The appearance of terrestrial plants and animals during the early Palaeozoic (570-245 million years ago/ma) was one of the most important developments in the history of life on Earth. All tetrapods (backboned animals; all amphibians, reptiles, birds and mammals) can ultimately trace their family tree to the first organisms which lurched out of the water and gasped for breath. It wasn’t a rapid process, of course. Prior to the appearance of the first land plants and amphibians and pursuant to the so-called ‘Cambrian Explosion’ c. 570 ma, life had primarily been dominated by primitive molluscs and fish (which in turn developed from the soft-bodied Ediacaran fauna). Before that, nondescript bacteria of various kinds quietly photosynthesised until we trace the line back to the origin of life itself, some 3.8 billion years ago.
But I digress - life on land, yes.
So, why did it happen? Well, the incentives were certainly there: land would initially have (initially) promised few predators, adequate food supplies (once plants became established) or direct sunlight for photosynthetic plants - despite the damage and dehydration which ultraviolet radiation (UVR) could cause - and would have alleviated the pressure upon overpopulated ocean environments. Ultraviolet radiation, though, could be fatal and it was the dry, arid conditions on land which the first terrestrial organisms battled the most fiercely. Before I proceed with a chronology of evolutionary processes, I shall present a summary of the primary biotic developments (generally though to be all that one needs to remember).
In plants, these included: Sporangia (spore cases, designed to shield spores from UVR).
Vertical growing stems (which required lignin, a substance conspicuously absent in the earliest plants. Upright plants were better able to harness sunlight).
Cuticles (waxy coverings designed to reduce unwanted evaporation and UVR damage).
Stoma (minute pores in the epidermis of a leaf which allow for the passage of gas and water vapour).
Root systems (for anchorage and the acquisition of nutrients from the soil).
Whereas in animals, these included: Appendages and stronger muscles (for locomotion and support; the loss of a buoyant environment was one of the primary flaws of this relocation).
Protective skin (UVR and dehydration were constant threats and it is for this reason that amphibians and reptiles developed water-retentive and UV-protective coverings).
A more complex neural system (enabling the organism to maintain its orientation on land)
Lungs (more oxygen was required for life on land - gills were inefficient).
Internal fertilisation and harder eggshells (animals had previously relied on water to distribute sex cells and prevent dehydration. This was no longer possible, although many animals returned to the water to procreate for a long time).
And now, the chronology…
The climate near the beginning and middle of the Ordovician period (505-438 ma) was generally warm and atmospheric moisture levels were high, a product of unrestricted ocean currents (in turn due to the continued sundering of Pangaea) which formed warm, shallow seas. Gondwana drifted toward the South Pole during this time, eventually coming to rest there. Large glaciers were formed and shallow seas were drained, exposing a significantly larger amount of land. It is at this time that we see the first plants emerging on land, evidenced by samples of cell microfossils, cuticles and sporangia, although these were very primitive and lacked most of the characteristics of modern plants. Ice cores and similar testing methods have revealed that the late Ordovician saw the atmospheric oxygen content leap about 15% from the beginning of the period as plants colonised the land far ahead of any animals (whose survival there was, in fact, reliant on plants).
Vascular plants (i.e. those with circulatory systems) did not emerge until the Silurian (438-408 ma). The Silurian was a climatically stable period and many of the larger glacial formations began to melt. Sea levels rose and jawless fish proliferated. Due to mutations, environmental conditions and geographical isolation, the first jawed and freshwater fish (the most likely candidates for becoming terrestrial organisms) came into being. The only mechanisms which were then truly necessary for these animals to emerge from the water were a more efficient gas exchange mechanism and a means of locomotion; lungs and legs, respectively. To this we shall return. Algae, liverworts and moss-like growths began to grow adjacent to land - these were fed upon by segmented sea animals (the ancestors of modern arachnids).
The appearance of vertebrate life marked the end of the Devonian period (408-360 ma). By this point, the freshwater fish had developed a floatation bladder and a complex vascular system which enabled a comparatively immense amount of oxygen storage (which ensured their survival when oxygen supplies in the water were low). Lungfish were the product of this biological innovation; with the ability to breathe outside water, tough, scaly skin (protective against UVR), rudimentary limbs and large muscles (suited both to buoyant aquatic and more burdensome terrestrial environments), these animals could spend at least some time outside of water; it must be borne in mind that despite these drastic changes, the lungfish’s skeleton was in places comprised only of cartilage and the respiratory system permitted only short stints on land.
This may have seemed an inauspicious beginning, and so it was. The real move onto land probably came when the descendents of the lungfish gained the ability to walk more fluidly - the further development of locomotive appendages has been theorised to be the product of the need for those pioneering organisms to manoeuvre through shallow and weed-choked water. These would be the first amphibians, heralding the beginning of the Carboniferous (360-290 ma). These organisms were generally distinguished by the presence of an air passage from the nostrils to the roof of the mouth and other anatomical differences such as skeletal structure (particularly strengthening endoskeletons and composition, number and shape of teeth and joints). Atmospheric oxygen content dropped 10% during the Devonian as lungs became prolific and the race to colonise the land saw the emergence of ecosystems very similar to those of modern Earth.
Sources:’Spotlight’ Earth and Environmental Science (HSC), written by David Heffernan, Rob Mahon, John McDougall and Kylie Gillies for the NSW HSC syllabus.
http://www.glg.ed.ac.uk/home/s9810658/eastlothian/timescale.gif (one of many geological timescales)