Before the Great Dying:
Life on Land in the Paleozoic
d.w.rowlands [at] gmail.com
The following is the material I covered in a one-hour class for high school students as part of the MIT Educational Studies Program's Splash 2014.
Since cells are basically membranes filled with aqueous solution, it's generally assumed that life originated in water. When the first living things moved on land is not really clear, but the first land life presumably consisted of single-celled organisms. Multicellular life complex enough to invade land really only began around the start of the Phanerozoic Eon, 541 million years ago. During this eon, which continues to the present day, multicellular life developed from a few fairly simple forms to a wide variety of organisms that have spread into most environments on the planet. The first half of the Phanerozoic, the Paleozoic Era, saw life on land go from simple microbial mats to a variety of forms that we would largely recognize today, or at least are relatively familiar with even if they're now extinct.
To adapt to life on land, organisms had to resolve several problems. First of all, they needed to manage to retain enough internal water without drying out. This is particularly difficult in dryer environments far from water. Even when they have partly resolved this, managing to reproduce without water is often a separate challenge, since gametes are usually smaller and much less protected than adult organisms.
Life on land has other problems as well. Without the buoyancy of water, an organism needs some form of rigid internal structure if it wants to be able to move or lift itself very far off the ground. It also needs protection against environmental conditions that water shields marine life from: the impact of rain and blown debris, more severe temperature changes, and UV radiation. Since shallow water offers much less protection against these than deeper water with more thermal inertia, it's unlikely that life crawled onto land from the sea. Rather, the first terrestrial organisms probably evolved from organisms adapted to lakes, swamps, or rivers.
Another thing that marine life uses water for is chemical exchange: animals extract dissolved oxygen from water and release waste carbon dioxide and nitrogen from broken-down proteins into it. Plants need to extract oxygen, carbon dioxide, and reduced nitrogen for photosynthesis, metabolism, and protein synthesis. Finally, without access to minerals dissolved in seawater, organisms need to make do with an often less-broad selection of minerals that can be found in soil. In particular, without access to as many inorganic reducing agents, life on land had to develop a number of new organic molecules to serve as anti-oxidants.
Cambrian (540-485 million years ago)
The first life on land necessarily consisted of autotrophs, since organisms that subsist on consuming organic material couldn't survive without some source of organic material. In all likelihood, this meant photosynthetic bacterial films. Films of "blue-green algae"---cyanobacteria---are still found on land in a few environments, mostly in deserts so extreme that vascular plants can't grow. These are likely the last survivors of terrestrial cyanobacteria ecosystems that existed before multicellular life ventured onto land.
The oldest fossils of terrestrial multicellular life, however, are fungi dating back to the Cambrian. Not much is known about these early fungal colonists: some may have been lichens, relying on cyanobacteria endosymbiotes to provide food via photosynthesis. Others may have been feeding of the bacterial-film ecosystem that had already existed for some time.
Ordovician (485-445 million years ago)
When the Ordovician began, sea levels were at the highest they've been since the start of the Paleozoic, and the planet was very warm: mean ocean temperature seems to have been around 45C. However, the spread of life onto land during the period changed the face of the planet significantly. During the Ordovician, the first land plants, or embryophytes, developed from green algae. Their tolerance of surface conditions was largely an extension of earlier adaptations to living in shallow ponds, but there were two major changes necessary to actually make the transition to life on land. First, embryophyte cells have one very large central vacuole in the center of the cell that serves to maintain pressure on the cell walls and help keep the cells and the plant rigid. Second, their fertilized eggs form embryos with protective waterproof layers that allow them to germinate on try land.
The spread of these plants, along with some early annelids and arthropods---probably chelicerata, the group that contains horseshoe crabs and arachnids---led to major changes on the surface. First of all, they began the creation of soil from the lifeless rock and sand that had previously covered all dry land on Earth. Second, the plants flourished in the high carbon dioxide atmosphere enough to cause the carbon dioxide levels to crash. By the end of the Ordovician, Earth had entered an ice age, causing sea levels to drop worldwide and resulting in the first mass extinction in the fossil record.
Silurian (445-420 million years ago)
The Earth returned to a greenhouse state with no glaciers and high sea levels in the Silurian, but did not get as hot as it had been in the early Ordovician. Land plants became even more widespread, as vascular plants evolved. Vascular plants have water-conducting tubes made of the hydrophobic compound lignin. These tubes allow water to be conducted much more efficiently than through tubes made of cells with membranes through which water can diffuse, and they also contribute structural strength. As a result, vascular plants are able to grow upward and not just cling to flat surfaces as non-vascular land plants must.
Four of the five major arthropod lineages---excepting the trilobites, which had been severely decimated in the Ordovician-Silurian mass extinction and never really recovered---made it onto land in the Silurian. In each case, they developed similar organs for eliminating nitrogenous wastes by producing uric acid, which is much less toxic and requires much less water to remove it than ammonia, which their marine ancestors had produced to eliminate nitrogen. They also evolved internal fertilization, though some group's eggs still needed to be laid in water or very damp environments.
The arthropod branch of the chelicerata became the best adapted to life on land: they are the only arthropod group to develop lungs, a simple system of "book lungs" that consist of alternating air pockets and tissues filled with blood connected to an opening along the bottom of the abdomen. The myriapoda---centipedes and millipedes---and hexapoda---insects---also got their start on land at this time and were quite successful in adapting to it, though the myriapoda remained relatively dependent on oxygen transmission through their skin and so never fully adapted to non-damp environments. Insects adapted more fully to land, though they never developed proper lungs and instead breathe only through air tubes called tracheae that expand cycle air as they move their bodies. A few crustacean groups also took to land at this time, though most of them did not adapt as fully to it.
Devonian (420-360 million years ago)
As the climate continued to warm in the Devonian, life on land began to become significantly more diverse, and began to have a larger effect on geography. Both lineages of vascular plants---the euphyllophytines, the ancestors of ferns and seed-bearing plants, and lycopodiophytes, the ancestors of a few small modern plants such as club mosses---began to develop into trees for the first time, with significant root systems that reduced erosion and decreased the amount of sediment in rivers, leading to a planet-wide transition from braided to meandering river systems.
In the late Devonian, seed-bearing plants developed for the first time. Like all plants, they reproduce through a process called "alternation of generations" in which adult plants are diploid (have two copies of each chromosome) and produce haploid (with one copy of each chromosome) spores that grow into haploid plants that produce the actual eggs and sperm that will grow into an adult plant. Seed-bearing plants have made themselves more drought-tolerant by containing this entire process within waterproof membranes. Male spores develop inside pollen grains to form sperm, while female spores develop inside seeds to produce eggs. The haploid male plants inside the sperm then drill through the shell of the unfertilized seeds to deposit sperm and fertilize the egg cell. Lycophytes nearly developed a similar system independently, but their pollen is incapable of drilling into the egg-containing capsule and so depends on it having an opening that keeps the seeds from being properly desiccation-resistant.
Arthropod life on land radiated significantly in the Devonian, although most modern groups did not yet exist. Meanwhile, in the late Devonian, vertebrates ventured onto land for the first time. The first tetropod amphibians seem to have developed in swampy areas where the water was fairly anoxic due to decomposing leaves from trees. They used their limbs to propel themselves through the tangle of roots and vegetation underwater and occasionally to jump above the surface to grab arthropods on floating vegetable matter and to gulp air to supplement the limited oxygen in the water, leading to the origin of amphibian positive-pressure respiration. Fish had already developed urea as a less toxic and more water-soluble alternative to ammonia as a means of disposing of nitrogenous waste, probably as an adaptation to their large size, and this relieved the early amphibians of the need to develop a new nitrogen-removal system as the arthropods had needed to.
Carboniferous (360-300 million years ago)
The sequestration of carbon in trees that had begun in the Devonian continued into the Carboniferous as trees got taller and taller, and the climate cooled to present-day temperatures. This effect was magnified by the fact that fungi had not yet developed lignin-digesting white rot, which meant that tree trunks couldn't fully decay, since there was no organism capable of breaking down lignin. Instead, large amounts of lignin-containing wood ended up being buried and becoming the coal fields of today. White rot evolved near the end of the Carboniferous and, ever since, coal has formed at much, much lower rates, since almost all vegetable matter rots before it gets the chance to become coal.
The carbon sequestration and increasingly high oxygen levels---over a hundred and fifty percent of the current atmospheric oxygen---allowed insects to become incredibly successful despite the limited efficiency of their lung-less breathing. A number of modern insect lineages first appear at this time, including cockroaches, crickets, and dragonflies, though wings that could be folded away when not in use didn't evolve until the late Carboniferous. Relatives of dragonflies called griffin flies developed multiple-foot wingspans and were the largest flying insects ever. Meanwhile, the palaeodictyoptera became the first really important terrestrial herbivores and pushed trees to develop very thick bark as a defensive measure.
Amphibians, which also have relatively inefficient respiratory systems, also proliferated in the Carboniferous's high-oxygen conditions and some grew to be apex predators several meters in length. The first land gastropods (slugs and snails) are found in the Carboniferous rainforests, but they are uncommon. While almost all other transitions to life on land seem to have occurred in the Paleozoic, most mollusc groups to come on land did so in the Mesozoic or even Cenozoic.
Near the end of the Paleozoic the growth of huge rainforests sequestering carbon and boosting oxygen levels collapsed. The higher oxygen levels made forest fires much more common, and the low carbon dioxide levels in the atmosphere plunged the planet into an ice age, resulting in the decimation of the large rainforests that had covered much of the planet.
Permian (300-250 million years ago)
The Permian climate was much dryer than the Carboniferous had been. While this decreased the ecological niches open to amphibians, it provided an opening for a group of tetrapods called amniotes. Like some amphibians, amniotes reproduced by internal fertilization. However, they laid eggs surrounded by an amnios, a sack that kept water in while allowing oxygen to diffuse through for the developing embryo. These eggs had leathery shells and could be laid on land. Amniotes also evolved improved lungs that inflated via negative pressure and no longer needed to keep their skin damp to enable respiration through it, making them the first vertebrates fully capable of life on land.
The main successful group of amniotes during the Permian was the synapsids or mammal-like reptiles. However, as the climate became even dryer in the Triassic period, they were decimated while the sauropsids, or bird-like reptiles, became the dominant land vertebrates. Sauropsids were particularly adapted for dry conditions because of their scaly skin, unlike the gland-filled skin of synapsids, and because they'd evolved the ability to excrete nitrogenous waste as uric acid, which is much more water soluble than urea.
The lower-oxygen and dryer Permian was not a particularly good time for insects, although beetles did originate during this time. Most modern insect groups, including butterflies, ants, bees, termites, and flies evolved in the Mesozoic in response to the Cretaceous flourishing of flowering plants.