The Colonization of Land: Environmental, Morphological, and Evolutionary Perspectives

Environmental Prerequisites and Continental Development during the Cambrian and Ordovician (541–443 Ma)

• Availability of Land Surfaces: While land surfaces were available for colonization early in the history of multicellular life, several environmental prerequisites had to develop before permanent plant establishment could occur. These included the formation of sizeable and stable near-shore environments, the development of soils, and the amelioration of atmospheric/climatic conditions suitable for survival.
• Tectonic Activity and Continental Reorganization: The Cambrian and Ordovician were periods of intense tectonic activity, resulting in the reorganization of continental plates.     * The supercontinent Rodinia fragmented, leading to a collision between East Gondwana and West Gondwana.     * Subsequent rotation and collision of West Gondwana relative to Laurasia resulted in the eventual assembly of the supercontinent Pangea.
• Sea-Level Fluctuations: Sea levels initially rose due to mantle upwelling associated with tectonic activity and the melting of ice sheets from late Proterozoic glaciations:     * Sturtian Glaciation: $-726-660\,Ma$.     * Marinoan Glaciation: $-655-635\,Ma$.     * Edicarian Glaciation: Approx. $-582-580\,Ma$.     * These events created vast areas of shallow continental shelf; for instance, more than two-thirds of North America was once covered by a shallow sea.
• Late Ordovician Glaciation: Around $443\,Ma$, a brief but severe glaciation period occurred (estimated to have lasted as little as $500,000$ years). This led to a dramatic reduction in global sea levels by as much as $60\,m$ and coincided with a severe marine extinction event lasting $1-2$ million years.

Formation and Biological Enrichment of Early Soils

• Limitations of Bare Rock: Earliest terrestrial environments consisted of bare rock surfaces lacking humic material or biologically available mineral elements such as $N$, $P$, $Fe$, and $S$.
• Evidence of Early Soil: Geological evidence of basalt-derived paleo-soils dates back to $2700\,Ma$. By the early Silurian ($440\,Ma$), well-established soil profiles existed, showing in situ oxidation of organic matter and signs of deep-burrowing organisms.
• Mineral Weathering Mechanisms:     * Biological Input: Organic acids secreted by microbes (cyanobacteria, non-photosynthetic bacteria, and eukaryotic algae) broke down rock to release $Fe$ and $P$.     * Siderophores: Specific organic molecules secreted by organisms to chelate $Fe(III)$ from rocks; these are then taken up by other organisms, releasing iron upon breakdown.     * Atmospheric Input: Weathering was also facilitated by acid rain.
• Nitrogen Acquisition: Inorganic nitrogen was available through planetary accretion and volcanic degassing. Biological availability (conversion to nitrates, $NO_3$) was achieved through:     * Lightning Strikes: A physical mechanism for nitrogen fixation.     * Biological Fixation: Carried out by nitrogen-fixing bacteria. The evolution of anoxygenic phototrophic bacteria (e.g., phototrophic purple bacteria) is estimated to have increased the flux of nutrients through the biosphere by a factor of up to $100$.
• Early Lichens: Symbiotic associations between a fungus and a nitrogen-fixing cyanobacterium (lichens) are found in deposits dated at $550\,Ma$ to $635\,Ma$ in South China. These early partnerships were vital for weathering bare rock prior to the evolution of land plants (embryophytes).

Atmospheric and Climatic Evolution (485–443 Ma)

• Comparison to Modern Atmosphere: In the modern atmosphere, there is $535$ times more oxygen than carbon dioxide. In contrast, the Ordovician atmosphere contained between $8$ and $35$ times more $O_2$ than $CO_2$.     * CO2 Levels: Estimated at $8$ to $15$ times higher than present levels ($PAL$).     * O2 Levels: Exceptionally low, estimated at only $4\%$ compared to the modern $21\%$.
• Biological Implications of Low O2:     * Anoxia in Tissue: Low atmospheric oxygen likely limited the body size and tissue complexity of early plants, as bulky tissues become anoxic even in modern concentrations.     * UV Radiation: A thinner ozone layer (due to low $O_2$) may have been a primary limitation to terrestrialization.
• Temperature and the 'Faint Young Sun Hypothesis': High greenhouse gas concentrations did not lead to extreme temperatures because solar radiation was significantly less than today. The early Ordovician global average surface temperature was approx. $21\,^{\circ}\text{C}$.     * Cooling Trend: Atmospheric $CO_2$ may have dropped to $8$ times higher than present around $460\,Ma$, triggering a $-6\,^{\circ}\text{C}$ drop in global temperature and subsequent glaciation.     * Gradients: Steep temperature gradients from the equator to the poles developed as the planet cooled, particularly across the Gondwana landmass.

Morphological Adaptations for Terrestrial Life

• Spore Evolution and Meiosis: Spores appear in the record at $-475\,Ma$.     * Arrangements: Early spores appeared as single enclosed monads, dyads (pairs), or tetrads (fours) within an envelope.     * Trilete Mark: A Y-shaped scar indicative of meiotic division where four haploid spores were attached in a pyramidal arrangement.     * Phylogenetic Significance: Tetrads are predominantly associated with non-vascular plants (bryophytes); trilete spores are typical of vascular tissue plants (ferns and lycophytes).
• Life Cycle Modification (Diplobiontic Advantage): Algal ancestors (e.g., Charales) had a haplobiontic life cycle (one dominant multicellular haploid generation). Land plants evolved a diplobiontic cycle (alternation between a multicellular diploid sporophyte and haploid gametophyte).     * Embryophytes: The name reflects the retention of the fertilized egg (zygote) within the gametophyte structure.     * Propagation Efficiency: A single successful sexual fusion in the gametophyte can lead to a sporophyte producing hundreds or thousands of spores, whereas in a haplobiontic cycle, one fusion leads to only one new plant.
• Desiccation Protection:     * Sporopollenin: A complex polymer in spore walls providing resistance to desiccation, abrasion, and UV radiation, enabling long-distance wind dispersal.     * Cuticle: A layer of wax and lipid polymers covering epidermal cells. First non-stomatiferous cuticles appeared $-450\,Ma$; cuticles with stomata appeared $-420\,Ma$.
• Stomatal Regulation: Early plants evolved few stomata (< 5\,\text{mm}^{-2}) because high atmospheric $CO_2$ required less pore area for photosynthesis, thus minimizing water loss via transpiration.

Specialized Water Transport and Mechanical Support

• Conducting Tissues: Specialized tissue is required for plants over $2\,cm$ in height. Evolution focused on hollow cells with walls strong enough to withstand negative internal pressure.
• Tracheid Cell Wall Types:     1. G-type: Annular-reticulate thickenings.     2. S-type: Helical thickenings combining features of tracheids and moss hydroids.     3. P-type: Scalariform pitting (transversely elongated parallel pits).
• Lignin: Chemical analysis confirms lignin in plants from the early Devonian ($415\,Ma$), such as Psilophyton. Lignin synthesis is highly oxygen-dependent and likely evolved initially as a defense compound before being co-opted for mechanical strength.
• Biomechanics and Height:     * Flexural Rigidity: Combines material properties with cross-sectional geometry.     * Optimal Shapes: Flat dorsiventral shapes maximize light interception for low-growth plants (e.g., Parka decipiens). Cylindrical stems are best for height; hollow, thick-walled cylinders can grow $26\%$ taller than solid ones.     * Tissue Organization: Early plants used a photosynthetic "rind" on the outside and a hydrostatic core (parenchyma) in the center. Eventually, mechanical tissue (collenchyma) was placed between these layers.
• Anchoring Mechanisms: The earliest unequivocal root traces appear in paleosoils from $408\,Ma$. These consisted of filaments $0.5-2\,cm$ in diameter and up to $90\,cm$ long.

Examples of Earliest Land Plants in the Record

• Cooksonia ($425\,Ma$): Simplest dichotomously branched body, leafless, terminal sporangia, approx. $6.5\,cm$ tall.
• Aglaophyton major ($407\,Ma$): From the Rhynie Chert. Morphologically complex aerial stems (up to $20\,cm$) from a horizontal rhizome. Lacks lignin; water-conducting cells resemble bryophyte hydroids.
• Rhynia gwynne-vaughanii ($400\,Ma$): Approx. $18\,cm$ tall. Possessed vascularized stems with helical S-type elements. Produced spores in tetrahedral tetrads.
• Zosterophyllum divaricatum ($400\,Ma$): Sporangia borne laterally along the stem. Approx. $30\,cm$ tall with H-type branching and G-type tracheids.
• Baragwanathia longifolia ($420-410\,Ma$): Robust stems ($1-2\,cm$ diameter) covered in long, slender leaves ($4\,cm$). Stems up to $1\,m$ long. Age is debated but implies early complexity on Gondwana.
• Psilophyton dawsonii ($395\,Ma$): Approx. $60\,cm$ tall, sophisticated vascular system (P-type cells), monopodial branching, terminal sporangia clusters.

Evolutionary Trends and Earth System Impacts

• Viridophytes (Green Plant Lineage):     * Chlorophytes: One major clade.     * Streptophytes: The clade containing land plants and Charales. Distinguished by the enzyme glycolate oxidase, essential for the photorespiration pathway which protects against UV and photoinhibition.
• Phylogenetic Sequence: Current consensus suggests a split from liverworts (which lack stomata), followed by mosses, then hornworts (which share an intercalary meristem with vascular plants), leading to polysporangiophytes.
• Impact on Earth Systems:     * River Systems: Pre-terrestrialization rivers were broad and unstable. Plants contributed to channeled rivers, stable banks, and muddy floodplains.     * Biogeographical Regions: Five regions identified in the Early Devonian: Siberian-North Laurussian, Kazakhstan, South Laurussian-China, Australian, and Gondwanan.     * Global Cooling and Glaciation: Experiments with Physcomitrellapatens show mosses increase silicate weathering by $1.4$ to $5$ times. This sequestrates atmospheric $CO_2$ into carbonates, potentially triggering the global glaciation at the end of the Ordovician ($443\,Ma$).