How Do Scientists Know When Various Kinds Of Plants And Animals Appeared

Institute and Animal Evolution

The History of Brute Evolution

For many people animals are perhaps the about familiar, and most interesting, of living things. This may be because we are animals ourselves. Equally such, we have a number of features in mutual with all the organisms placed in the creature kingdom, and these common features betoken that we have a shared evolutionary history.

All animals and plants are classified as multicellular eukaryotes: their bodies are fabricated upwardly of large numbers of cells, and microscopic inspection of these cells reveals that they contain a nucleus and a number of other organelles . Compared to prokaryotic organisms such as bacteria, plants and animals accept a relatively recent evolutionary origin. Dna bear witness suggests that the first eukaryotes evolved from prokaryotes, betwixt 2500 and k million years ago. That is, eukaryotes as a taxon date from the Proterozoic Era, the final Era of the Precambrian. Fossils of both elementary unicellular and more than complex multicellular organisms are establish in abundance in rocks from this period of time. In fact, the name "Proterozoic" means "early life".

Plants and animals both owe their origins to endosymbiosis , a process where ane cell ingests another, but for some reason then fails to digest it. The evidence for this lies in the way their cells function. Both found and animal rely on structures called mitochondria to release energy in their cells, using aerobic respiration to produce the energy-carrying molecule ATP . There is considerable evidence that mitochondria evolved from free-living aerobic bacteria: they are the size of bacterial cells; they divide independently of the prison cell by binary fission ; they have their own genome in the form of a single circular Dna molecule; their ribosomes are more similar to those of bacteria than to the ribosomes institute in the eukaryote cell'south cytoplasm; and like chloroplasts they are enclosed by a double membrane as would be expected if they derived from bacterial cells engulfed by another cell.

Like the plants, animals evolved in the sea. And that is where they remained for at least 600 meg years. This is considering, in the absence of a protective ozone layer, the land was bathed in lethal levels of UV radiation. Once photosynthesis had raised atmospheric oxygen levels high enough, the ozone layer formed, meaning that it was and then possible for living things to venture onto the land.

The oldest fossil evidence of multicellular animals, or metazoans , is burrows that appear to have been made past smoothen, wormlike organisms. Such trace fossils have been plant in rocks from Cathay, Canada, and India, simply they tell us piffling nigh the animals that fabricated them apart from their basic shape.

The Ediacaran animals
The Cambrian "explosion" and the Burgess Shale
What caused the Cambrian "explosion"?
A foot on the land
The earliest vertebrates
Appearance of the fish
The jawless fish
Colonisation of the state
Problems encountered in the motility to land
The evolution of amphibians
What drove amphibian development?
Early reptiles and the amniotic egg
The early on mammals
Developments in the dinosaur lineage
Farther developments in the early mammals
Taking wing: Archaeopteryx and the origin of the birds
The end of the dinosaur age
The appearance of modern mammal groups

The Ediacaran animals

Between 620 and 550 million years agone (during the Vendian Period) relatively large, complex, soft-bodied multicellular animals appear in the fossil record for the first fourth dimension. While found in several localities around the globe, this detail group of animals is generally known equally the Ediacaran beast, subsequently the site in Commonwealth of australia where they were first discovered.

The Ediacaran animals are puzzling in that there is little or no evidence of any skeletal hard parts i.east. they were soft-bodied organisms, and while some of them may accept belonged to groups that survive today others don't seem to acquit any relationship to animals we know. Although many of the Ediacaran organisms have been compared to modern-24-hour interval jellyfish or worms, they have also been described equally resembling a mattress, with tough outer walls around fluid-filled internal cavities - rather like a sponge.

As a group, Ediacaran animals had a apartment, quilted appearance and many showed radial symmetry. They ranged in size form 1cm to >1m, and have been classified into three main groups on the basis of their shape: discoidal, frond-like, or ovate-elongate. The large variety of Ediacaran animals is pregnant, as it suggests there must have been a lengthy period of evolution prior to their first appearance in the fossil record.

The Cambrian "explosion" and the Burgess Shale

The Ediacaran animals disappear from the fossil record at the end of the Vendian (544 million years ago). In their identify we notice representatives of virtually all the modern phyla recognised today: sponges, jellyfish and corals, flatworms, molluscs, annelid worms, insects, echinoderms and chordates, plus many "lesser" phyla such as nemertean worms. These "modern" organisms announced relatively quickly in the geological time scale, and their abrupt appearance is ofttimes described as the "Cambrian explosion" however, bear in mind that the fossil record of the "explosion" is spread over nigh xxx million years. I continue taking things out of brackets because it is interesting relevant and memorable

I of the almost famous assemblages of Cambrian fossils comes from the Burgess Shale of British Colombia. The rocks of the Burgess Shale were laid downward in the middle Cambrian, when the "explosion" had already been underway for several one thousand thousand years. They contain familiar animals such as trilobites, molluscs and echinoderms, just also the get-go appearance of brachiopods, and some odd animals, e.g. Opabinia, that may have belonged to extinct phyla. Even an early chordate, Pikaia, has been found in this fossil assemblage.

The Burgess Shale fossils are important, non simply for their evidence of early variety among brute forms, simply also considering both soft parts of animals and their hard bodies (i.east. the whole beast) is preserved, and animals that were entirely soft-bodied. Preservation of soft-bodied organisms is rare, and in this case seems to have occurred when the animals were rapidly buried in a mudslide downwards into deep, anaerobic waters, where there was niggling bacterial decay. Prior to the discovery of this fossil aggregation, early in the 20th century, there was no evidence of soft-bodied animals from the Cambrian (retrieve that this is before the Ediacaran fauna were establish).

These fossils also provide expert evidence of predatory animals (e.1000. Anomalocaris ), and therefore of complex predator-casualty relationships. They also requite insights into how evolution might accept progressed relatively early in the history of multicellular animals, and in fact some authors view the Cambrian as a period of extreme "experimentation" and diversity.

What acquired the Cambrian "explosion"?

The cause of the proliferation of creature forms in the Cambrian is a matter of considerable debate among scientists. Some point to the increment in atmospheric oxygen levels that began around 2000 million years agone, supporting a higher metabolic rate and assuasive the evolution of larger organisms and more complex body structures. Changed body of water chemistry would take played a office here, allowing for the offset time the development of difficult torso parts such as teeth and supporting skeletons based on calcium carbonate (CaCO₃), and besides supporting higher levels of primary product equally a effect of increased concentrations of phosphates and nitrates. The mass extinction that marked the end of the Vendian catamenia would have opened up ecological niches that the new animals exploited, equally would habitat changes wrought by continental migrate.

Genetic factors were also crucial. Contempo research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes (the homeobox or "hox" genes ) that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented menstruum of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Future evolutionary modify was so limited to acting on the torso plans that remained in existence.

Recently many scientists have begun to question whether the Cambrian explosion was a real event, or a reflection of the patchiness of this ancient fossil tape. Genetic data advise that multicellular animals evolved around one thousand million years ago; this is supported by fossil embryos from rocks in China that appointment back 600 million years. These embryos are more than complex than those of simple organisms such every bit sponges and jellyfish, which suggests that multicellular animals must take evolved much further back in time. In addition, trilobites were a very diverse group fifty-fifty early in the Cambrian, and some scientists suggest that this indicates that the arthropod group must have had a much earlier evolutionary origin.

A foot on the land

Any their origins, animals may accept ventured onto land early in the Cambrian. Previously scientists believed that animals did not begin to colonise the land until the Silurian (440 - 410 million years ago). However, the 2002 discovery of the footprints of animals that scuttled about on sand dunes nigh 530 1000000 years agone has changed this view. These animals were arthropods, and resembled centipedes nigh the size of crayfish. They probably didn't live on land, instead coming aground to mate or evade predators. At this time the just country plants appear to have resembled mosses .

The earliest vertebrates

Animals connected to diversify in the Ordovician seas (505 - 440 million years ago). They were by and large invertebrates, including graptolites , which were stick-like branching colonies of tiny animals, together with brachiopods , trilobites, cephalopods , corals, crinoids and conodonts . We now place the conodonts with the chordates, but for a long time they were known simply by their tiny, but very common, teeth.

In terms of number of species invertebrates were by far the nigh common Ordovician animals - as they still are today. Nonetheless, members of another taxon were as well evolving in the Ordovician seas. These were the fish.

Appearance of the fish

Like the conodonts, fish are members of the chordate phylum because they display certain defining characteristics: a dorsal stiffening rod called the notochord, a dorsal nerve cord, pharyngeal gill slits and a tail that extends beyond the anus. Nevertheless, fish are placed in the subphylum Vertebrata , considering they as well show the development of skeletal features such every bit a courage, skull, and limb bones.

Not all the modern groups of fish were represented in the Ordovician oceans. At this fourth dimension just the jawless fish had evolved from a chordate ancestor. The sharks and their relatives and 2 extinct groups, the placoderms (which had bony plates covering their heads) and the acanthodians (the first known jawed vertebrates, with a skeleton of cartilage) made their advent in the Silurian. However, neither the sharks nor the agnathans became common until the Devonian. The other ii living lineages, the ray-finned (e.k. carp and kahawai) and the lobe-finned fish (e.g. lungfish and the coelacanth), evolved during the Devonian menses.

The jawless fish

Agnathans , or jawless fish, were the earliest fish: an fantabulous fossil of Haikouichthys ercaicunensis dates back about 530 meg years, to the Cambrian. Previously the primeval-known agnathans were dated to effectually 480 1000000 years ago. Agnathans have traditionally been placed with the vertebrates due to the presence of a skull, although the modern forms such every bit hagfish lack a vertebral column. The earliest agnathans were Ostracoderms. They were bottom-feeders and were virtually entirely covered in armour plates. When the sharks and bony fish began to evolve, around 450 million years ago, most ostracoderms became extinct. Merely the lineage that produced the modernistic hagfish and lampreys survived.

Colonisation of the land

Fish continued to evolve during the Silurian catamenia (440 - 410 one thousand thousand years ago). At the same time some groups of plants and animals took a major step as they colonised the land for the showtime time. Nosotros are non sure why this advance occurred, but it was probably the result of contest in the marine ecosystems, plus the opportunity to escape predators and the availability of new terrestrial niches.

Arthropods, which had ventured temporarily onto state 100 million years earlier, were the starting time animals to become more permanent colonists. Fossil footprints fabricated in the sandy flats surrounding temporary lakes dating back about 420 meg years have been found in Western Australia.

The arthropods were pre-adapted to life on land. By the time they moved ashore, they had already evolved lighter bodies and slim, potent legs that could back up them against the pull of gravity. Their difficult outer exoskeletons provided protection and would help to retain water, although the development of a waxy, waterproof cuticle was necessary for efficient water conservation.

Spiders, centipedes and mites were among the earliest country animals. Some of them were giants: the largest was Slimonia, the size of a human and a relative of the scorpions. This creature was still too big and too heavy and the walking legs too small-scale to venture onto country for whatsoever length of time then they lived in marginal marine (deltaic) environments.

Problems encountered in the motion to country

These early state animals had to solve the same problems that plants faced when they moved to the country: h2o conservation, gas exchange, reproduction and dispersal, and the fact that water no longer buoyed them up against the pull of gravity. Like plants, animals evolved waterproof external layers, internal gas exchange systems, means of reproducing that did not involve water, and strong back up systems ( endoskeletons and exoskeletons) that allowed them to move well-nigh on country. Remember that not all fauna taxa were every bit successful in solving these problems.

The evolution of amphibians

By the Devonian period two major animal groups dominated the state: the tetrapods (4-legged terrestrial vertebrates) and the arthropods, including arachnids and wingless insects. The start tetrapods were amphibians , such as Ichthyostega, and were closely related to a group of fish known as lobe-finned fish e.chiliad. Eusthenopteron . Once thought to be extinct, the coelacanth is a living representative of this group.

Eusthenopteron had a number of exaptations that pre-adapted it to life on land: information technology had limbs (with digits) that allowed it to movement around on the bottom of pools, lungs - which meant information technology could gulp air at the surface, and the beginnings of a neck. This final is important equally a terrestrial predator cannot rely on h2o current to bring food into its rima oris, only must movement its head to catch prey. And the basic in Eusthenopteron's fins are almost identical to those in the limbs of the primeval amphibians, an example of homology .

Ichthyostega'due south skull was almost identical to that of the lobe-finned fish Eusthenopteron, a definite cervix separated its body from its head, and it retained a deep tail with fins. While Ichthyostega had 4 strong limbs, the course of its hind legs suggests that it did not spend all its time on land.

All modern tetrapods have a maximum of 5 digits on each limb, and are thus said to have a pentadactyl limb. For a long time scientists believed that pentadactyly was the ancestral state for tetrapods. Withal, careful exam of the fossils of early amphibians such as Ichthyostega and Acanthostega has revealed the presence of up to 8 toes on each foot!

In addition, these early on amphibians were large-bodied animals with stiff bodies and prominent ribs - quite different in advent from modern representatives such equally frogs and axolotls.

What drove amphibian development?

It was originally believed that the tetrapods evolved during periods of drought, when the ability to move between pools would be an reward. The animals would also have been able to have reward of terrestrial prey, such as arthropods. Juvenile animals could avoid predation by the land-based adults by living in shallow water.

However, fossil and geological evidence tells us that the early tetrapods lived in lagoons in tropical regions, and then that drought was not an upshot. They were unlikely to be feeding on country: arthropods are pocket-sized and fast-moving, unlikely prey for large, sluggish amphibians. But amphibians that laid their eggs on land, rather than in h2o, would be at a selective advantage, avoiding predation past aquatic vertebrates (such every bit other amphibians and fish) on gametes, eggs and hatchlings.

Even today some amphibians e.g. the Eleutherodactylid frogs of Australia and Indonesia lay their eggs in soil on the country. However, they must still be in a moist surround, and the size of the egg is restricted to less than ane.5cm in bore. This is because the egg is dependent on diffusion alone for gas substitution, and means that the embryo must develop rapidly into a nutrient-seeking larval form rather than undergo prolonged development inside the egg.

In the Devonian seas, brachiopods had become a ascendant invertebrate group, while the fish connected to evolve, with sharks becoming the ascendant marine vertebrates. The placoderms and acanthodian fish were quite various during the Devonian, simply their numbers then dwindled rapidly and both groups became extinct past the end of the Carboniferous period. Lobe-finned fish as well peaked in numbers during the Devonian.

Early reptiles and the amniotic egg

Ane of the greatest evolutionary innovations of the Carboniferous period (360 - 268 million years ago) was the amniotic egg , which immune early reptiles to motility away from waterside habitats and colonise dry regions. The amniotic egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the embryo inside from drying out, and then eggs could exist laid abroad from the water. It also meant that in dissimilarity to the amphibians the reptiles could produce fewer eggs at any one time, because there was less risk of predation on the eggs. Reptiles don't get through a larval food-seeking stage, simply undergo straight evolution into a miniature adult form while in the egg, and fertilisation is internal.

The primeval engagement for evolution of the amniotic egg is almost 320 million years ago. However, reptiles didn't undergo whatsoever major adaptive radiations for some other twenty million years. Current thinking is that these early amniotes were withal spending time in the water and came aground mainly to lay their eggs, rather than to feed. Information technology wasn't until the evolution of herbivory that new reptile groups appeared, able to accept reward of the abundant plant life of the Carboniferous.

Early on reptiles belonged to a grouping chosen the cotylosaurs. Hylonomus and Paleothyris were two members of this group. They were small, lizard-sized animals with amphibian-like skulls, shoulders, pelvis and limbs, and intermediate teeth and vertebrae. The balance of the skeleton was reptilian. Many of these new "reptilian" features are also seen in little, modern, amphibians (which may also have straight-developing eggs laid on land e.one thousand. New Zealand's leiopelmid frogs, then perhaps these features were simply associated with the small body size of the offset reptiles.

The early on mammals

A major transition in the development of life occurred when mammals evolved from ane lineage of reptiles. This transition began during the Permian (286 - 248 million years ago), when the reptile group that included Dimetrodon gave rise to the "fauna-faced" therapsids. (The other major branching, the "lizard-faced" sauropsids, gave ascent to birds and mod reptiles). These mammal-like reptiles in turn gave ascension to the cynodonts e.g. Thrinaxodon during the Triassic period.

Early adaptive radiation among the reptiles

This lineage provides an excellent series of transitional fossils . The development of a key mammalian trait, the presence of just a single bone in the lower jaw (compared to several in reptiles) tin be traced in the fossil history of this group. It includes the excellent transitional fossils, Diarthrognathus and Morganucodon, whose lower jaws have both reptilian and mammalian articulations with the upper. Other novel features institute in this lineage include the development of different kinds of teeth (a feature known as heterodonty), the beginnings of a secondary palate, and enlargement of the dentary bone in the lower jaw. Legs are held directly underneath the body, an evolutionary accelerate that occurred independently in the ancestors of the dinosaurs.

The stop of the Permian was marked by perhaps the greatest mass extinction always to occur. Some estimates propose that up to 90% of the species and so living became extinct. (Recent enquiry has suggested that this effect, like the better-known cease-Cretaceous outcome, was acquired past the bear on of an asteroid.) During the subsequent Triassic period (248 - 213 one thousand thousand years ago), the survivors of that consequence radiated into the large number of now-vacant ecological niches.

Notwithstanding, at the finish of the Permian information technology was the dinosaurs, non the mammal-like reptiles, which took advantage of the newly available terrestrial niches to diversify into the dominant land vertebrates. In the sea, the ray-finned fish began the major adaptive radiation that would see them become the most species-rich of all vertebrate classes.

Developments in the dinosaur lineage

One major change, in the grouping of reptiles that gave rising to the dinosaurs, was in the animals' posture. This changed from the usual "sprawling" mode, where the limbs jut sideways, to an erect posture, with the limbs held straight under the body. This had major implications for locomotion, as it allowed much more energy-efficient move.

The dinosaurs , or "terrible lizards", fall into two major groups on the ground of their hip structure: the saurischians (or "lizard-hipped" dinosaurs) and the ornithischians (misleadingly known every bit the "bird-hipped" dinosaurs). Ornithischians include Triceratops, Iguanodon, Hadrosaurus, and Stegosaurus). Saurischians are further subdivided into theropods (such as Coelophysis and Tyrannosaurus rex) and sauropods (e.g. Apatosaurus). Most scientists agree that birds evolved from theropod dinosaurs.

Although the dinosaurs and their firsthand ancestors dominated the world'southward terrestrial ecosystems during the Triassic, mammals continued to evolve during this time.

Farther developments in the early mammals

Mammals are advanced synapsids . Synapsida is 1 of two great branches of the amniote family tree. Amniotes are the group of animals that produce an amniotic egg i.e. the reptiles, birds, and mammals. The other major amniote group, the Diapsida, includes the birds and all living and extinct reptiles other than the turtles and tortoises. Turtles and tortoises belong in a third group of amniotes, the Anapsida. Members of these groups are classified on the basis of the number of openings in the temporal region of the skull.

Synapsids are characterised by having a pair of extra openings in the skull behind the optics. This opening gave the synapsids (and similarly the diapsids, which have ii pairs of openings) stronger jaw muscles and better biting ability than earlier animals. (The jaw muscles of a synapsid are anchored to the edges of the skull opening). Pelycosaurs (like Dimetrodon and Edaphosaurus) were early synapsids; they were mammal-like reptiles. Afterward synapsids include the therapsids and the cynodonts , which lived during the Triassic.

Cynodonts possessed many mammalian features, including the reduction or complete absence of lumbar ribs implying the presence of a diaphragm; well-adult canine teeth, the development of a bony secondary palate and then that air and food had separate passages to the dorsum of the pharynx; increased size of the dentary - the primary bone in the lower jaw; and holes for fretfulness and blood vessels in the lower jaw, suggesting the presence of whiskers.

Past 125 million years ago the mammals had already become a various group of organisms. Some of them would have resembled today's monotremes (e.g. platypus and echidna), but early marsupials (a grouping that includes modern kangaroos and possums) were likewise present. Until recently it was thought that placental mammals (the group to which nigh living mammals belong) had a much subsequently evolutionary origin. However, recent fossil finds and Dna evidence suggest that the placental mammals are much older, perhaps evolving more than 105 million years agone. Notation that the marsupial and placental mammals provide some excellent examples of convergent evolution , where organisms that are non peculiarly closely related have evolved like torso forms in response to similar environmental pressures.

However, despite the fact that the mammals had what many people regard every bit "advanced" features, they were withal only small players on the earth stage. As the globe entered the Jurassic period (213 - 145 million years agone), the ascendant animals on state, in the ocean, and in the air, were the reptiles. Dinosaurs, more numerous and more extraordinary than those of the Triassic, were the principal land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited by the pterosaurs .

Taking fly: Archaeopteryx and the origins of the birds

In 1861 an intriguing fossil was found in the Jurassic Solnhofen Limestone of southern Germany, a source of rare simply exceptionally well-preserved fossils. Given the proper name Archeopteryx lithographica the fossil appeared to combine features of both birds and reptiles: a reptilian skeleton, accompanied by the clear impression of feathers. This made the find highly significant equally it had the potential to support the Darwinians in the debate that was raging following the 1859 publication of "On the origin of species".

While information technology was originally described as simply a feathered reptile, Archaeopteryx has long been regarded as a transitional form between birds and reptiles, making it 1 of the most important fossils always discovered. Until relatively recently it was also the earliest known bird. Lately, scientists take realised that Archaeopteryx bears even more resemblance to the Maniraptora, a group of dinosaurs that includes the infamous velociraptors of "Jurassic Park", than to modern birds. Thus the Archaeopteryx provides a strong phylogenetic link between the two groups. Fossil birds take been discovered in Prc that are fifty-fifty older than Archaeopteryx, and other discoveries of feathered dinosaurs support the theory that theropods evolved feathers for insulation and thermo-regulation earlier birds used them for flight. This is an case of an exaptation .

Closer examination of the early history of birds provides a good example of the concept that evolution is neither linear nor progressive. The bird lineage is messy, with a variety of �experimental� forms appearing. Not all achieved powered flying, and some looked quite different mod birds e.thou. Microraptor gui, which appears to have been a gliding animal and had asymmetric flight feathers on all four limbs, while its skeleton is essentially that of a pocket-size dromaeosaur. Archaeopteryx itself did not vest to the lineage from which modern birds (Neornithes) have evolved, simply was a member of the now-extinct Enantiornithes. A reconstruction of the avian family tree would show a many-branched bush, not a unmarried straight trunk.

The terminate of the dinosaur historic period

Dinosaurs spread throughout the world - including New Zealand, which had its ain dinosaur fauna - during the Jurassic, but during the subsequent Cretaceous flow (145 - 65 meg years ago) they were failing in species multifariousness. In fact, many of the typically Mesozoic organisms - such as ammonites, belemnites, gymnosperms, ichthyosaurs, plesiosaurs, and pterosaurs - were in decline at this time, despite the fact that they were still giving rising to new species.

The origin of flowering plants (the angiosperms) during the early Cretaceous triggered a major adaptive radiation amongst the insects: new groups, such as butterflies, moths, ants and bees arose and flourished. These insects drank the nectar from the flowers and acted every bit pollinating agents in the process.

The mass extinction at the stop of the Cretaceous period, 65 million years ago, wiped out the dinosaurs along with every other country animate being that weighed much more than 25 kg. This cleared the way for the expansion of the mammals on land. In the ocean at this time, the fish again became the dominant vertebrate taxon.

The appearance of modernistic mammal groups

At the beginning of the Palaeocene epoch (65 - 55.5 million years agone) the world was without larger-sized terrestrial animals. This unique situation was the starting point for the groovy evolutionary diversification of the mammals, which upwardly until and then had been nocturnal animals the size of small rodents. By the cease of the epoch, mammals occupied many of the vacant ecological niches. While mammal fossils from this period of fourth dimension are scarce, and often consist largely of their characteristic teeth, we know that minor, rodent-like insectivorous mammals roamed the forests, the first big herbivorous mammals were browsing on the arable vegetation, and carnivorous mammals were stalking their casualty.

The oldest confirmed primate fossils date to nigh 60 million years ago, in the mid-Palaeocene. The early on primates evolved from primitive nocturnal insectivores, something like shrews, and resembled lemurs or tarsiers (the prosimians ). They were probably arboreal , living in tropical or subtropical forests. Many of their characteristic features are well suited for this habitat: easily specialised for grasping, rotating shoulder joints, and stereoscopic vision. They also take a relatively large brain size and nails on their digits, instead of claws.

The earliest known fossils of most of the modern orders of mammals appear in a brief catamenia during the early Eocene (55.v - 33.7 million years ago). Both groups of modern hoofed animals, the Artiodactyla ("even-toed" taxa such every bit cows and pigs) and Perrisodactyla ("odd-toed" taxa, including the horses), became widespread throughout North America and Europe. The evolutionary history of the horses is particularly well understood: Stephen Jay Gould (1983) provides an excellent discussion of it in his volume "Hens' teeth and horses' toes".

At the same time every bit the mammals were diversifying on land, they were also returning to the ocean. The evolutionary transitions that led to the whales have been closely studied in recent years, with extensive fossil finds from India, Pakistan, and the Center East. These fossils chronicle the change from the land-dwelling mesonychids, which are the likely ancestors of whales, through animals such equally Ambulocetus , which was still a tetrapod but which likewise has such whale-similar features as an ear capsule isolated from the rest of its skull, to the archaic whales called the Archaeocetes.

The trend towards a cooler global climate that occurred during the Oligocene epoch (33.vii - 23.viii million years ago) saw the appearance of the grasses, which were to extend into vast grasslands during the subsequent Miocene (23.8 - 5.iii million years ago). This modify in vegetation collection the evolution of browsing animals, such equally more mod horses, with teeth that could deal with the high silica content of the grasses. The cooling climate trend also affected the oceans, with a decline in the number of marine plankton and invertebrates.

While Deoxyribonucleic acid prove suggests that the great apes evolved during the Oligocene, abundant fossils practice not appear until the Miocene. Hominids, on the evolutionary line leading to humans, first announced in the fossil record in the Pliocene (5.3 - 1.8 1000000 years agone). The story of human evolution is covered hither - Human Development material.

New Zealand, by virtue of its isolation and its relatively recent geological evolution, was non the centre of any novel evolutionary development. However, many of the species that date back to Gondwanaland, or that arrived more recently as migrants, accept undergone significant adaptive radiation in their new homeland. Some of the all-time examples of this tin be related to the major ecological changes that accompanied the Pleistocene Ice Ages.

Throughout the Pleistocene there were about 20 cycles of cold glacial ("Ice Age") and warm interglacial periods at intervals of about 100,000 years. During the Ice Ages glaciers dominated the landscape, snow and ice extended into the lowlands, transporting huge quantities of rock with them. During these periods the Southward Island was extensively glaciated, and there were small glaciers on the Tararua Ranges and Central Plateau. Because a lot of water was locked upward in ice, the sea levels dropped during the glacials (upwardly to 135m lower than at nowadays). All-encompassing country bridges joined the main and many offshore islands, assuasive the migration of plants and animals. During the warmer periods large areas became submerged over again nether water. These repeated episodes of environmental fragmentation drove rapid adaptive radiation in many NZ species, particularly (but not exclusively) the alpine plants.

For example, speciation patterns in the native Placostylus flax snails of Northland can be related to changes in sea level. Originally ii-three species were widespread at a time of low sea levels. Rising seas at the terminate of the glacial menstruum isolated these as populations on offshore islands, where differential natural selection pressures led to the evolution of a greater number of split species.

The distribution of land snails such as Powelliphanta in Marlborough and the southern Due north Island likewise offers evidence for the presence of land bridges and the possibility of future speciation. The same varieties are found both north and south of Cook Strait, implying a continuous land bridge in the past as the animals die in salt h2o. The fact that no further speciation has occurred in this case suggests that the land bridge was recently submerged by rising seas, mayhap just 10,000 years ago.

New Zealand Example

For more information on NZ examples of development, click here.

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