21 History of Life and Terrestrial Evolution Study Guide
Geologic Timeline and the History of Life
The history of life on Earth is categorized into broad eons and specific periods. The Earth formed approximately years ago, marking the beginning of the Precambrian, which lasted until approximately years ago. During the Precambrian, the earliest organic structures emerged between and years ago. The Phanerozoic Eon follows the Precambrian and is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era begins with the Cambrian Period ( years ago), followed by the Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian periods. The Mesozoic Era, often associated with dinosaurs, includes the Triassic, Jurassic, and Cretaceous periods, ending roughly years ago. The Cenozoic Era encompasses the Tertiary and Quaternary periods, the latter of which includes the Pleistocene and Holocene epochs. The transition to terrestrial life occurred progressively throughout the Paleozoic, specifically during the Ordovician, Silurian, and Devonian periods.
The Phylogenetic Tree of Eukaryotes
The evolution of life is represented by a general tree of life that branches from a common ancestor into three main domains: Bacteria, Archaea, and Eukarya. Within the Eukarya, several major lineages exist. The Unikonta group includes Amoebozoa (various amoebas and slime molds) and Opisthokonta, which contains the kingdoms Fungi and Animals. The Chromalveolata group is highly diverse, including Alveolata (Ciliates and Dinoflagellates), Stramenopila (Diatoms, Brown algae, and Coccolithophores belonging to Haptophyta), and Rhizaria (Foraminifera and Radiolarians). The Plantae lineage comprises Red algae, Green algae, and Land plants. It is important to note that "algae" is a broad term for organisms that are not always closely related; for instance, Brown and Green algae are likely as distantly related to one another as humans are to fungi. Many of these organisms were traditionally grouped under the kingdom Protista.
Comparative Biology of Marine Algae
Marine macro-algae, commonly known as seaweeds, are eukaryotes that lack the true leaves, stems, and roots found in land plants, though they possess analogous structures. The main body of the seaweed is called the thallus. The blade or frond provides the surface area for photosynthesis. The stipes serve as stem-like supports, and the holdfast is used to anchor the organism in place; the holdfast is distinct from a root system as it does not function in nutrient or water uptake. Some seaweeds possess pneumatocysts, which are gas-filled bladders that help the blades float. Crucially, seaweeds lack conducting (vascular) tissues.
Brown algae, or Chromista, are a group of single-celled and multicellular species with distinctive photosynthetic organelles called plastids. They are likely polyphyletic. They utilize chlorophyll , which is chemically distinct from the chlorophyll found in land plants, and their characteristic gold or brown color comes from the carotenoid pigment fucoxanthin. Red algae, or Rhodophyta, conduct photosynthesis using chlorophyll and , as well as phycobilins (extensively phycoerythrin). They are adaptable to various water temperatures and can survive at low levels of solar radiance. Green algae, or Chlorophyta, are the closest relatives to land plants. They use chlorophyll and along with carotenoids, store excess energy as starch, and some species, such as Codium fragile, utilize cellulose in their cell walls.
Advantages and Challenges of Terrestrial Colonization
Until the Middle Paleozoic, the evolution of multicellular life took place almost exclusively in the oceans. The colonization of land offered distinct advantages for photosynthetic organisms. Sunlight is much stronger on land because it is not filtered or diminished by depth of water. Furthermore, CO2, which is essential for photosynthesis, is much more concentrated in the atmosphere than in seawater. However, moving from an aquatic to a terrestrial environment presented significant biological hurdles. Organisms had to develop mechanisms for the control of gas exchange to prevent drying out, a reliable water supply for nutrient transport, physical support to counteract gravity in a less dense medium (air vs. water), and internal fluid movement to transport nutrients effectively against the pull of gravity.
In aquatic environments, gas exchange occurs easily across any surface in contact with water, and support is provided by the density of the medium (neutral buoyancy). On land, tissues exposed to air will desiccate, and the lack of medium density causes organisms to collapse without rigid structures. Furthermore, while water and nutrients are omnipresent in the ocean, terrestrial organisms must find a source and prevent loss once obtained.
Evolutionary Adaptations for Land Plants
To overcome the challenges of land, plants evolved several key structures. Lignin and cellulose are incorporated into cell walls to provide the necessary rigidity and support to stand upright. Lignin is a complex organic polymer with a structure composed of various phenolic units and carbohydrate linkages. For internal fluid transport, plants developed vascular tissues: the xylem is responsible for transporting water and minerals from the roots upward, while the phloem transports the products of photosynthesis and other nutrients throughout the plant. Roots evolved to provide access to water and minerals in the soil while acting as an anchor for support. To prevent desiccation, plants developed a cuticle, which is a waxy covering that seals moisture inside internal tissues. To allow the plant to "breathe" through this seal, stomata evolved; these are specialized cells that regulate gas exchange by opening to allow gases in or closing to prevent moisture loss.
Fossil Record of Land Colonization
There is no evidence for complex life on land during the Cambrian or early Ordovician. The earliest evidence of land plants comes from reproductive systems, specifically cryptospores found in the Middle Ordovician, roughly years ago. These early plants were limited, often non-vascular, lacking roots, and potentially lacking stomata. Identified cryptospore types include Chamotriletes? sp., Greudhaspora (Laevolancis) divellamedia, Laevolancis chibrikovae, Sphaerasacus glabellus, and Tetrahedraletes f. medinensis. The first fossils of complete plants appear in the Middle Silurian, such as Cooksonia from Ireland and Baragwanathia from the Late Silurian of Australia. By the Early Devonian, more advanced vascular plants like Rhynia appear in the fossil record.
Questions & Discussion
Evidence for terrestrial animals is also found in the Paleozoic. Pneumodesmus, a genus of millipede from the Late Silurian of Scotland, was once considered the earliest land animal, though newer studies suggest it may be Devonian in age. There is also evidence of animal activity in the form of tracks; Scoyenia tracks from the Late Ordovician of Pennsylvania suggest early land movement. The colonization of land by animals followed plants. While vertebrates began to appear on land by the Devonian, arthropods such as centipedes, spiders, and insects had already been established for a significant period. In a hypothetical dialogue between early land-dwellers, a vertebrate might claim to be among the "first ones" on land during the Devonian, only to be corrected by arthropods (centipedes, spiders, and insects) claiming they had already inhabited the terrestrial environment for sixty million years.