Chapter 25: The History of Life on Earth

Introduction

The history of life on Earth is marked by the emergence, diversification, and extinction of various groups of organisms. Large-scale processes such as continental drift, mass extinctions, and adaptive radiations have played significant roles in shaping the diversity of life over geological time.

How Has Life on Earth Changed Over Time?

Macroevolutionary Patterns

• Macroevolution refers to evolutionary changes above the species level, including the origin of new groups, mass extinctions, and the emergence of key adaptations.

• The fossil record documents the rise and fall of different lineages, showing that groups of organisms arise, diversify, and often go extinct.

• Major processes influencing these patterns include:

  • Continental drift: The movement of Earth's continents over time, affecting climate and habitats.

  • Mass extinction: Periods when large numbers of species become extinct globally.

  • Adaptive radiation: Rapid evolution of diversely adapted species from a common ancestor, often following mass extinctions or the colonization of new environments.

• Example: The transition of whales from land to sea, as documented by fossils found in the Saharan Desert.

Conditions on Early Earth and the Origin of Life

Stages in the Origin of Life

• Life is hypothesized to have originated through a sequence of stages:

  1. Abiotic synthesis of small organic molecules (e.g., amino acids, nucleotides)

  2. Joining of these molecules into macromolecules (e.g., proteins, nucleic acids)

  3. Packaging of molecules into protocells (membrane-bound droplets with internal chemistry different from their surroundings)

  4. Origin of self-replicating molecules (e.g., RNA)

• Earth's early atmosphere was likely reducing or neutral, with little oxygen and abundant water vapor and volcanic gases (e.g., nitrogen, carbon dioxide, methane, ammonia).

• Miller-Urey experiment (1953): Demonstrated that organic molecules could form abiotically under early Earth-like conditions.

• Other sources of organic molecules may have included deep-sea hydrothermal vents and meteorites (e.g., the Murchison meteorite contained amino acids and other organic compounds).

Abiotic Synthesis of Macromolecules

• Organic monomers can spontaneously polymerize on hot surfaces such as sand, clay, or rock.

• Protocells may have formed from vesicles with lipid bilayers, capable of maintaining an internal environment and exhibiting simple metabolism and replication.

Origin of Genetic Material

• The first genetic material was likely RNA, which can both store genetic information and catalyze chemical reactions (ribozymes).

• Ribozymes can catalyze the replication of RNA molecules, providing a template for the eventual evolution of DNA, which is more stable and can be replicated more accurately.

The Fossil Record

Formation and Interpretation

• Fossils are primarily found in sedimentary rock layers (strata).

• The fossil record is incomplete and biased toward species that were abundant, widespread, and had hard parts (e.g., shells, bones).

• Radiometric dating is used to determine the age of fossils by measuring the decay of radioactive isotopes.

• Half-life is the time required for half of a radioactive isotope to decay.

• For example, carbon-14 dating can be used for fossils up to about 75,000 years old.

Major Events in the History of Life

First Single-Celled Organisms

• The oldest known fossils are stromatolites (layered rocks formed by bacterial mats), dating back 3.5 billion years.

• Prokaryotes were Earth's sole inhabitants for over 1.5 billion years.

Photosynthesis and the Oxygen Revolution

• Oxygenic photosynthesis by cyanobacteria led to the accumulation of atmospheric oxygen (~2.7 billion years ago).

• Oxygen reacted with dissolved iron, forming banded iron formations.

• The rise in oxygen levels caused the extinction of many anaerobic organisms and enabled the evolution of aerobic respiration.

First Eukaryotes

• The oldest eukaryotic fossils are about 1.8 billion years old.

• Eukaryotic cells have a nuclear envelope, mitochondria, endoplasmic reticulum, and a cytoskeleton.

• Endosymbiosis hypothesis: Eukaryotes originated when a prokaryotic cell engulfed another cell that became a mitochondrion (and later, in some lineages, a chloroplast).

• An endosymbiont is a cell that lives within another cell (the host).

Multicellular Eukaryotes

• The oldest multicellular eukaryote fossils are red algae from about 1.2 billion years ago.

• The Ediacaran biota (635–541 million years ago) were diverse, soft-bodied organisms.

Cambrian Explosion

• The Cambrian explosion (535–525 million years ago) marks the rapid appearance of most major animal phyla in the fossil record.

• First evidence of predator-prey interactions, with adaptations such as claws and defensive spines.

• Molecular and fossil data suggest a "long fuse" leading up to the explosion.

Colonization of Land

• Fungi, plants, and animals began to colonize land about 500 million years ago.

• Adaptations such as vascular systems and mechanisms to prevent dehydration were crucial for terrestrial life.

Plate Tectonics and Continental Drift

Earth's Changing Surface

• Earth's crust is composed of plates that float on the mantle (plate tectonics).

• Continental drift has repeatedly formed and broken up supercontinents (e.g., Pangaea).

Consequences of Continental Drift

• Changes in ocean basins, climate, and habitats.

• Organisms must adapt, migrate, or face extinction as continents move.

• Separation of landmasses can lead to allopatric speciation (formation of new species due to geographic isolation).

Extinctions and Adaptive Radiations

Mass Extinctions

• Most species that have ever lived are now extinct.

• Five major mass extinctions have occurred, each eliminating at least 50% of marine species.

• Permian extinction (252 million years ago): Caused by volcanic activity, global warming, and ocean acidification; resulted in the loss of about 96% of marine species.

• Cretaceous extinction (66 million years ago): Likely caused by a meteorite impact; led to the extinction of dinosaurs (except birds) and many other groups.

• Current extinction rates are much higher than background rates, suggesting a possible sixth mass extinction driven by human activities.

Adaptive Radiations

• Adaptive radiation is the rapid evolution of many diverse species from a common ancestor.

• Often follows mass extinctions, the evolution of novel traits, or the colonization of new environments.

• Example: Mammals diversified rapidly after the extinction of non-avian dinosaurs.

Key Terms and Concepts Table

Key Equations

• Radioactive Decay (Half-life):

• Where is the amount of parent isotope remaining after time , is the initial amount, and is the half-life.

Summary

• The history of life on Earth is shaped by evolutionary, geological, and environmental processes.

• Major transitions include the origin of life, the rise of eukaryotes, multicellularity, colonization of land, and the impact of mass extinctions and adaptive radiations.

• Understanding these patterns helps explain the diversity and distribution of life observed today.