.

  • Extraterrestrial Origin: Early life or organic molecules arrived on Earth via meteorites, supported by the discovery of organic molecules in some meteorites.

3.2 The Miller-Urey Experiment

Conducted in 1953, this experiment simulated early Earth's reducing atmosphere with water and electrical sparks (lightning). It successfully produced organic compounds such as formaldehyde, hydrogen cyanide, formic acid, urea, and amino acids (glycine, alanine), demonstrating that key molecules for life could have formed spontaneously.

3.3 Emergence of Metabolic Pathways

  • Autotrophic vs. Heterotrophic: Early life forms may have been autotrophic (synthesizing complex molecules from inorganic ones) or heterotrophic (acquiring molecules directly from the environment).

  • RNA World Hypothesis: Suggests RNA was the first molecule to catalyze peptide synthesis and store genetic information. RNA can catalyze reactions and form in the absence of pure ribose, with clay and ice crystals facilitating bonding.

3.4 Single Cells and Membranes

Primitive life forms were single cells. Membranes confine molecules, increasing the probability of metabolic reactions. Early membranes may have been simpler, composed of fatty acids rather than the modern phospholipid bilayer.

3.5 Key Eukaryotic Developments

Several innovations contributed to eukaryotic success and diversity:

  • Compartmentalized Cells: Enabled larger size and specialized functions.

  • Endomembrane System: Divided the cell into functional compartments.

  • Nuclear Membrane: A unique eukaryotic feature, not found in bacteria or archaea.

  • Endosymbiosis: The process by which eukaryotes acquired energy-producing organelles like mitochondria and chloroplasts.

  • Multicellularity: Arose multiple times, facilitating diversification.

  • Cellular Communication: Evolved to coordinate gene expression and cell-to-cell interaction.

  • Sexual Reproduction: A eukaryotic characteristic that significantly increased genetic diversity.

4. Evidence for Early Life

  • Carbon Fixation: Living organisms preferentially incorporate carbon-12 (C12) into their cells. Analyzing carbon isotope ratios in rocks and fossils can indicate biological activity. Evidence of carbon fixation is found in rocks as old as 3.8 billion years.

  • Biomarkers: Organic molecules (e.g., hydrocarbons) whose carbon isotope ratios can confirm a biological origin.

  • Microfossils: The microfossil dated to 3.5 billion years ago is often cited as the first fossil evidence of life, though its identification is debated due to its small size, simplicity, and potential for non-biological mimics. The earliest definitive fossil evidence of life is dated to 3.2 billion years ago.

  • Stromatolites: Structures created by mats of bacteria trapping mineral deposits, serving as another indicator of biological activity, often pointing to very hot environments.

5. Earth's Changing Systems and Their Impact on Evolution

Earth's systems have profoundly shaped evolution through climate changes, atmospheric shifts, continental drift, and the influence of life forms themselves.

5.1 Early Earth Climate and Atmosphere

Early Earth featured a very hot climate with high levels of carbon dioxide. Cooling occurred as CO_2 was removed from the atmosphere:

  • CO_2 combined with water to form carbonic acid.

  • Carbonic acid broke down rocks, releasing HCO_3 and calcium.

  • Calcium washed into the ocean, forming calcium carbonate, which sequestered CO_2 as it precipitated.

  • Decreased CO_2 led to less absorbed radiant energy and atmospheric cooling.

Earth's temperature has varied drastically, from 2000^ extrm{o}C to a mean of -50^ extrm{o}C, with periods of glaciation decimating life.

5.2 Continental Drift

The Earth's crust is made of shifting plates (plate tectonics). Over geological time, continents have merged into supercontinents (e.g., Rodinia, Gondwana, Pangea). This drift led to reproductive isolation and divergent evolution, evidenced by the distribution of similar fossils on presently separated continents.

6. Colonization of Land and Oxygen Atmosphere
6.1 The Move to Land

Following the Cambrian explosion, the colonization of land was a critical step. Terrestrial environments offered new niches but presented challenges:

  • Preventing desiccation (drying out).

  • Efficient gas exchange.

  • Developing structural durability without the buoyancy of water.

6.2 The Oxygen-Rich Atmosphere

Photosynthesis gradually transformed Earth's atmosphere. Initially, oxygen released by photosynthesis precipitated as iron oxide in oceans. Once this capacity was exhausted, free oxygen accumulated, leading to the formation of the ozone layer, which protects life from UV radiation.

Plants also contributed to glaciation events by removing CO_2 from the atmosphere:

  • The first major glaciation occurred after initial land colonization by plants.

  • The second coincided with vascular plant diversification.

7. Organizing Life: Taxonomy

To study life's history and evolution, organisms are grouped into three monophyletic domains: Bacteria, Archaea, and Eukaryotes.

Life is organized hierarchically:

  1. Domain

  2. Kingdom

  3. Phylum

  4. Class

  5. Order

  6. Family

  7. Genus

  8. Species

Species are given binomial names (genus and species), providing a standard for identification. Each rank becomes more specific, with organisms at lower ranks being more closely related.

8. Human Impact: The Anthropocene

The Earth continues to change due to natural forces, but human activities now exert significant influence on biodiversity, climate, and the introduction of novel radioactive materials. Many argue that our impact warrants defining a new geological epoch, the Anthropocene, though debate over its precise criteria continues.