Terrestrialization: How plants and animals emerged from the water to conquer land

A learner-centric microbiology education framework 1 Terrestrialization: How plants and animals emerged from the water to conquer land Mom, Dad, I read that all plants and animals were in the ocean before living on land. What happened to them? How do they come out? Photo credit: Marco Fusi Marco Fusi1, Ramona Marasco2 1Dove Marine Laboratory, School of Natural and Environmental Science, Newcastle University, UK. 2Biological and Environmental Sciences and Engineering Division (BESE), King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia. A learner-centric microbiology education framework 2 Terrestrialization: How plants and animals emerged from the water to conquer land Storyline Long before forests, insects, or animals filled the land, all life lived in the oceans. Terrestrialization is the name we give to the incredible process that changed everything, during which some life forms learned how to breathe, move, and survive on land. In fact, billions of years ago, Earth was almost entirely covered by oceans—the cradle where life first appeared in the form of tiny microorganisms (or microbes) like bacteria. As time passed, around 500 million years ago, continents slowly rose from the sea, creating rocky coastlines, sandy beaches, and river deltas. At first, these lands were harsh and barren, too dry and unstable for most life, but tiny pioneers, such as cyanobacteria, algae, and fungi living in the oceans, began to colonise the rocks. They formed microbial mats and biological crusts that trapped dust, broke down minerals, and created the first thin soil layers. Gradually, plants followed, spreading across damp areas and bringing shade, oxygen, and food to the new landscape. As the land became greener and more prosperous, some marine species began to evolve features that helped them survive in shallow or temporary waters: stronger fins for pushing, tougher skins to prevent drying, and lungs or air sacs to breathe when oxygen levels were low. These adaptations were the first steps toward life on land. Then, about 200 million years later, some “brave” organisms began their journey to fully explore life beyond water by evolving new body parts and behaviours that enabled them to breathe air, move on solid ground, eat new foods, and reproduce on land. This transition from water to land is one of the most important steps in the history of life on Earth, and microorganisms played a significant role in it. Yet even after plants and animals arrived, microorganisms continued to play vital roles by forming microbe-host partnerships. The Microbiology and Societal Context The microbiology: plant and animal microbial symbiont (or symbiome); plant/animal metaorganism (or holobiont); plant-growth-promoting microorganisms; microbe-mediated adaptation. Sustainability issues: terrestrial and marine biodiversity; ecosystem functioning. 2: End hunger Microbiology theme and SDGs treated in this chapter 3: Healthy lives 14: Life below water 15: Life on land 11: Safe settlements Human well-being Planet Earth Water Plants & crops Animals Nutrition Biotechnology Microbiology Literacy Topics Sustainable Development Goals 12: Sustainable consumption 13: Combat climate change A learner-centric microbiology education framework 3 1. The terrestrialization process: the timeline of events in a nutshell. Terrestrialization is the name scientists give to the big evolutionary shift when living things moved from water to land. This incredible journey began hundreds of millions of years ago and completely changed life on Earth. Before animals and plants took their first “steps” on land, tiny organisms like bacteria, fungi, algae, and lichens were already there. These microscopic pioneers lived on rocks and soil, forming crusts that helped hold the ground together. Some of them, like cyanobacteria, have the capacity to synthesise oxygen, a gas that would later be essential for animals to breathe. Later, around 480 million years ago, small plants began to grow on land. They not only made the landscape greener but also created shade, food, and shelter, setting the stage for animals to follow. The earliest land animals were arthropods, creatures with jointed legs and hard outer shells, such as millipedes and ancient insect relatives. One fossil millipede, Pneumodesmus newmani, is about 428 million years old and shows tiny breathing holes called spiracles, proving it could breathe air. Other animals, like snails and slugs (called molluscs), adapted by developing air-breathing lungs to survive on land. Much later in evolutionary history, some fish developed air-breathing lungs, this time derived from their swim bladders, along with stronger, limb-like fins that could support their bodies. One famous example is the Coelacanth, often called a “living fossil”, which still inhabits the deep waters of the Indian Ocean. Unlike typical ray-finned fish, the Coelacanth has robust, lobed fins with a bone-like structure, an essential step toward the evolution of limbs capable of supporting terrestrial movement. Fossils such as Tiktaalik, Acanthostega, and Ichthyostega reveal how vertebrates (animals with backbones) gradually adapted to life on land. These transitional species show a mix of Form water to land Terrestrialization of arthropods Terrestrialization of tetrapod Fresh water Network of fungi supporting primordial plant to establish on land Algal mat Terrestrialization of plants Oxygen production and Ozon formation Sea water UV radiation Unicellular algae Archean Headen Proterozoic Palaeozoic CenozoicMesozoic From sea to land: the journey of microbes, plants, and animals Life’s transition from aquatic to terrestrial environments occurred in multiple stages over hundreds of millions of years. Oxygenic photosynthesis by cyanobacteria and algae enriched the atmosphere and led to ozone formation, reducing harmful UV radiation. Microbial mats and lichens were among the first to colonize land, stabilizing surfaces and initiating soil formation. Green algae (Charophyceae) adapted to fluctuating wet–dry conditions, giving rise to early land plants. These plants transformed landscapes, enabling the later terrestrialization of arthropods (~400 Ma) and vertebrates (~375– 400 Ma), culminating in fully developed terrestrial ecosystems. Image credit: panel available under common license (Google Images). A learner-centric microbiology education framework 4 aquatic and terrestrial features, including limbs with digits and lungs for breathing air. By around 395 million years ago, fossilised footprints with distinct toes were discovered in rocks in Poland, providing evidence that tetrapods, i.e., four-limbed vertebrates, were already venturing onto dry land. 2. Why terrestrialization matters in Earth’s evolution Terrestrialization stands as one of the most profound evolutionary shifts in Earth’s history. When life ventured from water onto land, it didn’t merely change its surroundings, but it redefined the architecture of ecosystems and the trajectory of biodiversity. This transition sparked the rise of land plants, which anchored soils, enriched the atmosphere with oxygen, and led to the formation of expansive forests. These new landscapes became the scaffolding for intricate food webs and ecological niches. From arthropods to amphibians, reptiles to mammals— including ourselves—every terrestrial creature owes its existence to this ancient leap. 3. How terrestrialisation unfolded Let’s take a closer look at the key stages of terrestrialization, focusing on the colonisation of land by (i) microorganisms, (ii) plants, and (iii) animals: a. Microorganisms, the invisible allies of life on land. Microorganisms were the first great engineers of our planet’s transformation. Around 3.5 billion years ago, simple microbial life already existed in Earth’s oceans, but it was the rise of cyanobacteria—microscopic organisms capable of oxygenic photosynthesis that changed everything. Before them, the atmosphere contained almost no oxygen. Cyanobacteria used sunlight, carbon dioxide (CO2), and water (H2O) to produce energy, releasing oxygen as a waste product (the photosynthesis reaction: CO2+H2O+light energy → sugars + O2). About 2.4 billion years ago, over a period of hundreds of millions of years, this process triggered the Great Oxidation Event, one of the most important turning points in Earth’s history. The oxygen released by these microbes slowly accumulated in the oceans and atmosphere, reacting with dissolved iron to form vast bands of rust-colored rock (the banded iron formations we still find today). As oxygen levels rose, the ozone layer (O₃) formed in the upper atmosphere, protecting Earth’s surface from harmful ultraviolet radiation. This protection was essential because it allowed living organisms to survive and thrive on land. Meanwhile, microalgae and algae, which evolved later in the oceans, joined this global photosynthetic revolution. They expanded oxygen production and formed the foundation of marine food webs, feeding animals and shaping the chemistry of coastal ecosystems. These photosynthetic microorganisms, along with other bacteria, fungi, and lichens (formed by fungi and algae), formed the first microbial mats and biological crusts by growing on bare rock surfaces. These living films trapped dust, broke down minerals, and created the first thin, nutrient-rich soils. They stabilised sediments against erosion and helped retain moisture— conditions that allowed mosses, liverworts, and other early plants to take root. Moreover, when these microorganisms died, they added organic matter to the soil, enriching it with nutrients and making it more conducive to plant growth. In short, microorganisms were responsible for oxygenating Earth’s atmosphere and preparing the land for colonisation. They transformed a lifeless, rocky surface into a dynamic and habitable environment, paving the way for plants, animals, and eventually humans. Without their ancient work, the story of terrestrialization—and indeed the story of life on Earth—would never have been possible. A learner-centric microbiology education framework 5 b. Plants, the green pioneers. Plants were the first complex organisms to colonise the land after microorganisms. When the first moss-like plants appeared on land about 480 million years ago, they began to cover bare rocks and soils. These plants trapped water, added organic matter, and produced additional oxygen. As plants grew taller and developed stems, roots, and leaves, they created shade, food, and shelter. Forests of giant ferns and trees soon covered large areas, shaping the landscape and providing new habitats for insects, amphibians, and reptiles. During this transition, plants evolved notable physiological and morphological modifications to survive. First, they developed waxy leaves and stems to prevent water loss. They also formed stomata, tiny pores that can open and close to control breathing and water loss. To stand tall and reach sunlight, plants evolved lignin, a strong material in their cell walls, which allowed them to grow upright and form stems and trunks. Roots and root hairs appeared, helping them anchor to the soil and absorb water and nutrients. Finally, plants developed spores and later seeds with protective coats, allowing them to reproduce without staying in water. These adaptations allowed plants to spread across land, create forests, and shape the environment that animals—including us—depend on today. However, these traits didn’t evolve in a single way; instead, they followed different evolutionary paths depending on the environment, driving the incredible diversity we see across Earth’s landscapes today. From damp riverbanks and shaded forests to deserts and high mountains, plants evolved distinct strategies to cope with temperature, light, and water availability. For example, desert plants evolved thick stems and tiny leaves to conserve water, mountain plants developed compact forms to resist cold and wind, and coastal species adapted to salty soils and shifting tides. Notably, Klebsormidium (often shortened to Klebs) belongs to the streptophyte algae, which represent the ancestral lineage from which land plants evolved. The genus Klebsormidium is among the best-known and most widely studied members of this group and is considered one of the closest living relatives of early land plants. It Photosynthetic bacteria and (micro)algae oxygenating our planet Cyanobacteria and microalgae are key drivers of oxygen production on Earth. Through photosynthesis, they capture sunlight and release oxygen, supporting ecosystems in aquatic and coastal environments. These microbial mats and biofilms, found in tidal flats and shallow waters, represent modern analogues of the ancient communities that first oxygenated our planet and made terrestrial life possible. Image credits: photographs by Ramona Marasco and Khaoula Lassoued. A learner-centric microbiology education framework 6 can still be found today in terrestrial and freshwater ecosystems across a wide range of climates, from polar to tropical regions. Its remarkable ability to withstand desiccation, intense light, and fluctuating water availability makes it a valuable modern analogue for studying the molecular, structural, and physiological traits that facilitated the terrestrialization of plants—see for more details EvolutionaryStar: Klebs (Klebsormidium flaccidum) in the Evolutionary Route Marker Star Microbes, Portrait Gallery. c. Animals followed. The arrival of animals on land was only possible thanks to the groundwork laid by microorganisms and plants, which transformed barren terrain into habitable Mesostigmatophyceae Chlorokybophyceae Klebsomidiophyceae Charophyceae Coleochaetophyceae Zygnematophyceae Land plants Bryophytes Tracheophytes vascular plants Chlorophytes Chloroplastida Streptophytes Interaction with substrate microbiome Plant terrestrialization: from green algae to vascular plants From aquatic algae to early land plants, evolution involved major innovations such as rhizoids, cuticles, stomata, and partnerships with fungi that allowed survival outside water. These adaptations transformed Earth’s surface, enabling the rise of terrestrial ecosystems. Image credit: illustration and plant photographs obtained under Creative Commons licence. A learner-centric microbiology education framework 7 ecosystems. Without these early pioneers stabilising soils, producing oxygen, and creating food sources, animals would have had neither the opportunity nor the means to leave the water. But how did animals adapt to life on land? This transition demanded a suite of evolutionary innovations, both behavioural and anatomical. From breathing air and moving without the buoyancy of water to sensing new environmental cues, animals had to reengineer their bodies and behaviours to survive in the air-exposed, gravity-bound world of terrestrial life. Below are some of the most essential evolutionary upgrades that made this leap possible: • Breathe air: gills worked well underwater, but on land, lungs were essential. Some fish evolved lung-like organs, allowing them to draw oxygen directly from the atmosphere. • Motility: fins transformed into limbs with elbows, wrists, and digits, seen in fossils like Tiktaalik, giving early tetrapods the ability to push, crawl, and eventually walk. But movement required strength, so animals evolved reinforced skeletons, including strong backbones, hips, and limb girdles, to support their body weight without the buoyancy of water. • Weeing: marine animals release ammonia directly into water, but land animals need kidneys and specialised excretory systems to conserve water and safely remove nitrogenous waste. This led to the production of concentrated urine, often aided by microbial partners that helped detoxify and recycle nutrients. • Dryness: on land, animals needed protective barriers—such as thicker skin, shells, or other coverings—to prevent dehydration and defend against both UV and new predators and microbes. A B FE C D Animal terrestrialization: Modern analogue and associated behaviors A – Coconut crabs, Birgus latro; BMudskipper, Periophthalmus barbarus, CLand crabs, Geocarcinus lateralis; D – Climbing crabs, Aratus pisonii; E – Ghost crabs, Ocypode macropthalmus; E – Climbing snail , Cerethidea decollate. All these animals are undergoing multiple adaptation to survive out of the water. Some, like the coconut crabs and the climbing crabs did so well that they live on the canopy of trees at several meters of height, feeding in fresh leaves. Although not visible with our eyes,! microbes are their best allies to made this happen. For examples, they help the digestion of fresh leaves, much more difficult to digest compared to algae, they help with breathing and weeing, and also, they will help them to protect from the ultraviolet rays of the sun by producing carotenoids on the surface of their carapace! Image credits: photographs are under creative common license (Google images) A learner-centric microbiology education framework 8 • Reproduction: eggs laid in water were vulnerable on land, so animals developed hard shells, nesting behaviours, and other strategies to protect their offspring. • Sensing the new environment: eyes and ears adapted to air rather than water, and brains evolved to process new kinds of information—from navigating terrain to spotting predators in open landscapes. Each of these adaptations was a solution to the unique challenges of life on land that fish, millipedes, centipedes, spiders, insects, and even snails overcame in their own distinct ways. Through countless evolutionary experiments, these pioneers laid the foundation for the rich and diverse ecosystems we see today. From tiny insects to towering mammals, every land-dwelling animal carries the imprint of this ancient transformation, a legacy written in bones, lungs, limbs, and instincts. 4. The microbial side of the story: how microorganisms contribute to plant and animal terrestrialization Which role did bacteria play in the terrestrialization of plants and animals? Microorganisms were the quiet engineers of terrestrialization, preparing the land, supporting the newcomers, and still sustaining ecosystem functionality and stability on Earth today. Besides their role in reshaping terrestrial ecosystems, microorganisms also helped plants and animals adapt out of water. a. Plant-microbe interaction. Interactions between plants and microbes were among the most crucial partnerships in Earth’s history, and fungi played a leading role in helping plants make the move from water to land. For the first moss-like organisms, the presence of mycorrhizal fungi—fungi forming close associations with plant tissues. These fungi, which were connected to the primitive rhizoids or root-like structures of early plants, formed a mutualistic relationship known as mycorrhiza, in which both partners benefited. The fungi’s fine, branching filaments (hyphae) spread through the soil, greatly expanding the plant’s ability to absorb water and essential minerals, especially phosphorus and nitrogen, from nutrient-poor environments. In exchange, plants supplied the fungi with sugars and organic compounds produced during photosynthesis, delivered through root exudates. This collaboration went far beyond nutrient exchange—it fundamentally shaped terrestrial ecosystems. Mycorrhizal fungi acted as biological extensions of the root system, exploring tiny soil spaces that plant cells could not reach. They also protected plants from pathogens and environmental stresses such as drought or salinity by improving water balance and enhancing resistance. Remarkably, scientists believe that these fungi began helping plants before true roots even evolved. Early land plants, small and simple in structure, lacked the deep roots of modern vegetation, yet fungal hyphae functioned like temporary roots, anchoring plants and facilitating mineral uptake from raw, rocky surfaces. Over time, these interactions drove co-evolution: as plants developed true roots and vascular systems, mycorrhizal fungi evolved alongside them, diversifying into arbuscular, ectomycorrhizal, and ericoid forms. This symbiosis not only enabled plants to colonise new habitats but also transformed barren land into fertile soil, stabilising sediments and fostering the establishment of larger plant communities. In essence, the partnership between plants and fungi was the biological bridge between the aquatic world and the terrestrial biosphere, laying the foundation for the prosperous and stable ecosystems that cover Earth today. Subsequently, other soil and root-associated microbes joined this alliance, giving rise to the plant microbiome—the complex community of bacteria, fungi, and archaea living in and around plant tissues. Some bacteria, such as rhizobia, formed nitrogen-fixing nodules in roots, A learner-centric microbiology education framework 9 providing essential nutrients in exchange for carbon. Others, like plant-growth-promoting rhizobacteria, released hormones that stimulated root elongation or protected against pathogens. Together, these microbial partners created a protective, nutrient-recycling network that continues to sustain plant life today, showing that terrestrialization was not just a story of plants conquering land, but of entire microbial communities working together to make it possible. b. Animal-microbe interaction. Microbes also played a crucial role in helping animals adapt to new environments. As animals began to be exposed to new (and challenging) conditions, including dry air, sunlight, and fluctuating temperatures, their interactions with microbes became crucial. Based on current research on frogs and salamanders, scientists have shown that skin microbes have become increasingly critical during terrestrialization. Skin stays naturally moist in water, but on land it dries quickly. Whereas some bacteria can form protective biofilms—natural shields that help prevent infections and dehydration—others secrete antimicrobial compounds that stop harmful fungi or viruses from growing, a defence strategy that persists today. Even reproductive success may have depended on microbes. When early animals began to lay eggs on land, soil bacteria and fungi likely helped recycle nutrients, keep the ground oxygen-rich, and prevent rotting processes or invasion by other microbes. They supported embryo development in drier conditions and, in some cases, facilitated the transmission of certain microbes to newborns that consume egg remnants after hatching. Besides these microbe-interactions, one of the most important is the one that evolved to help digest new complex materials and types of food on land, such as fibrous plants, tough leaves, and insects with hard shells. At first, the microbial colonisation of the stomach and/or gut probably occurred as microbes entered the digestive tracts passively through food, water, or sediment. Over time, certain microbial species formed stable symbiotic relationships with their How microbe contribute to plant adaptation from water to land Fungal hyphae expanding plant roots Phyllosphere microbiome •Plant growth promotion • Disease control • Bioreactions Rhizobacteria •Plant growth promotion • Nitrogen fixation • Potassium solubilization • Phosphate solubilization • Phytohormone production Mycorrhizal colonization • Increased absorptive surface area • Enhance water sequestration • Enhanced mineral uptake Image credit: illustrations are under Creative Commons licence and fungal photographs by Mohamed Hijri. A learner-centric microbiology education framework 16 For example, microbial partnerships with plants—such as mycorrhizal fungi—enhance soil fertility and carbon storage, which are crucial for sustainable agriculture and climate resilience. Studying how these relationships have evolved helps us design more effective strategies for ecosystem restoration, carbon sequestration, and biodiversity conservation. Similarly, microbes that protect amphibians from disease or help herbivores digest plants are key to maintaining healthy wildlife populations and food webs. From a Grand Challenges perspective, microbial terrestrialization research addresses fundamental questions about the origins of complex life, the evolution of symbiosis, and how life adapts to new environments. It also informs applied fields such as biotechnology, soil science, and microbiome health, offering tools to address pressing issues such as land degradation, food security, and emerging diseases. Pupil participation 1. Class discussion on the role of microbes in terrestrialization Activity I. Objective: To help pupils understand how microbes prepared the Earth for life on land and continue to support all living things today. Suggested activities: Begin with a short recap of the story of terrestrialization, showing how life moved from oceans to land. Then ask: • What would the world look like without microbes? • How did microbes make it possible for plants and animals to survive on land? • Can we see examples of microbial life helping nature today (for example, in soil, roots, or animals)? Show images or animations of microbial mats, cyanobacteria, and early land plants to spark discussion. Discussion: Divide the class into small groups; each group lists three ways microbes helped terrestrialization (e.g., oxygen production, soil formation, plant partnerships). Bring everyone together to share answers and create a “Microbes made it possible!” wall chart that summarises their ideas. Learning outcomes: Pupils recognise that microbes were essential to the colonisation of land. They understand that microorganisms remain vital to ecosystems, agriculture, and health. Activity II. Objective: To help pupils understand how food chains and food webs work and how microbes support all living things, even those at the top of the chain. Suggested activities: Choose your favourite animal! Each student picks an animal they like (for example: lion, penguin, cow, bee, frog, crab). Then draw your food chain or web by placing your chosen animal in the middle of the page; above it, draw what might eat it (predators) and below it, draw what it eats (plants, smaller animals, or detritus). Discussion: Think about what happens before and after your animal — what decomposes waste or dead organisms? Discuss as a class where microbes fit in your web: label where these microbes act and how they keep the ecosystem healthy. Learning outcomes: Understand that microbes are the foundation of every food chain and keep ecosystems functioning. Recognise that energy and nutrients flow through interconnected living systems, not isolated organisms. 2. Pupil awareness Objective: To inspire pupils to connect what they learn with their daily lives and to appreciate A learner-centric microbiology education framework 17 the ongoing importance of microbes in the environment. Suggested activities: • Observation task - ask pupils to look around their environment (soil, plants, insects) and imagine the hidden microbial world beneath • Creative reflection - pupils draw or write a short piece titled “A day in the life of a microbe” describing how a tiny organism helps the world • Awareness campaign - as a class, design posters or short presentations on “Why microbes matter” to share with the school community. Encourage pupils to think about how human actions (pollution, waste, antibiotic use) can harm or benefit microbial life, and how this affects their interactions with animals and plants. Learning outcomes: Pupils become aware that microbes are everywhere and play a central role in life on Earth. They develop a sense of responsibility for protecting ecosystems — from the smallest bacteria to the largest animals. The evidence base, further reading, and teaching aids Videos Terrestrialisation https://www.youtube.com/watch?v=g6NtQ3XNLW0 Tree of Life https://www.youtube.com/watch?v=ii4510LeRXo Plants https://www.youtube.com/watch?v=ONVpFtiD-fo Fungi https://www.youtube.com/watch?v=5FqFg-rjzPo Books Chipman, Ariel D., 'Terrestrialization', Organismic Animal Biology: An Evolutionary Approach (Oxford, 2024; online edn, Oxford Academic, 30 Apr. 2024), https://doi.org/10.1093/oso/9780192893581.003.0022, accessed 30 Oct. 2025. Burggren, Warren W., and Brian R. McMahon, eds. Biology of the land crabs. Cambridge University Press, 1988. References Boyce C. Kevin and Matthew P. Nelsen (2025). Terrestrialization: toward a shared framework for ecosystem evolution. Paleobiology 51(1):174-194. Fusi, M., Ngugi, D. K., Marasco, R., Booth, J. M., Cardinale, M., Sacchi, L., and Daffonchio, D. (2023). Gill-associated bacteria are homogeneously selected in amphibious mangrove crabs to sustain host intertidal adaptation. Microbiome 11(1):189. Puginier, Camille, Jean Keller, and Pierre-Marc Delaux (2022). Plant–microbe interactions that have impacted plant terrestrializations. Plant Physiology 190(1):72-84. Vermeij, Geerat J., and Victoria M. Watson-Zink (2022). Terrestrialization in gastropods: lineages, ecological constraints and comparisons with other animals. Biological Journal of the Linnean Society 136(3):393-404. A learner-centric microbiology education framework 18 Glossary Adaptations: Physical or behavioural changes that help an organism survive better in its environment. Algae/Microalgae: Simple aquatic organisms, often green, that make their own food using sunlight; they include tiny plankton and larger seaweeds. Arthropods: Animals with jointed legs and hard outer skeletons, such as insects, spiders, and crabs. Bacteria: Single-celled microorganisms that live almost everywhere on Earth; some cause disease, but many are helpful to humans, animals, and the environment. Biodiversity: The variety of all living things on Earth, from tiny bacteria to giant trees and animals. Biological crust (or soil crust): A living skin of microbes, algae, lichens, and mosses that covers soil in deserts and helps hold it together. Cyanobacteria: Ancient bacteria that can do photosynthesis, releasing oxygen and helping create Earth’s atmosphere billions of years ago. DNA (Deoxyribonucleic Acid): The molecule that carries the genetic code containing the instructions for life in all organisms. Ecosystem functionality/stability: How well and how consistently an ecosystem performs its natural roles, even when facing change or disturbance. Ecosystem functioning: The way living organisms and their environment work together, cycling nutrients and energy. Evolution/Evolutionary shift: The gradual change of species over time as they adapt and new forms of life appear. Food webs: Networks that show how energy and nutrients pass from one organism to another through eating relationships. Fungi: Organisms such as moulds, yeasts, and mushrooms that absorb nutrients from their surroundings and often live in partnership with plants. Genetic code: The set of instructions in DNA that tells cells how to build proteins and control life processes. Great Oxidation Event (GOE): The time about 2.4 billion years ago when cyanobacteria began producing oxygen, transforming Earth’s atmosphere and enabling complex life. Lichens: Partnerships between fungi and algae (or cyanobacteria) that can live on rocks, bark, or soil and help form early ecosystems. Metagenomics: The study of all the DNA in a sample (like soil or water) to learn which microbes live there and what they do. Metaorganism (or holobiont): The idea that every animal or plant and all its associated microbes together form a single biological unit. Microbe-host partnerships: Relationships in which microbes live with plants or animals, often providing protection or nutrients. Microbe-mediated adaptation: A change or ability in a plant or animal that happens thanks to the help of its microbial partners. Microbial mat: A thin, colourful layer of microbes (often cyanobacteria and algae) growing together on rocks or sediments in moist places. A learner-centric microbiology education framework 19 Microbial symbiont (or symbiome): A microbe that lives closely with another organism, forming a partnership that benefits one or both. Microorganisms (or microbes): Tiny living things such as bacteria, fungi, or algae that are too small to see without a microscope. Molecular tools: Laboratory methods that let scientists study genes, proteins, or other molecules to understand how life works. Molluscs: Soft-bodied animals, often with shells, such as snails, clams, and octopuses. Morphological: Related to the shape, structure, or appearance of an organism. Mutualistic relationship: A partnership between two organisms in which both benefit. Mycorrhiza: from Greek: “mykes” = fungus, “rhiza” = root, refers to a special group of fungi that helps plants absorb water and minerals (especially phosphorus) from the soil. This relationship improves plant growth and soil health and is found in most land plants. Photosynthesis: The process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to make food (sugars) and oxygen. Phylogenetics: The study of evolutionary relationships among species, often shown as a branching “family tree” based on DNA. Physiological: Related to how the body or cells of a living organism function. Plant-growth-promoting microbes: Beneficial bacteria or fungi that help plants grow by improving nutrient uptake or protecting against disease. Spiracles: Small openings on the body of insects or millipedes that allow them to breathe air. Synthesise: To make or build something; in biology, the process by which living organisms make complex molecules needed for growth, repair, and energy, such as proteins, vitamins, or natural chemicals, from simpler substances. Terrestrialization: The long process through which life that once lived only in water gradually adapted to survive and thrive on land. Tetrapods: Four-limbed vertebrates (amphibians, reptiles, birds, and mammals) that descended from ancient fishes.