Colonization of the land

The advent of animals and plants on land during the Ordovician period added new complexity to preexisting ecosystems of microbes. The newly increased mass of vegetation on land served to stabilize soils against erosion and promoted the weathering of their nutrient minerals. Arthropods, too, found a place in this early ecosystem of nonvascular plants on land.

Appearance of Life on Land

The appearance of animals and plants on land by the Middle Ordovician period, some 450 million years ago, was a major event in the evolution of terrestrial ecosystems. Nevertheless, they probably were not the Earth's first inhabitants. There is a fossil record of blue-green algae and other microscopic life well back into Precambrian time, as much as 3.5 billion years ago. Indeed, it is doubtful that plants and animals visible to the naked eye could have lived on land without preexisting microbial ecosystems, which served to stabilize minerals in soils, decompose and circulate organic matter of dead organisms, and oxygenate the atmosphere by photosynthesis. The increased mass of more complex animals and plants on land during Ordovician time further stabilized soils, invigorated the recycling of organic matter, and boosted atmospheric oxygenation. In addition, large plants provided greater depth and structure to terrestrial ecosystems than was possible with microbes and so may have promoted photosynthetic efficiency, biological diversity, and perhaps also resistance to disturbance by floods and storms. This self-reinforcing boost to terrestrial productivity firmly established life on land.

Because there are marine fossils of plants and animals visible to the naked eye in Precambrian rocks (at least 600 million years old), it has commonly been assumed that the earliest creatures on land during the Ordovician and Silurian periods invaded from the sea. Reasons advanced to explain why the land was unavailable for marine creatures for more than 200 million years include the lack of available oxygen, the poverty of terrestrial microbial photosynthetic productivity, and an unpredictable land surface of flash floods and erosional badlands. This view of an invasion from the sea has been used to explain the origins of the earliest land animals, which probably were arthropods, such as millipedes and spiders. Various fossil arthropods have been found in Cambrian, Ordovician, and Silurian deposits of shallow seas and estuaries. Like modern marine crabs, these creatures may have ventured out to a limited extent on land, and some may have become more fully adapted to more difficult conditions there. The external skeletons of arthropods, important for defense in the sea, also are effective for support, movement, and preventing desiccation on land.

In contrast, millipedes and spiders are not very closely related either to any known fossil or to living aquatic arthropods. A reassessment of the earliest fossil scorpions, formerly regarded as possible early land animals, has shown that they had a breathing apparatus that would have been effective only in water. Substantial evolution on land must have occurred to produce the earliest spiders and millipedes, perhaps from microscopic early microbial feeders that have left no fossil record.

Immigrant Versus Indigenous Evolution

The idea of invasion of the land by marine and freshwater algae is supported to some extent by the close biochemical similarities between modern land plants and charophytes (a kind of pond weed commonly called stonewort because of its calcified egg cells). Charophytes, however, are very different from land plants, and it is unlikely that such soft-bodied aquatic algae in the geological past were any more successful in colonizing the land than are the mounds of rotting seaweed now thrown up on beaches by storms. Land plants differ from stoneworts and seaweeds in many ways: They have a waxy and proteinaceous outer coating (cuticle) to prevent desiccation and to allow the plant body to remain turgid through internal water pressure; they have small openings (stomates) surrounded by cells that can open and close the opening to control loss of water and oxygen and intake of carbon dioxide; they have internal systems of support and water transport, which include tubular thick-walled cells (hydroids) in nonvascular plants, such as mosses and liverworts, and elongate cells with helical or banded woody thickenings (tracheids) in vascular plants; they have roots, unicellular root hairs, or rootlike organs (rhizoids) that gather water and nutrients from soil; and they have propagules (spores) protected from desiccation and abrasion by proteinaceous envelopes. To some botanists, the coordinated evolution of all these features from aquatic algae is extremely unlikely, notwithstanding the impressive diversity of algae today. This consideration, plus the simple nature of the earliest fossil land plants, has led to the argument that land plants evolved on land from microscopic algae already accustomed to such conditions.

While immigrant versus indigenous evolutionary origins of the earliest land creatures remains a theoretical problem, there is fossil evidence of very early land ecosystems. In Late Ordovician rocks are found the earliest spores of land plants. Most of them are smooth and closely appressed in groups of four, somewhat similar to spores of liverworts and mosses today. This is not to say that they belonged to liverworts and mosses; no clear fossils of land plants visible to the naked eye have yet been found in rocks of this age. Early moss and liverwort ancestors are found in Silurian rocks, but so are extinct nonvascular plants, such as nematophytes. These early experiments in the evolution of land plants had tissues supported by densely interwoven proteinaceous tubes. In life, they had the rubbery texture of a mushroom and various bladelike and elongated forms like those of modern algae.

Although the botanical affinities of the earliest spores of nonvascular land plants remain unclear, there is evidence that they grew in clumps. Buried soils of the Late Ordovician age have been found with surficial erosion scours formed by wind around clumps of vegetation. The clumps are represented by gray spots from the reducing effect of remnant organic matter. Burrows also have been found in Late Ordovician buried soils to indicate animals in these early land ecosystems. The fossil burrows are quite large (two to tweny-one millimeters). They are similar in their clayey linings, backfill structures, and fecal pellets to the burrows of modern roundback millipedes. The buried soils are calcareous and strongly ferruginized—indications that they were nutrient-rich, periodically dry, and well-drained, as are modern soils preferred by millipedes. Actual fossils of millipedes have not yet been found in rocks older than Late Silurian, so all that can be said at present is that these very early animals on land were, in some ways, like millipedes.

Diversification of Life on Land

By Silurian time (some 438 million years ago), there was a considerable diversification of life on land. Spores of fungi and of vascular land plants have been found fossilized in Early Silurian rocks. During the Mid-Silurian time, there were small, leafless plants with bifurcating rhizomes and photosynthetic stems terminated by globular, spore-bearing organs. These matchstick-sized fossil plants have been called Cooksonia. Although not so well preserved as to show their water-conducting cells, they have been regarded as the earliest representatives of the extinct group of vascular plants called rhyniophytes. In Devonian rocks (some 408 million years old), some well-preserved rhyniophytes are known to have been true vascular plants, but there are other plants similar in general appearance that had simpler thick-walled conducting cells like those of nonvascular plants. By Devonian time, there were also vascular plants with spore-bearing organs borne above lateral branches (zosterophylls), plants with true roots and spore-bearing organs borne in clusters (trimerophytes), and spore-producing plants with woody roots and tree trunks (progymnosperms). The evolution of the earliest vascular plant cover on land, and of the first forests, involved different kinds of plants now extinct.

The fossils of millipedes were found as early as the Silurian age. Over time, millipedes diversified. By the Devonian age there were spiders, centipedes, springtails (Collembola), and bristletails (Thysanura). The earliest land vertebrates were identified from the bones of extinct amphibians (Ichthyostegalia) and from footprints of Devonian age. Some of the bones and footprints were 370 million years old.

This great Silurian and Devonian evolutionary radiation promoted environmental changes similar to those initiated by the first colonization of land by plants and animals, as well as some new changes. For example, the formation of charcoal from wildfires in woodlands and the accumulation of peat in swamps were ways of burying carbon that otherwise might have decayed or been digested into carbon dioxide in the atmosphere. This removal of carbon dioxide allowed increased oxygenation of the atmosphere. Oxygenation was kept within bounds by increased flammability of woodlands when oxygen reached amounts much more than modern atmospheric level.

Late Devonian ecosystems were very different from modern ones. Major ecological roles, such as insect-eating large land animals, were still being added. More changes were to come, but the world at that time would have seemed a much more familiar place than the meadows of Cooksonia during the Silurian, the patchy cover of Ordovician nonvascular plants, and the red and green microbial earths of earlier times.

Principal Terms

arthropods: a group of animals lacking backbones but with rigid, proteinaceous, and mineralized external skeletons that are jointed at the limbs and at other points of movement; includes crabs, spiders, and insects

charophyte: a kind of freshwater green alga with filamentous branches arranged in whorls around a threadlike central stem, characterized by egg cells that are large compared with those of other algae

cuticle: an outer, thin, waterproof, waxy, and proteinaceous cover to the bodies of land plants and arthropods

millipede: a kind of arthropod with an elongated body of many nearly identical segments, each with a pair of limbs on each side

nematophyte: an extinct kind of nonvascular land plant consisting of flat sheets, stems, or trunks, supported by densely interwoven, microscopic, proteinaceous tubes

photosynthesis: the process by which plants and some microbes create organic matter and oxygen from carbon dioxide and water, using the energy of the sun and a catalytic pigment

rhizome: an organ of vascular plants with the anatomy of a stem rather than of a root but running along or under the ground more like a root than a stem

rhyniophyte: an extinct kind of vascular land plant with rhizomes and simple spore-bearing organs but lacking true roots or leaves

spore: a reproductive propagule with a tough, acid- and desiccation-resistant, external, proteinaceous coat in land plants lacking seeds

stomate: an opening in the surface of the green parts of vascular land plants that can be closed by the flexing of adjacent cells to control gas exchange between the atmosphere and the plant interior

vascular plants: those plants with elongate, woody, water-conducting, tubular cells (tracheids) in their stems or veins of leaves; now includes ferns, conifers, and flowering plants

Bibliography

Benton, Michael J. Vertebrate Palaeontology. 4th ed., Blackwell Science, 2015.

Friedman, Matt. “Emerging onto a Tangled Bank.” Science, vol. 324, 2009, pp. 341-42.

Geological Society of London. A Global Synthesis of the Ordovician System. Part 2. The Geological Society, 2023.

Hill, Christopher L. "Geoarchaeology, History." Encyclopedia of Geoarchaeology. Cham: Springer International Publishing, 2024. p. 1-12.

Pires, Nuno D., and Liam Dolan. “Morphological Evolution in Land Plants: New Designs with Old Genes.” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, vol. 367, no. 1588, 2012, pp. 508-18. doi:10.1098/rstb.2011.0252.

Prothero, D. R., and Robert H. Dott. Evolution of the Earth. 8th ed., McGraw-Hill, 2009.

Schopf, J. William, editor. Major Events in the History of Life. Boston: Jones and Bartlett, 1992.

Stebbins, G. L., and G. J. C. Hill. “Did Multicellular Plants Invade the Land?” American Naturalist, vol. 115, 1980, pp. 342-53. doi.org/10.1086/283565.

Tinkler, Keith J. A Short History of Geomorphology. Routledge, 2020.

Wicander, Reed, and James S. Monroe. Historical Geology. 8th ed., Brooks/Cole, Cengage Learning, 2016.

Wang, Xianyan, et al. Special External Effects on Fluvial System Evolution. MDPI AG, 2019.

Zimmer, Carl. At the Water's Edge: Macroevolution and the Transformation of Life. New York: Free Press, 1998.