Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size must create special structures to withstand gravity;
respiration and
gas exchange systems have to change; reproductive systems cannot depend on water to carry
eggs and
sperm towards each other. Although the earliest good evidence of land plants and animals dates back to the Ordovician period (), and a number of microorganism lineages made it onto land much earlier, modern land
ecosystems only appeared in the Late
Devonian, about . In July 2018, scientists reported that the earliest life on land may have been
bacteria living on land 3.22 billion years ago. In May 2019, scientists reported the discovery of a
fossilized
fungus, named
Ourasphaira giraldae, in the
Canadian Arctic, that may have grown on land a billion years ago, well before
plants were living on land.
Evolution of terrestrial antioxidants Oxygen began to accumulate in Earth's atmosphere over 3 Ga, as a by-product of
photosynthesis in cyanobacteria (blue-green algae). However, oxygen produces destructive chemical
oxidation which was toxic to most previous organisms. Protective endogenous antioxidant enzymes and exogenous dietary antioxidants helped to prevent oxidative damage. For example, brown algae accumulate inorganic mineral
antioxidants such as
rubidium,
vanadium,
zinc, iron,
copper,
molybdenum,
selenium and
iodine, concentrated more than 30,000 times more than in seawater. Most marine mineral antioxidants act in the cells as essential
trace elements in
redox and antioxidant
metalloenzymes. When plants and animals began to enter rivers and land about 500 Ma, environmental deficiency of these marine mineral antioxidants was a challenge to the evolution of terrestrial life. Terrestrial plants slowly optimized the production of new endogenous antioxidants such as
ascorbic acid,
polyphenols,
flavonoids,
tocopherols, etc. A few of these appeared more recently, in the last 200–50 Ma, in
fruits and
flowers of
angiosperm plants. In fact, angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the
Late Jurassic period. Plants employ antioxidants to defend their structures against
reactive oxygen species produced during photosynthesis. Animals are exposed to the same oxidants, and they have evolved endogenous enzymatic antioxidant systems.
Iodine in the form of the iodide ion I− is the most primitive and abundant electron-rich essential element in the diet of marine and terrestrial organisms; it acts as an
electron donor and has this ancestral antioxidant function in all iodide-concentrating cells, from primitive marine algae to terrestrial vertebrates.
Evolution of soil Before the colonization of land there was no
soil, a combination of mineral particles and decomposed
organic matter. Land surfaces were either bare rock or shifting sand produced by
weathering. Water and dissolved nutrients would have drained away very quickly. It has been argued that in the late Neoproterozoic
sheet wash was a dominant process of erosion of surface material due to the
lack of plants on land. s growing on
concrete Films of cyanobacteria, which are not plants but use the same photosynthesis mechanisms, have been found in modern deserts in areas unsuitable for
vascular plants. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to
intertidal zones and then to land.
Lichens, which are
symbiotic combinations of a fungus (almost always an
ascomycete) and one or more photosynthesizers (green algae or cyanobacteria), are also important colonizers of lifeless environments,
Plants and the Late Devonian wood crisis '', a
vascular plant from the
Silurian s from the Middle
Devonian Gilboa Fossil Forest In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land requires plants to become internally more complex and specialized: photosynthesis is most efficient at the top; roots extract water and nutrients from the ground; and the intermediate parts support and transport. By the Late Devonian , abundant trees such as
Archaeopteris bound the soil so firmly that they changed river systems from mostly
braided to mostly
meandering. This caused the "Late Devonian wood crisis" because: • They removed more carbon dioxide from the atmosphere, reducing the
greenhouse effect and thus causing an
ice age in the
Carboniferous period. This did not repeat in later ecosystems, since the carbon dioxide "locked up" in wood was returned to the atmosphere by decomposition of dead wood, but the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian. • The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused
algal blooms whose high consumption of oxygen caused
anoxic events in deeper waters, increasing the extinction rate among deep-water animals. Its air-breathing, terrestrial nature is evidenced by the presence of
spiracles, the openings to
tracheal systems. However, some earlier
trace fossils from the Cambrian-Ordovician boundary about are interpreted as the tracks of large
amphibious arthropods on coastal
sand dunes, and may have been made by
euthycarcinoids, which are thought to be evolutionary "aunts" of
myriapods. Other trace fossils from the Late Ordovician a little over probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and
alluvial plains shortly before the Silurian-Devonian boundary, about , including signs that some arthropods ate plants. Arthropods were well
pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. The
fossil record of other major invertebrate groups on land is poor: none at all for non-
parasitic flatworms,
nematodes or
nemerteans; some parasitic nematodes have been fossilized in
amber; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of
gastropods on land date from the Late Carboniferous, and this group may have had to wait until
leaf litter became abundant enough to provide the moist conditions they need. The earliest confirmed fossils of flying
insects date from the Late Carboniferous, but it is thought that insects developed the ability to fly in the Early Carboniferous or even Late Devonian. This gave them a wider range of
ecological niches for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment. About 99% of modern insect species fly or are descendants of flying species.
Amphibians '' changed views about the early evolution of
tetrapods. The early groups are grouped together as
Labyrinthodontia. They retained aquatic, fry-like
tadpoles, a system still seen in
modern amphibians. Iodine and
T4/T3 stimulate the amphibian metamorphosis and the
evolution of nervous systems transforming the aquatic, vegetarian tadpole into a "more developed" terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting. From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of
Acanthostega from about showed that this Late Devonian
transitional animal had
legs and both
lungs and
gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that
Acanthostega, which was about long, was a wholly aquatic predator that hunted in shallow water. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the
shoulder girdle and it has a distinct neck. The Devonian proliferation of land plants may help to explain why air breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less
dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen. Unfortunately, there is then a gap (
Romer's gap) of about 30 Ma between the fossils of ancestral tetrapods and Middle Carboniferous fossils of vertebrates that look well-adapted for life on land, during which only some fossils are found, which had five digits in the terminating point of the four limbs, showing true or crown
tetrapods appeared in the gap around 350 Ma. Some of the fossils after this gap look as if the animals which they belonged to were early relatives of modern amphibians, all of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the
amniotes, whose waterproof skin and egg membranes enable them to live and breed far from water. The
Carboniferous Rainforest Collapse may have paved the way for amniotes to become dominant over amphibians.
Reptiles Possible family tree of
dinosaurs,
birds and
mammals The synapsid
pelycosaurs and their descendants the
therapsids are the most common land vertebrates in the best-known Permian (298.9 to 251.9 Ma) fossil beds. However, at the time these were all in
temperate zones at middle
latitudes, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians. The
Permian–Triassic extinction event wiped out almost all land vertebrates, as well as the great majority of other life. During the slow recovery from this catastrophe, estimated to have taken 30 million years, a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of
archosauriformes ("ruling lizard forms") have been found in Late Permian rocks, but, by the
Middle Triassic, archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods ().
Birds During the Late Jurassic, birds evolved from small, predatory
theropod dinosaurs. The first birds inherited teeth and long, bony tails from their dinosaur ancestors, and short
pygostyle tails by the
Early Cretaceous.
Mammals While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the
mammaliaform successors of the therapsids evolved into small, mainly nocturnal
insectivores. This ecological role may have promoted the
evolution of mammals, for example nocturnal life may have accelerated the development of
endothermy ("warm-bloodedness") and hair or fur. By in the
Early Jurassic there were animals that were very like today's mammals in a number of respects. Unfortunately, there is a gap in the fossil record throughout the Middle Jurassic. However, fossil teeth discovered in
Madagascar indicate that the split between the lineage leading to
monotremes and the one leading to other living mammals had occurred by . After dominating land vertebrate niches for about 150 Ma, the non-avian dinosaurs perished in the Cretaceous–Paleogene extinction event () along with many other groups of organisms. Mammals throughout the time of the dinosaurs had been restricted to a narrow range of
taxa, sizes and shapes, but increased rapidly in size and diversity after the extinction, with
bats taking to the air within 13 million years, and
cetaceans to the sea within 15 million years.
Flowering plants The first flowering plants appeared around 130 Ma. The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from , and that their rise was associated with that of
pollinating insects. The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual
haplodiploid method of
sex determination, which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed. However,
E. O. Wilson and
Bert Hölldobler argue that this explanation is faulty: for example, it is based on
kin selection, but there is no evidence of
nepotism in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages through co-operative
foraging. In support of this explanation they cite the appearance of eusociality in
bathyergid mole rats, The earliest fossils of insects have been found in Early Devonian rocks from about , which preserve only a few varieties of flightless insect. The
Mazon Creek lagerstätten from the Late Carboniferous, about , include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as
herbivores,
detritivores and insectivores. Social termites and ants first appeared in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle
Cenozoic.
Humans The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by
Robert Chambers in 1844 and taken up by
Charles Darwin in 1871. Modern humans evolved from a lineage of upright-walking
apes that has been traced back over to
Sahelanthropus. The first known
stone tools were made about , apparently by
Australopithecus garhi, and were found near animal bones that bear scratches made by these tools. The earliest
hominines had
chimpanzee-sized brains, but there has been a fourfold increase in the last 3 Ma; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence. There is a long-running debate about whether modern humans evolved
all over the world simultaneously from existing advanced hominines or are descendants of a
single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species. There is also debate about whether anatomically modern humans had an intellectual, cultural and technological
"Great Leap Forward" under 40,000–50,000 years ago and, if so, whether this was due to neurological changes that are not visible in fossils. == Mass extinctions ==