1.5: First Life on Land
This epoch (spans Silurian and Devonian periods) began more than 440 million years ago and took about 85 million years. The Earth’s climate was gradually warmer, starting with a small glaciation of Gondwana (the South Pole was in Brazil), climatic situation slowly reversed, and during the Devonian period, the world was dominated by abnormally high temperatures and extremely high ocean level. This time was ended with Caledonian orogeny, the result of proto-North America and proto-Europe collision, when mountains of Scandinavia, Scotland, and eastern North America have risen.
On land, there was a radiation (i.e., evolution in different directions) of terrestrial plants. There were already several biomes : bog communities, semi-aquatic ecosystems, and more dry plant associations with domination of mosses. Once the plants have “learned” how to make chemicals that make their cell walls much stronger ( lignin and suberin ), they started to make “skyscrapers” to escape competition for the light; this allowed them to grow up to the almost unlimited height. By the end of epoch, first forests appeared, which consisted of marattioid ferns (Fig. 2.2.11, middle left), giant horsetails, mosses, and first seed plants .
Origin of seed was most likely connected with the origin of trees . Ancestors of the seed plants (it is possible that they were close to modern tongue ferns, Fig. 2.2.11) were among the first plants to acquire the cambium , “stem-cell” tissue, and, consequently, the ability of the secondary thickening their trunk. After that, growth in height was virtually unrestricted. But there was another problem: the huge ecological gap between the giant sporophyte and minuscule, short-lived gametophyte dramatically reduced protection capabilities of the sporophyte and the overall plants’ viability (a similar thing happened with dinosaurs in the Late Cretaceous).
Seed plants solved the problem and found the room for gametophyte right on the sporophyte . However, this change required plenty of coordination in the development (e.g., pollination), and initially, seed plants (like contemporary ginkgo, Fig. 2.2.12, top left, and cycads) were not much better than their sporic competitors.
At the seas, predatory vertebrates, armored fish “pushed” the old dominants, chelicerates (Fig. 2.2.26, bottom right) into the land. The last group became first terrestrial predators . There was already plenty of prey in the terrestrial fauna, in particular, millipedes and wingless proto-insects (Fig. 2.2.29, the middle). The last group (in order to escape predators) was likely forced to migrate to live on trees , and true insects appeared in the next epoch.
Shallow-water communities were dominated by advanced fish groups. The most important were ancestors of terrestrial vertebrates, lobe-finned fish (Fig. 2.2.17, 4th from top). These predatory animals, probably in order to “catch up” with the retreating water (as the tides at that time apparently extended for kilometers into the “land”), and also in the search for more food, started to develop adaptations to the terrestrial lifestyle. At the end of the epoch, they “made” organisms similar to modern amphibians, labyrinthodonts. They had many characters of terrestrial animals but likely spent most of their life in water.