Macroevolution

Conditions on Early Earth and the Origin of Life

• Early Earth Conditions

  • Earth formed approximately $4.6$ billion years ago.

  • Early atmosphere was low in oxygen, primarily consisting of water vapor and volcanic gases ($N<em>{2}$, $CO</em>{2}$, $CH<em>{4}$, $NH</em>{3}$, and $H_{2}$).

  • A. I. Oparin and J. B. S. Haldane proposed a reducing atmosphere facilitating organic molecule formation; Miller and Urey demonstrated this abiotic synthesis in $1953$.

• Protocells

  • Definition: Abiotically produced vesicles with a membrane-like structure that maintain distinct internal chemistry.

  • Key properties include simple growth, reproduction, and metabolism, often aided by montmorillonite clay.

• Early Genetic Material

  • The first genetic material was likely RNA, specifically ribozymes, which can catalyze their own replication.

The Fossil Record and Dating

• Nature of the Fossil Record

  • Most fossils are found in sedimentary rock layers called strata, which show the relative sequence of life.

  • Incompleteness: The record is incomplete because few organisms become fossils, many fossils are destroyed by geological processes, and many remain undiscovered.

  • Bias: The record favors species that lived for long periods, were abundant/wide-ranging, and possessed hard structures like shells or skeletons.

• Radiometric Dating

  • Used to determine absolute ages based on the decay of radioactive isotopes.

  • Half-life: The time required for $50\%$ of a parent isotope to decay into a daughter isotope.

  • Carbon Dating: Uses the ratio of Carbon-14 (decays with a half-life of $5,730$ years) to Carbon-12 (stable) for fossils up to $75,000$ years old.

Key Events in the History of Life

• Stromatolites

  • Fossilized mats formed by prokaryotes layering sediment, dating back $3.5$ billion years.

• Photosynthesis and Banded-Iron Formations

  • Early photosynthetic bacteria produced $O_{2}$, which reacted with dissolved iron to create iron oxide precipitates (banded-iron formations).

  • The subsequent "oxygen revolution" ($2.7$ to $2.4$ billion years ago) caused mass extinctions of many prokaryotes and the adaptation of others for aerobic respiration.

• Origin of Eukaryotes (Endosymbiosis)

  • Endosymbiont Hypothesis: Eukaryotes evolved when a prokaryotic host cell engulfed aerobic bacteria (mitochondria) or photosynthetic bacteria (plastids).

  • Evidence:

  • Homologous membrane proteins.

  • Circular DNA and independent replication similar to bacteria.

  • Ribosome structure and size similar to prokaryotic ribosomes.

• Multicellularity and Diversification

  • Ediacaran Biota: Larger, soft-bodied multicellular organisms appearing roughly $600$ million years ago.

  • Cambrian Explosion: Rapid diversification of animal phyla ($535-525$ million years ago).

  • Post-Cambrian Adaptation: Emerging predation led to novel defenses (claws, shells, spines).

Macroevolutionary Dynamics

• Plate Tectonics

  • Theory of Earth's crustal plates drifting; this process moves continents (Continental Drift).

  • Pangaea: The supercontinent formed roughly $250$ million years ago, altering habitats and climates dramatically.

• Mass Extinctions

  • Permian Extinction ($252$ mya): Caused by extreme volcanism in Siberia, leading to global warming, ocean acidification, and $96\%$ loss of marine species.

  • Cretaceous Extinction ($66$ mya): Attributed to a large meteorite impact (evidenced by the Iridium layer), which wiped out non-avian dinosaurs.

• Adaptive Radiation

  • Rapid diversification into new ecological niches.

  • Conditions: Occurs after mass extinctions, the evolution of key innovations, or colonization of new regions with little competition.

  • Example: Mammals underwent massive adaptive radiation after the extinction of dinosaurs opened previously occupied niches.

Developmental Changes and Evolution

• Gene Effects on Morphology

  • Heterochrony: An evolutionary change in the timing or rate of developmental events (e.g., human vs. chimp skull growth).

  • Paedomorphosis: The retention of juvenile features in the adult stage due to accelerated reproductive development.

  • Homeotic Genes: Master regulatory genes that determine the spatial organization of body parts.

  • Hox Genes: A class of homeotic genes; changes in their expression or sequence can lead to major morphological changes (e.g., limb placement).

• Exaptations

  • Structures that evolve for one purpose but are later co-opted for another function (e.g., feathers initially for insulation, later for flight).