The Colonization of Land

The Greening of Earth

  • Earth’s history shows lifeless terrestrial surfaces for the first 2 billion years.
  • Cyanobacteria and Protists emerged on land about 1.2 billion years ago.
  • Small plants, fungi, and animals colonized land around 500 million years ago, leading to the formation of the first forests about 385 million years ago.

Plant and Fungal Partnerships

  • Plants and fungi colonized land together, functioning as partners.
  • Plants produce oxygen and serve as the primary food source for land animals.
  • Fungi decompose organic material, helping recycle nutrients.

Fossil Evidence of Algal Ancestry

  • Plant fossils date over 470 million years ago.
  • Closest living relatives of land plants are charophytes, a type of green algae.
  • Shared characteristics between land plants and charophytes include:
    • Rings of cellulose-synthesizing complexes.
    • Structure of flagellated sperm.

Adaptations for Life on Land

  • Sporopollenin, a durable polymer, protects zygotes from drying out.
  • Adaptations from charophytes to land include:
    • Unfiltered sunlight, CO2, and nutrient-rich soil, but challenges included water scarcity and lack of structural support.
  • Plants are defined as embryophytes, indicating they possess embryos.

Derived Traits of Land Plants

  • Key characteristics of nearly all land plants absent in charophytes include:
    • Alternation of generations.
    • Multicellular, dependent embryos.
    • Walled spores from sporangia.
    • Apical meristems allowing continuous growth.

Alternation of Generations

  • The haploid gametophyte produces haploid gametes.
  • The diploid sporophyte develops from the fusion of gametes, producing spores via meiosis.

Multicellular, Dependent Embryos

  • The diploid embryo is retained within the maternal gametophyte, receiving nourishment via placental transfer cells.

Walled Spores from Sporangia

  • Sporangia are multicellular organs that generate spores with sporopollenin walls for environmental protection.

Apical Meristems

  • Regions of growth at the tips of roots and shoots.
  • Enable continual growth from these specific areas.

Early Plants

  • Fossil evidence indicates land colonization began at least 470 million years ago.
  • Specialized structures like Cooksonia's sporangium were recorded 425 million years ago.
  • Early plants developed specialized tissues for water transport, cuticle protection, stomata for gas exchange, and branching sporophytes.

Role of Fungi in Colonization

  • Fungi, while not closely related to plants, were crucial for the colonization of land, forming associations with early plants for nutrient acquisition.
  • Mycorrhizae involve fungi aiding plants in nutrient uptake, especially when early plants lacked true roots and leaves.

Morphological Features of Fungi

  • Multicellular fungi consist of branched hyphae, enhancing nutrient absorption.
  • Mycelium structure optimizes surface-to-volume ratio for efficiency.

Evolution of Vascular Plants

  • Land plants can be grouped into vascular and nonvascular plants (bryophytes).
  • Vascular plants possess xylem (for water/mineral conduction) and phloem (for transporting nutrients), allowing for greater height and competitive advantages in their environments.

Bryophytes and Seedless Vascular Plants

  • Bryophytes include liverworts, mosses, and hornworts; and rely on water for fertilization due to flagellated sperm.
  • Seedless vascular plants emerged after bryophytes, evolving into taller forms with dominant sporophytes.

Key Adaptations: Seeds and Pollen

  • Seed plants arose about 360 million years ago, utilizing seeds to expand across terrestrial environments.
  • Seeds: Multicellular structures containing an embryo and a food supply surrounded by a protective coat.
  • Angiosperms (flowering plants) evolved distinct reproductive structures for efficient pollination and seed dispersal.

Advantages of Seeds

  • Seeds allow dormancy and provide a food supply, giving a reproductive edge over spores.

Impact on Chemical Cycling and Ecosystems

  • Plants and fungi transformed ecosystems, influencing chemical cycles by stabilizing soil, providing nutrients and oxygen, and facilitating animal habitats.
  • Lichens demonstrate roles in pioneering new land and soil formation.

Biotic Interactions

  • Relationship dynamics between plants and fungi can be mutualistic (beneficial to both) or parasitic (harmful to one).
  • Plant-animal interactions influence evolutionary traits in both groups, such as defenses against herbivores affecting plant morphology.

Conservation Issues

  • Extensive deforestation driven by human activity threatens plant and animal species diversity, with alarming extinction rates possible within centuries.