One of the most significant evolutionary events that occurred on Earth was the transition of water-dwelling fish to terrestrial tetrapods (four-limbed organisms with backbones). Fish probably originated in the oceans, and our first records of them are in marine rocks. However, by the Devonian Period (408 million to 362 million years ago), they had radiated into almost all available aquatic habitats, including freshwater settings. One of the groups whose fossils are especially common in rocks deposited in fresh water is the lobe-finned fish.

The freshwater Devonian lobe-finned fish rhipidistian crossopterygian is of particular interest to biologists studying tetrapod evolution. These fish lived in river channels and lakes on large deltas. The delta rocks in which these fossils are found are commonly red due to oxidized iron minerals, indicating that the deltas formed in a climate that had alternate wet and dry periods. If there were periods of drought, any adaptations allowing the fish to survive the dry conditions would have been advantageous. In these rhipidistians，several such adaptations existed. It is known that they had lungs as well as gills for breathing. Cross sections cut through some of the fossils reveal that the mud filling the interior of the carcass differed in consistency and texture depending on its location inside the fish. These differences suggest a sadlike cavity below the front end of the gut that can only be interpreted as a lung. Gills were undoubtedly the main source of oxygen for these fish, but the lungs served as an auxiliary breathing device for gulping air when the water became oxygen depleted, such as during extended periods of drought. So, these fish had already evolved one of the prime requisites for living on land: the ability to use air as a source of oxygen.

A second adaptation of these fish was in the structure of the lobe fins. The fins were thick, fleshy, and quite sturdy, with a median axis of bone down the center. They could have been used as feeble locomotor devices on land, perhaps good enough to allow a fish to flop its way from one pool of water that was almost dry to an adjacent pond that had enough water and oxygen for survival. These fins eventually changed into short, stubby legs. The bones of the fins of a Devonian rhipidistian exactly match in number and position the limb bones of the earliest known tetrapods, the amphibians. It should be emphasized that the evolution of lungs and limbs was in no sense an anticipation of future life on land. These adaptations developed because they helped fish to survive in their existing aquatic environment.

What ecological pressures might have caused fishes to gradually abandon their watery habitat and become increasingly land-dwelling creatures? Changes in climate during the Devonian may have had something to do with this if freshwater areas became progressively more restricted. Another impetus may have been new sources of food. The edges of ponds and streams surely had scattered dead fish and other water-dwelling creatures. In addition, plants had emerged into terrestrial habitats in areas near streams and ponds, and crabs and other arthropods were also members of this earliest terrestrial community.Thus, by the Devonian the land habitat marginal to freshwater was probably a rich source of protein that could be exploited by an animal that could easily climb out of water. Evidence from teeth suggests that these earliest tetrapods did not utilize land plants as food; they were presumably carnivorous and had not developed the ability to feed on plants.

How did the first tetrapods make the transition to a terrestrial habitat? Like early land plants such as rhyniophytes, they made only a partial transition; they were still quite tied to water. However, many problems that faced early land plants were not applicable to the first tetrapods. The ancestors of these animals already had a circulation system, and they were mobile, so that they could move to water to drink. Furthermore, they already had lungs, which rhipidistians presumably used for auxiliary breathing. The principal changes for the earliest tetrapods were in the skeletal system—changes in the bones of the fins, the vertebral column, pelvic girdle, and pectoral girdle.

水中栖息的鱼类进化为陆地四足动物（有脊椎的四肢生物）是地球上发生过的重大进化事件之一。鱼类很可能起源于海洋，有关鱼类的最早记录是在海洋岩石中。然而，到了泥盆纪（4.08亿至3.62亿年前），鱼类的栖息地已经扩展到几乎所有可能的水生环境，包括淡水水域。淡水沉积岩石中十分常见的是一种叶状鳍鱼化石。

研究四足动物进化的生物学家对泥盆纪时期生活在淡水水域的总鳍鱼纲、扇鳍鱼目叶状鳍鱼尤其感兴趣。这种鱼生活在大型三角洲的河道和湖泊里。它们的化石常见于三角洲岩石中，这类岩石因为含有氧化铁矿物，故而呈红色，这也表明三角洲形成时已经有干湿季节的交替。如果有干旱期存在，任何有助于鱼类度过干旱的进化都会是有利的。这些扇鳍鱼身上也出现了几处这样的进化。我们知道扇鳍鱼的呼吸器官除了鳃还有肺。一些化石的横截面显示，扇鳍鱼尸体内的泥的稠度和质地根据其在体内位置的不同而不同。这些差异表明在鱼肠前端下面像腔一样的东西只可能是肺。鳃无疑是这些鱼的主要氧气来源，但当水中含氧量大幅减少时，比如在长期干旱的情况下，肺就会作为辅助呼吸系统来吸气。可见，这些鱼已经进化出陆地生存的必备条件之一：将空气作为氧气来源的能力。

这些鱼的另一个适应是在叶状的鳍。这些鳍很厚、很有肉质感、而且十分结实，内有一条轴心骨。在陆地上鱼鳍从一定程度上可以作为运动的装置，或许能使鱼为了生存从几近干涸的池塘挪动到相邻的有足够水和氧气的池塘。这些鳍最终进化成了短而健壮的腿。泥盆纪扇鳍鱼的鳍骨在数量和位置上与已知的最早的四足动物的肢骨完全匹配。应该强调的是，肺和四肢的进化并不是因为它们预料到未来要在陆地上生活。这些适应是为了帮助它们在当时的水生环境中存活下来。

是什么样的生态压力导致鱼类逐渐放弃在水中的栖息地，逐渐成为陆生生物呢？如果淡水区域逐步缩减，泥盆纪的气候变化可能与此相关。另一个动力可能是新的食物来源。池塘和溪流的边缘无疑会散落有死掉的鱼和其他水栖生物。此外，溪流与池塘附近的陆地开始有植物出现，蟹和其他节肢动物也加入了这一最早的陆地群落。因此，在泥盆纪时期，对于能够轻易爬出水面的生物，淡水边缘的陆地很可能是丰富的蛋白质食物来源。对四足动物的牙齿的研究表明，最早的四足动物并不食用陆生植物；它们可能是食肉动物，而且也尚未形成食用植物的能力。

第一批四足动物是怎样向陆地环境过渡的呢？就像早期的陆生植物莱尼蕨类一样，这些四足动物只是产生了部分变化，这个阶段，它们仍然很依赖于水。不过，早期陆生植物面临的许多问题并不会困扰到它们。这些动物从祖先起就已经有了一个流通系统，它们可以移动，所以可以挪到水边饮水。另外，它们已经有肺了，当时扇鳍鱼可能都已经将其用于辅助呼吸了。最早期的四足动物主要进化的是骨骼系统，即鳍骨、脊柱、骨盆、肩胛骨发生变化。