Which of the following traits was most important in enabling the first plants to move onto land?

In evolutionary theory, adaptation is the biological mechanism by which organisms adjust to new environments or to changes in their current environment. Although scientists discussed adaptation prior to the 1800s, it was not until then that Charles Darwin and Alfred Russel Wallace developed the theory of natural selection.

Wallace believed that the evolution of organisms was connected in some way with adaptation of organisms to changing environmental conditions. In developing the theory of evolution by natural selection, Wallace and Darwin both went beyond simple adaptation by explaining how organisms adapt and evolve. The idea of natural selection is that traits that can be passed down allow organisms to adapt to the environment better than other organisms of the same species. This enables better survival and reproduction compared with other members of the species, leading to evolution.

Organisms can adapt to an environment in different ways. They can adapt biologically, meaning they alter body functions. An example of biological adaptation can be seen in the bodies of people living at high altitudes, such as Tibet. Tibetans thrive at altitudes where oxygen levels are up to 40 percent lower than at sea level. Breathing air that thin would cause most people to get sick, but Tibetans’ bodies have evolved changes in their body chemistry. Most people can survive at high altitudes for a short time because their bodies raise their levels of hemoglobin, a protein that transports oxygen in the blood. However, continuously high levels of hemoglobin are dangerous, so increased hemoglobin levels are not a good solution to high-altitude survival in the long term. Tibetans seemed to have evolved genetic mutations that allow them to use oxygen far more efficently without the need for extra hemoglobin.

Organisms can also exhibit behavioral adaptation. One example of behavioral adaptation is how emperor penguins in Antarctica crowd together to share their warmth in the middle of winter.

Scientists who studied adaptation prior to the development of evolutionary theory included Georges Louis Leclerc Comte de Buffon. He was a French mathematician who believed that organisms changed over time by adapting to the environments of their geographical locations. Another French thinker, Jean Baptiste Lamarck, proposed that animals could adapt, pass on their adaptations to their offspring, and therefore evolve. The example he gave stated the ancestors of giraffes might have adapted to a shortage of food from short trees by stretching their necks to reach higher branches. In Lamarck’s thinking, the offspring of a giraffe that stretched its neck would then inherit a slightly longer neck. Lamarck theorized that behaviors aquired in a giraffe's lifetime would affect its offspring. However, it was Darwin’s concept of natural selection, wherein favorable traits like a long neck in giraffes suvived not because of aquired skills, but because only giraffes that had long enough necks to feed themselves survived long enough to reproduce. Natural selection, then, provides a more compelling mechanism for adaptation and evolution than Lamarck's theories.

1. Place land plants on a phylogenetic tree
2. Recognize adaptations common to (nearly all) land plant taxa (cuticle, stomata, roots/root-like structures, mycorrhizal fungi)
3. Identify specific, key land plant adaptations (true roots, vascular tissue, lignin, pollen, seeds, flowers) and explain why they are adaptations to drier environments
4. Define, draw, and label the general alternation of generations life cycle
5. Differentiate major plant taxa (bryophytes, lycophytes, gymnosperms, and angiosperms) using the key adaptations to life on land and the dominant life cycle stage (gametophyte or sporophyte)
6. Identify the geologic time periods when the major land plant taxa were dominant and why they are important to humans

We have already spent quite a bit of time considering the evolutionary tree of life and the three domains of life. Now we will narrow in on one specific lineage of eukaryotes within the domain Eukarya: land plants. Note that we are specifically referring to LAND plants throughout this reading, such as mosses, ferns, conifers, and flowering plants. Algae, which are aquatic, photosynthetic eukaryotes, are also typically considered to be plants (though obviously not land plants); however, the term “algae” refers to a large and diverse group of photosynthetic eukaryotes that includes green, brown, and red algae that do not have a single common photosynethic ancestor (in other words, the term “algae” is not monophyletic). But green algae and land plants do share a common photosynthetic ancestor: land plants evolved from a group of green algae 480-470 MYA during the Ordovician Period in the Paleozoic Era in the Phanerozoic Eon. The common ancestry with green algae places plants on the phylogenetic tree of life as seen below:

A more simplified tree of life, which does not show protist lineages, would look like this:

Universal challenges and common adaptations to life on land

The information below was adapted from OpenStax Biology 25.1

The ancestor of all land plants was an aquatic, green algal-like species. Living in the water provides a number of advantages compared to life on land:

• In water or near it, plants can absorb water from their surroundings with no need for any special absorbing organ or tissue to prevent desiccation (drying out).
• Water provides a sort of external structure and buoyancy to living things; living on land requires additional structural support to avoid falling over.
• Sperm and egg can easily find each other through swimming in a water environment, and do not need protection from desiccation. Sperm and egg require alternative strategies for a) finding each other and b) avoiding drying out when on land.
• Water filters out a significant amount of ultraviolet-B (UVB) light, which is destructive to DNA. No such filtering occurs in air, so terrestrial organisms require alternative strategies for protection against UV irradiation.

If life on land presents so many challenges, why did any land plants evolve to live on land? Life on land offers several advantages—especially 470 MYA during the Ordovician Period:

• Sunlight is abundant in air compared to water. Water acts as a filter, altering the spectral quality of light absorbed by the photosynthetic pigment chlorophyll.
• Carbon dioxide is more readily available in air than in water, since it diffuses faster in air.
• Land plants evolved before land animals; therefore, no predators threatened early plant life. This situation changed as animals colonized land, where they fed on the abundant sources of nutrients in the established flora. As a result of this selective pressure by plant-eating animals, plants evolved adaptations to deter predation, such as spines, thorns, and toxic chemicals.

The transition from an aquatic to a terrestrial environment occurred as a result of a number of specific adaptations to the above challenges to survival on land. In fact, modern land plants have an array of adaptations to life on land, but they did not evolve all at once. In addition, different adaptations are present in different plant lineages. The adaptations and characteristics which ARE present in (nearly) all land plants include:

1. A waxy cuticle that covers the outer surface of the plant and prevents drying out through evaporation. The cuticle also partially protects against radiation damage from UV light.
2. Stomata (singular: stoma) are present in all land plant lineages except liverworts (similar to -but not the same as! – mosses). Stomata are pores or holes which allow for exchange of gasses (such as oxygen and carbon dioxide) between the plant cells and the environment. Stomata or similar structures are necessary in land plants because the waxy cuticle blocks free-flow of gasses.
3. Roots (or root-like structures) anchor plants to the soil and—in plants with true roots— serve as conduits for water absorption. All land plants except Bryophytes (mosses, liverworts, and hornworts) have true roots. Bryophytes have root-like structures called rhizoids that anchor them to their substrate but are not involved in water absorption (which is less important for Bryophytes because they can only survive in very moist environments).
4. Mutualistic association with mycorrhizal fungi, which are tightly associated with plant roots. Mychorrhizal fungi are associated with approximately 80% of all land plant species and provide additional surface area for absorption of both water and nutrients from the soil. The fungi share these resources with the plant roots, and—in exchange— the plant shares photosynthetic sugar products with the fungi.
5. The alternation of generations life cycle, which includes both a multicellular haploid stage and a multicellular diploid stage. Why is this an adaptation to life on land? It isn’t, in and of itself—in fact, it also occurs in *some* green algae, which are aquatic but share a common ancestor with all land plants. But specific adaptations to the alternation of generations life cycle have occurred in different lineages of plants, and those adaptations DO function as adaptations to life on land. We’ll consider these adaptations later in this reading.

Key adaptations to (increasingly drier) life on land

The information below was adapted from OpenStax Biology 25.1

Early land plants could not live very far from an abundant source of water. Over evolutionary time, land plants evolved strategies to survive in increasing degrees of dryness:

• Nonvascular plants, or Bryophytes (liverworts, mosses, and hornworts) are, in many ways, physically tied to water. Their major adaptions to life on land include a waxy cuticle and root-like structures (rhizoids). Other than those two traits, they are heavily dependent on water for their life cycle: they must live in very moist environments near sources of water. They are very short because they have no mechanism for moving water against gravity. Their sperm and eggs require water for mating: the gametes are not protected from desiccation, and the flagellated sperm must swim in water to find the egg.
• Seedless vascular plants (lycophytes, ferns, and horsetails) have two major adaptations compared to nonvascular plants: true roots and vascular tissue. These adaptations allowed seedless vascular plants to outcompete nonvascular plants in early colonization of life on land.
  • True roots grow deeper into the soil than rhizoids, allowing for better extraction of water and nutrients from the soil.
  • Vascular tissue (xylem and phloem) consists of tube-like cells that allow for transport of water (in xylem) from roots to leaves and transport of sugars (in phloem) from leaves to the rest of the plant tissues. The adaptation of vascular tissue meant that these plants could grow taller than bryophytes (and thus get more access to sunlight for photosynthesis). Lignin, a rigid component of some plant cell walls that provides structural rigidity and allows for higher movement of water against gravity and thus taller plant growth, will first evolve in these groups. 
  • Aside from these two adaptations, seedless vascular plants are still tied to the water for reproduction: like Bryophytes, their sperm and eggs are sensitive to desiccation, and the sperm must swim through water to get to the egg.
• Seeded, nonflowering plants, or gymnosperms, (gingkos, cycads, and conifers) are trees that grow to greater heights on land by combining the strength of lignin with the phenomenon of secondary growth (e.g. tree rings). They have two additional adaptations beyond seedless vascular plants, which allowed them to colonize drier habitats than nonvascular and seedless vascular plants:
  • Pollen, a mechanism for delivering sperm to egg in the absence of water. Many sources state that pollen is the same thing as sperm, which is not true. In fact, pollen produce sperm. We’ll revisit this point later on in this course, so for now, just know that pollen protects sperm from desiccation and provides a means for sperm to reach the egg in the absence of water. Seeded nonflowering plants rely on wind to transport pollen (and therefore sperm) to eggs.
  • Seeds, which protect the fertilized egg on land in multiple ways. Most obviously, the seed is a hard physical structure which protects the fertilized egg (embryo) against desiccation. But in addition, and less obviously, the seed is a form of ‘suspended animation’ for the embryo that pauses development until environmental conditions are favorable for seed germination (emergence of the embryo from the seed to start growing as a plant).
• Flowering plants, or angiosperms, possess the most recent adaptations to life on land: the flower, double fertilization and the endosperm, and fruit:
  • Flowers might not seem like an obvious adaptation to living on land, but flowers rely on pollinators (such as insects, birds, bats, and other animals) to move pollen (and therefore sperm) to eggs. Reliance on pollinators is much less random than reliance on wind, representing an important adaptation to live on land—as well as co-evolution with specific pollinators.
  • Double fertilization and the endosperm: Double fertilization is a unique process in flowering plants, where one sperm fertilizes the egg to create an embryo, and a second sperm fertilizes another structure next to the egg to create an endosperm. The endosperm undergoes a sort of pseudo-development, where it increases in mass and contents to later provide nutrients to the developing embryo during germination. We will discuss these concepts in much greater detail later in the course.
  • Fruits are any structure that aid in seed dispersal, such as something sweet that is eaten by an animal so that the seed is deposited somewhere new in the feces (and with its own personal supply of fertilizer!). Fruits thus provide a mechanism for seeds to colonize new territories away from the parent plant.

The phylogenetic tree below shows the evolutionary relationships between modern plants, as well as the origins of adaptations in each plant lineage:

Adaptations to alternation of generations in land plants

As we’ve previously discussed, all eukaryote life cycles include a haploid stage and a diploid stage. Usually one of these stages is large and multicellular (the organism we can see by eye), while the other is small and unicellular. All land plants (and *some* green algae) reproduce via the alternation of generations life cycle, where both the haploid and the diploid stage of an organism are multicellular: the haploid multicellular form, known as a gametophyte, is followed in the life cycle sequence by a multicellular diploid form: the sporophyte. The gametophyte gives rise to the gametes (reproductive cells) by mitosis. This can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain, in the vascular plants. The sporophyte stage is barely noticeable in nonvascular plants. Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines. The image below shows a simplified version of the alternation of generations life cycle:

Though all plants display an alternation of generations life cycle, there are significant variations in different lineages of plants, consistent with their evolutionary history and order of origination:

• In seedless non-vascular plants, or bryophytes, the haploid gametophyte is larger than the sporophyte (the plant structure that you see is the gametophyte); this is a gametophyte-dominated life cycle. (By “dominated” we mean “the stage of the plant you can see by eye.”) The sporophyte is attached to and dependent on the gametophyte for water and nutrients. 
• In seedless vascular plants (such as ferns), the sporophyte is larger than the gametophyte (the plant structure that you see is the sporophyte), but the gametophyte is free-living and independent from the diploid sporophyte.
• The life cycle of gymnosperms (conifers) and angiosperms (flowering plants) is dominated by the sporophyte stage (the plant structure that you see is the sporophyte), with the gametophyte remaining attached to and dependent on the sporophyte (reverse of bryophytes).
• Though they both have sporophyte-dominated life cycles, angiosperms and gymnosperms differ in that angiosperms have flowers, fruit-covered seeds, and double fertilization, while gymnosperms do not have flowers, have “naked” seeds, and do not have double fertilization.

The video below describes the features of nonvascular plants (mosses, liverworts, hornworts), and their alternation of generations life cycle:

The video below describes the features of vascular plants and their alternation of generations life cycle:

Plant evolution over geologic time

The information below was adapted from OpenStax Biology 25.1

Before we discuss evolution of plant lineages over geologic time, first let’s briefly review the relevant eras and periods of the Phanerozoic. The early era, known as the Paleozoic, is divided into six periods. It starts with the Cambrian period, followed by the Ordovician, Silurian, Devonian, Carboniferous, and Permian. The major event to mark the Ordovician, more than 500 million years ago, was the colonization of land by the ancestors of modern land plants. Fossilized cells, cuticles, and spores of early land plants have been dated as far back as the Ordovician period in the early Paleozoic era. These earliest plants to colonize land would have been nonvascular plants, lacking true leaves or roots and living in extremely damp environments close to water.

The oldest-known vascular plants have been identified in deposits from the Devonian. These now-extinct vascular plants probably lacked true leaves and roots and formed low vegetation mats similar in size to modern-day mosses, although fossils indicate that some reached up to one meter in height. Fossil evidence indicates that, by the end of the Devonian period, ferns, horsetails, and seed plants populated the landscape, giving rise to trees and forests throughout the Carboniferous. The club mosses and other seedless vascular plants dominated the landscape of the Carboniferous, growing into tall trees and forming large swamp forests alongside horsetails—some specimens reaching heights of more than 30 m (100 ft)—covering most of the land. These forests gave rise to the extensive coal deposits that gave the Carboniferous its name. The video below describes the impact and legacy of vegetation during Carboniferous period:

The vegetation covering the Earth in the Devonian and Carboniferous periods helped enrich the atmosphere in oxygen, making it easier for air-breathing animals to colonize dry land. Plants also established early symbiotic relationships with fungi, creating mycorrhizae. In the mycorrhizal relationship, the fungal network of filaments increases the efficiency of the plant root system, and the plants provide the fungi with byproducts of photosynthesis.

Gymnosperms, the earliest seed plants, also first appeared in the fossil record during the Devonian. Seedless vascular plants had previously colonized land, and the wet Devonian climate allowed the seedless plants to proliferate quickly. However, the Permian period at the end of the Paleozoic era saw much drier climates, and the dry climate provided gymnosperms an advantage over seedless plants because plants with seeds are better able to survive dry periods due to reproduction with pollen and seeds. Gymnosperms expanded in the Mesozoic era (about 240 million years ago), supplanting ferns in the landscape, and reaching their greatest diversity during this time. The Jurassic period of the Mesozoic era was as much the age of the cycads (palm-tree-like gymnosperms) as the age of the dinosaurs.

Angiosperms (flowering plants) are the most recent lineage of land plants to evolve. Fossil evidence indicates that flowering plants first appeared in the Lower Cretaceous, about 125 million years ago, and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago. Earlier traces of angiosperms are scarce, although fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, including pollinating insects that played a key role in ecology and the evolution of flowering plants. The video below describes evolution of flowering plants:

Here is the summary of these significant events in plant evolutionary history (in blue) on our geologic time scale: