History of Earth & Human Society: Biodiversity and Climate Trends

Origins of Animal Life and the Ediacaran Period

• Timeline of Early Life: Life on Earth is estimated to have originated more than $3.5\,\text{billion years ago}$.

• Microbial Dominance: For the vast majority of Earth’s history, life consisted exclusively of microorganisms.

• First Animal Fossils: Macroscopic fossils of organisms categorized as animals first appeared in the rock record approximately $565\,\text{million years ago (MYA)}$, specifically in the Ediacaran period.

• Characteristics of Ediacaran Fossils: These early animals possessed simple body shapes that do not clearly correspond or align with the body plans of modern animals.

• The Question of Timing: A central evolutionary question remains: Why did animals arrive so late in the overall history of life on Earth?

The Cambrian Explosion: Morphological Innovation and Diversification

• Timeline: The Cambrian Explosion occurred between $541\,\text{MYA}$ and $520\,\text{MYA}$.

• Rapid Diversification: This period was marked by an "explosion" of aquatic life diversity; at this time, there were no land plants or land animals.

• Appearance of Modern Phyla: Most modern body plans emerged during this window, including:     - Bilaterians: Animals with bilateral symmetry.     - Early Deuterostomes: Includes the ancestors of modern echinoderms.     - Arthropods and Mollusks.

• Key Biological and Morphological Innovations:     - Hard Parts (Biomineralization): Fossils from the Cambrian commonly include skeletons composed of minerals such as silica, calcium carbonate ($CaCO_3$), and calcium phosphate ($Ca_3(PO_4)_2$).     - Sensory Systems: Development of complex eyes.     - Coordination: Advancement in nervous and muscular system coordination.     - The Substrate Revolution: The advent of active burrowing by organisms.

• Ecological Arms Race: The rise in predation drove the evolution of defensive traits, including shells, spines, and specific defensive behaviors.

Colonization of Land and Transitional Evolution

• Terrestrial Diversification Timeline: Life began to colonize and diversify on land between $467\,\text{MYA}$ and $350\,\text{MYA}$.

• Sequence of Colonization: Land was colonized by fungi, plants, and arthropods, followed later by vertebrates.

• Evolution of Land Plants: Phylogenetic evidence suggests that green algae are the ancestors of land plants. Fungi and arthropods appeared near land roughly at the same time as the earliest plants.

• Vertebrate Evolution and Lobe-Finned Fish: Land vertebrates evolved from lobe-finned fish. All land vertebrates, from amphibians to mammals, are hypothesized to be descendants of fish.

• Tiktaalik (Transitional Fossil): This fossil represents a critical link between fish and early tetrapods. Its features include:     - Wrist-like bones that provided support in shallow water.     - Gills for underwater respiration.     - Primitive lungs for breathing air.

The Impact of Woody Plants and Historical Atmospheric Cycles

• Evolution of Woody Plants: The evolution of woody plants occurred between $400\text{--}350\,\text{MYA}$.

• Impact on $CO_2$: This evolutionary milestone caused a dramatic decline in atmospheric carbon dioxide ($CO_2$) levels.

• Atmospheric Modeling: Atmospheric $CO_2$ levels older than $1\,\text{million years}$ are estimated using computer simulations and geological analyses.

• Cycles of Control: While geological processes are generally the primary drivers of these cycles, biological organisms (like woody plants) exert significant influence.

Mass Extinctions: The "Big Five"

• Definition: Mass extinctions are rare events involving the loss of at least $75\%$ of species over a short geological period (Barnosky et al., 2011).

• Impact on Evolution: These events eliminate previously dominant groups but create "new possibilities" by opening ecological niches for survivors to radiate.

• The Permian Extinction ("The Great Dying"):     - Timing: Occurred $252\,\text{MYA}$.     - Cause: Massive volcanic eruptions in Siberia.     - Environmental Consequences: Release of enormous quantities of $CO_2$, leading to global warming, ocean acidification, and anoxic oceans (oxygen-depleted).     - Biological Toll: Approximately $90\text{--}96\%$ of marine species and $70\%$ of terrestrial species went extinct. It is the most devastating mass extinction on record.

• The Cretaceous Extinction:     - Timing: Occurred $65\,\text{MYA}$.     - Cause: A combination of a large asteroid impact (the Chicxulub crater) and volcanic activity (the Deccan Traps).     - Environmental Consequences: Global darkness, cooling, and the collapse of ecosystems.     - Biological Toll: Approximately $75\%$ of all species were eliminated, most notably the non-avian dinosaurs.     - Outcome: The extinction of dinosaurs cleared the way for the radiation of mammalian lineages (which originated around $210\,\text{MYA}$) and the avian dinosaur group.

The Modern Biodiversity Crisis: A Sixth Mass Extinction?

• Current Status: While Earth is experiencing a major loss in biodiversity, we may not yet be in a 6th mass extinction, though we may be on the brink.

• Extinction Rates: Modern extinction rates for vertebrates are up to $114 \times$ the historical background rate.

• Wildlife Decline: The Living Planet Report indicates that global wildlife populations have shrunk by an average of $73\%$ over the last $50\,\text{years}$.

• Amphibians as Indicators: Amphibians are often called the "canary of the gold mine" for ecosystems.     - One-third of amphibian species are currently threatened.     - They are highly sensitive due to narrow environmental tolerances and the fact that they exchange gases through their skin.     - Threats include habitat destruction, pesticide use, and fungal infections.

The Carbon Cycle and Greenhouse Gases

• Greenhouse Gases (GHGs): Essential for life by trapping heat, but dangerous in excess. Key GHGs include water vapor, methane ($CH_4$), and carbon dioxide ($CO_2$).

• Carbon Reservoirs: Carbon cycles between inorganic (rocks, air, oceans) and organic (living organisms, soil) sources.     - Sedimentary rock is the largest long-term carbon reservoir.

• The Keeling Curve: Data recording atmospheric $CO_2$ since 1958 shows more than a $30\%$ increase.     - Seasonal Pattern: $CO_2$ levels drop in spring/summer as plants absorb it for photosynthesis; levels rise in fall/winter as decaying matter releases it.     - Long-term Pattern: Each year ends with a higher $CO_2$ concentration than the previous year due to human activities like fossil fuel combustion and deforestation.

• Historical Context: For the past $400,000\,\text{years}$, Earth has shown a cyclical pattern between glacial (cold) and interglacial (warm) periods regulated by the long-term carbon cycle, independent of human input.

Impacts on Oceans and Positive Feedback Loops

• The "Deadly Trio": Increasing atmospheric $CO_2$ affects oceans in three ways:     1. Warming: Increase in ocean temperature (leads to coral bleaching).     2. Ocean Acidification: Decrease in the $pH$ of seawater.     3. Deoxygenation: Warmer water has a decreased capacity to store oxygen.

• Positive Feedback Loops (Destabilizing):     - Soil Respiration: Higher temperatures increase microbial activity, causing soils to release more $CO_2$, which further increases warming.     - Permafrost Melt: Warming thaws permafrost, releasing $CO_2$ and $CH_4$ (a powerful greenhouse gas), leading to more warming.

Carbon Dioxide and Agriculture

• CO2 Fertilization: Increased atmospheric $CO_2$ can stimulate plant growth and photosynthesis.

• Nutrient Quality Trade-off: While crop output may increase, the nutrient quality of those crops tends to decrease.

• Carbon Sequestration: Terrestrial ecosystems continue to act as a carbon sink, absorbing a portion of the $CO_2$ released by human activity.

Questions & Discussion

• Why does CO2 concentration change over the year? It fluctuates seasonally; plants remove $CO_2$ during the summer growing season for photosynthesis, but this is not fully balanced by the winter increases from decay.

• How do we know if we are in the 6th mass extinction? We would need to confirm that at least $75\%$ of all species have been lost over a short geological timeframe.

• Why care if extinctions are natural? While natural, the current rate is vastly higher than background levels, and the rapid pace limits the ability of ecosystems to adapt or for new species to radiate in time to provide essential ecosystem services.