Diversity of Species: Eukaryotes, Fungi, and Plants

Evidence for Ancient Life on Earth

• Timeline of Early Life:
      * The oldest fossil evidence of life on Earth consists of prokaryotes.
      * Records exist in the form of microfossils dating back to approximately $3.5 \text{ bya}$ (billion years ago).
      * Research indicates that life began even before the $3.5 \text{ billion year}$ threshold.
• Geological Significance:
      * Apex Chert: This is a rock formation located in Western Australia. It is regarded as one of the oldest and best-preserved rock deposits globally.
      * Research Field Work: In $2010$, geoscience researchers from UW-Madison, including John Valley, conducted field trips to the Apex Chert to study these early life signatures.

The Evolution of Eukaryotic Cells

• Emergence Timeline: Eukaryotes first emerged approximately $2.1 \text{ bya}$.
• Development of Membrane-Bound Organelles: Eukaryotic organelles developed through two distinct biological processes:
      * Infolding of the Plasma Membrane: This process involved the inward folding of the ancestral prokaryotic cell's plasma membrane. It gave rise to the endomembrane components including:
          * The Nucleus (protected by an inner and outer nuclear membrane).
          * The Endoplasmic Reticulum (ER).
          * The Golgi apparatus.
          * Nuclear pores and the cytosol were cũng defined through this structural differentiation.
      * Endosymbiosis: This process was responsible for the formation of mitochondria and chloroplasts.

Endosymbiotic Theory and Biological Evidence

• Definition and Process:
      * Mitochondria Origin: Mitochondria originated from an aerobic bacterial species that was engulfed by a host cell. Initially, this was a symbiotic relationship that eventually evolved into an obligate one where they could not live apart.
      * Chloroplast Origin: Chloroplasts originated from a cyanobacterium species that was engulfed by an early eukaryote in a separate endosymbiotic event.
      * Transformation: Over a long period, small changes occurred in the original bacteria. Today, they are no longer classified as bacteria but as specialized organelles (mitochondria and chloroplasts).
• Categorization of Events:
      * First Endosymbiotic Event: An ancestral eukaryote consumed aerobic bacteria, which evolved into mitochondria, leading to modern heterotrophic eukaryotes.
      * Second Endosymbiotic Event: An early eukaryote consumed photosynthetic bacteria, which evolved into chloroplasts, leading to modern photosynthetic eukaryotes.
• Evidence Supporting the Theory:
      * Size: Organelles are similar in size to bacteria.
      * Membranes: Both mitochondria and chloroplasts are double-membraned. The inner membrane's composition differs from the outer membrane.
      * Genetic Material: These organelles contain their own circular DNA, characteristic of prokaryotes.
      * Molecular Sequences: Molecular sequence comparisons demonstrate that chloroplasts are highly similar to cyanobacteria.
      * Ribosomes: The ribosomes found within these organelles are of the bacterial type rather than the eukaryotic type.

Broad Classification of Eukaryotes and Others

• Eukaryotic Characteristics:
      * Includes Protists (mostly aquatic and highly diverse), Fungi, Plantae, and Animalia.
      * While all are multicellular (with some exceptions in protists/fungi), they differ by nutrient acquisition:
          * Fungi: Absorb nutrients.
          * Plants: Photosynthesize.
          * Animals: Ingest nutrients.
      * Modern classification uses "Super groups" to organize eukaryotes.
• The Three Domains of Life:
      * Bacteria: Includes organisms such as Spirochetes, Chlamydia, Cyanobacteria (which contain plastids/chloroplasts), Green sulfur bacteria, and Green nonsulfur bacteria. Mitochondria are phylogenetically linked to this domain.
      * Archaea: Includes Sulfolobus, Thermophiles, Halophiles, and Methanobacterium.
      * Eukarya: Includes Land plants, Dinoflagellates, Green algae, Forams, Ciliates, Diatoms, Red algae, Amoebas, Cellular slime molds, Animals, Fungi, Euglena, Trypanosomes, and Leishmania.

Biological Characteristics of Fungi

• Taxonomic Relation: Fungi are eukaryotes and are more closely related to animals than they are to plants.
• Nutritional Mode: They cannot photosynthesize. Most are saprobes, meaning they derive nutrients from decaying organic matter (decomposers).
• Reproduction: Most fungi produce spores that are dispersed by the wind.

Essential Traits and Evolution of Land Plants

• Evolutionary Origin: Land plants evolved from green algae.
• Terrestrial Adaptations:
      * Protection from desiccation (drying out).
      * Protected embryos.
      * Walled spores.
      * Sporangia: Reproductive sacs specifically for spore production.
      * Gametangia: Structures where gametes (sperm and egg) are formed.
      * Alternation of Generations: A cycle involving distinct sexual (haploid) and asexual (diploid) stages.

Plant Life Cycles: Alternation of Generations

• Two Multicellular Forms:
      * Gametophyte ($1n$):
          * Haploid stage.
          * Develops from a spore.
          * Produces gametes (sperm and egg) through the process of mitosis.
      * Sporophyte ($2n$):
          * Diploid stage.
          * Develops from a zygote.
          * Produces spores through the process of meiosis.
• The Fern Life Cycle Example:
      * Step 1: The Sporophyte ($2n$) contains sori with sporangia.
      * Step 2: Meiosis occurs within the sporangia to produce haploid spores ($1n$).
      * Step 3: Spores undergo germination and mitosis to form the Gametophyte ($1n$).
      * Step 4: The Gametophyte produces sperm and eggs via mitosis.
      * Step 5: Fertilization occurs, combining gametes to form a Zygote ($2n$).
      * Step 6: The zygote undergoes mitosis to develop back into a new Sporophyte ($2n$).

Diversity and Classification of Major Plant Groups

• 1. Nonvascular Plants (Bryophytes):
      * Includes liverworts, hornworts, and mosses.
      * Characteristics: Generally small, lack vascular tissue, and possess flagellated sperm requiring a wet habitat for movement.
• 2. Seedless Vascular Plants:
      * Includes ferns, horsetails, and club mosses.
      * History: They dominated the Carboniferous period and were responsible for creating extensive coal deposits.
      * Reproduction: Possess flagellated sperm and require wet habitats.
• 3. Seeded Vascular Plants:
      * Gymnosperms ("Naked seeds"): Seeds develop into cones. There are four groups:
          * Conifers: Evergreen trees featuring pine cones.
          * Cycads: Plants that resemble short palm trees.
          * Gingkophytes: Only one surviving species exists: Ginkgo biloba.
          * Gnetophytes: Primarily vines found in tropical or subtropical zones.
      * Angiosperms (Flowering plants): Seeds develop into flowers and fruits. They represent approximately $\frac{2}{3}$ of all flowering plants. Divided into two groups:
          * Monocots: Includes grasses and lilies. Features one cotyledon, parallel leaf veins, and lacks a major taproot.
          * Dicots: Features two cotyledons and leaf veins in a network pattern. They can be herbaceous or woody and usually have a root system anchored by one main taproot.