The First Land-Dwelling Organisms

Although amphibians were the first vertebrates to live on land, they were not the first land-living organisms. Land plants, which probably evolved from green algae, first evolved during the Ordovician (490 million-443 million years ago). Furthermore, insects, millipedes, spiders, and even snails invaded the land before amphibians did. Fossil evidence indicates that such land-dwelling arthropods as scorpions and flightless insects had evolved by at least the Devonian (416 million-359 million years ago).

The transition from water to land required the amphibians to surmount several barriers. The most critical were extreme dryness, reproduction, the effects of gravity, and the extraction of oxygen from the atmosphere by lungs rather than from water by gills. These problems were partly solved by the crossopterygians (fleshy-finned fish known only in fossil form): they already had a backbone and limbs that could be used for walking and lungs that could extract oxygen

Nevertheless, the question remains as to when animals made the transition from water to land, and how and why it came about. The discovery in 1992 of fossilized footprints of tetrapods (four-limbed vertebrates) more than 365 million years old forced paleontologists to rethink how and when animals emerged onto land. The footprint path that Swiss geologist Iwan Stossel discovered that year on Valentia Island, off Ireland's coast, has shed light on the early evolution of tetrapods. From these footprints, geologists estimate that the creature was longer than 1 meter and had fairly large back legs. Furthermore, instead of walking on dry land, this animal was probably walking or wading around in a shallow tropical stream filled with aquatic vegetation and predatory fish. This hypothesis is based on the fact that the trackway showed no evidence of a tail being dragged behind it. Unfortunately, no hones are associated with the tracks to help reconstruct what this primitive tetrapod looked like.

One of the intriguing questions paleontologists ask is, why did limbs evolve in the first place? Probably not for walking on land.In fact, many scientists think aquatic limbs made it easier for animals to move around in streams, lakes, or swamps that were choked with water plants or other debris.The scant fossil evidence also seems to support this hypothesis.Fossils of Acanthostega, a tetrapod found in 360-million-year-old rocks from Greenland, reveal an animal with limbs but one clearly unable to walk on land.Paleontologist Jenny Clack, who recovered hundreds of specimens of Acanthostega, points out that Acanthostega's limbs were not strong enough to support its weight on land, and its rib cage was too small for the necessary muscles needed to hold its body off the ground. In addition, Acanthostega had gills and lungs, meaning it could survive on land but it was more suited for the water. Clack thinks Acanthostega used its limbs to maneuver around in swampy, plant-filled waters, where swimming would be difficult and limbs were an advantage. These shallow, aquatic environments became increasingly common during the Late Devonian, and they may have helped to drive the evolution of tetrapod limbs.

Fragmentary fossils from other tetrapods living at about the same time as Acanthostega suggest, however, that some of these early tetrapods may have spent more time on dry land than in the water. These amphibians, many of which are also found in the Upper Devonian Old Red Sandstone of eastern Greenland, had streamlined bodies, long tails, and fins along their backs. In addition to four legs, they had a strong backbone, rib cage, and pelvic and pectoral girdles, all of which were structural adaptations for walking on land. Many of these earliest amphibians seem to have inherited numerous characteristics from the crossopterygians with little modification.

The Late Paleozoic amphibians did not at all resemble the familiar frogs, toads, newts, and salamanders that make up modern amphibians. Rather, they displayed a broad spectrum of sizes, shapes, and modes of life. One group of amphibians was the labyrinthodonts, most of which were large animals, as long as 2 meters. These typically sluggish creatures lived in swamps and streams, eating fish, vegetation, insects, and other small amphibians. Labyrinthodonts were abundant during the Carboniferous (359 million-299 million years ago), when swampy conditions were widespread, but they soon declined in abundance during the Permian (299 million-251 million years ago, following the Carboniferous), perhaps in response to changing climatic conditions.