What Is The Origin Of Animal Life

Plant and Animal Development

| Classification | Establish Evolution | Animate being Evolution | Homology |

The History of Brute Evolution

For many people animals are mayhap the most familiar, and most interesting, of living things. This may exist because nosotros are animals ourselves. As such, we have a number of features in common with all the organisms placed in the beast kingdom, and these mutual features indicate that we accept a shared evolutionary history.

All animals and plants are classified as multicellular eukaryotes: their bodies are made up 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 every bit bacteria, plants and animals have a relatively recent evolutionary origin. Deoxyribonucleic acid evidence suggests that the showtime eukaryotes evolved from prokaryotes, betwixt 2500 and 1000 million years agone. That is, eukaryotes every bit a taxon date from the Proterozoic Era, the final Era of the Precambrian. Fossils of both uncomplicated unicellular and more complex multicellular organisms are found in abundance in rocks from this flow of time. In fact, the name "Proterozoic" means "early life".

Plants and animals both owe their origins to endosymbiosis , a process where 1 cell ingests some other, but for some reason and so fails to digest it. The evidence for this lies in the way their cells office. Both constitute and animal rely on structures chosen mitochondria to release energy in their cells, using aerobic respiration to produce the energy-conveying molecule ATP . There is considerable evidence that mitochondria evolved from gratuitous-living aerobic bacteria: they are the size of bacterial cells; they divide independently of the jail cell by binary fission ; they have their ain genome in the form of a single round Dna molecule; their ribosomes are more than like to those of bacteria than to the ribosomes found in the eukaryote cell's cytoplasm; and similar chloroplasts they are enclosed by a double membrane every bit would be expected if they derived from bacterial cells engulfed past 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 state was bathed in lethal levels of UV radiations. Once photosynthesis had raised atmospheric oxygen levels loftier plenty, the ozone layer formed, meaning that it was then possible for living things to venture onto the state.

The oldest fossil bear witness of multicellular animals, or metazoans , is burrows that appear to have been fabricated by smooth, wormlike organisms. Such trace fossils have been institute in rocks from China, Canada, and Bharat, but they tell usa footling virtually the animals that fabricated them apart from their bones 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 land
• Problems encountered in the move to land
• The evolution of amphibians
• What collection amphibian evolution?
• Early on reptiles and the amniotic egg
• The early mammals
• Developments in the dinosaur lineage
• Further developments in the early mammals
• Taking fly: Archaeopteryx and the origin of the birds
• The terminate of the dinosaur age
• The appearance of modern mammal groups

The Ediacaran animals

Between 620 and 550 million years ago (during the Vendian Flow) relatively big, complex, soft-bodied multicellular animals appear in the fossil tape for the first time. While found in several localities effectually the world, this detail group of animals is generally known as the Ediacaran fauna, subsequently the site in Australia where they were commencement discovered.

The Ediacaran animals are puzzling in that there is little or no show of whatever skeletal hard parts i.eastward. they were soft-bodied organisms, and while some of them may have belonged to groups that survive today others don't seem to bear any human relationship to animals we know. Although many of the Ediacaran organisms take been compared to modern-day jellyfish or worms, they accept also been described every bit resembling a mattress, with tough outer walls around fluid-filled internal cavities - rather similar a sponge.

As a group, Ediacaran animals had a flat, quilted appearance and many showed radial symmetry. They ranged in size course 1cm to >1m, and have been classified into 3 main groups on the basis of their shape: discoidal, frond-similar, or ovate-elongate. The large variety of Ediacaran animals is significant, equally it suggests there must have been a lengthy period of evolution prior to their commencement appearance in the fossil record.

The Cambrian "explosion" and the Burgess Shale

The Ediacaran animals disappear from the fossil tape at the stop of the Vendian (544 one thousand thousand years ago). In their identify we find representatives of about 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 appear relatively quickly in the geological fourth dimension scale, and their sharp appearance is oft described as the "Cambrian explosion" however, comport in heed that the fossil tape of the "explosion" is spread over nigh 30 1000000 years. I go on taking things out of brackets because it is interesting relevant and memorable

One of the nearly famous assemblages of Cambrian fossils comes from the Burgess Shale of British Colombia. The rocks of the Burgess Shale were laid down in the center Cambrian, when the "explosion" had already been underway for several million years. They contain familiar animals such equally trilobites, molluscs and echinoderms, but also the offset appearance of brachiopods, and some odd animals, eastward.g. Opabinia, that may have belonged to extinct phyla. Even an early chordate, Pikaia, has been found in this fossil aggregation.

The Burgess Shale fossils are important, not only for their bear witness of early variety amid creature forms, only also because both soft parts of animals and their hard bodies (i.e. the whole animal) is preserved, and animals that were entirely soft-bodied. Preservation of soft-bodied organisms is rare, and in this example seems to have occurred when the animals were rapidly buried in a mudslide down into deep, anaerobic waters, where there was little 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 (remember that this is before the Ediacaran fauna were found).

These fossils also provide good testify of predatory animals (e.g. Anomalocaris ), and therefore of circuitous predator-prey relationships. They too give insights into how evolution might take progressed relatively early on in the history of multicellular animals, and in fact some authors view the Cambrian as a menstruum of extreme "experimentation" and diversity.

What caused the Cambrian "explosion"?

The cause of the proliferation of animal forms in the Cambrian is a affair of considerable fence among scientists. Some point to the increase in atmospheric oxygen levels that began around 2000 million years ago, supporting a higher metabolic rate and assuasive the evolution of larger organisms and more complex body structures. Changed ocean chemistry would have played a part here, allowing for the first time the development of difficult body parts such as teeth and supporting skeletons based on calcium carbonate (CaCO₃), and also supporting higher levels of primary production every bit a result of increased concentrations of phosphates and nitrates. The mass extinction that marked the end of the Vendian menstruum would accept opened up ecological niches that the new animals exploited, as would habitat changes wrought past 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. In one case assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Future evolutionary change was then express to acting on the body plans that remained in existence.

Recently many scientists take begun to question whether the Cambrian explosion was a real event, or a reflection of the patchiness of this ancient fossil record. Genetic data suggest that multicellular animals evolved around 1000 meg years ago; this is supported by fossil embryos from rocks in China that date dorsum 600 million years. These embryos are more complex than those of unproblematic organisms such as sponges and jellyfish, which suggests that multicellular animals must have evolved much farther back in time. In addition, trilobites were a very diverse group fifty-fifty early on 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 state

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

The primeval vertebrates

Animals continued to diversify in the Ordovician seas (505 - 440 million years ago). They were more often than not 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, only very mutual, teeth.

In terms of number of species invertebrates were by far the most common Ordovician animals - every bit they still are today. Withal, members of another taxon were also 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. Even so, fish are placed in the subphylum Vertebrata , because they also show the development of skeletal features such as a backbone, skull, and limb bones.

Not all the modern groups of fish were represented in the Ordovician oceans. At this time but the jawless fish had evolved from a chordate antecedent. The sharks and their relatives and two 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 appearance in the Silurian. Notwithstanding, neither the sharks nor the agnathans became mutual until the Devonian. The other two living lineages, the ray-finned (due east.g. bother and kahawai) and the lobe-finned fish (east.g. lungfish and the coelacanth), evolved during the Devonian period.

The jawless fish

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

Colonisation of the land

Fish continued to evolve during the Silurian period (440 - 410 million years ago). At the same time some groups of plants and animals took a major footstep every bit they colonised the country for the first time. We are not sure why this advance occurred, just information technology was probably the result of competition in the marine ecosystems, plus the opportunity to escape predators and the availability of new terrestrial niches.

Arthropods, which had ventured temporarily onto land 100 1000000 years earlier, were the first animals to go more than permanent colonists. Fossil footprints made in the sandy flats surrounding temporary lakes dating back nigh 420 million 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, strong legs that could support them against the pull of gravity. Their hard 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 state animals. Some of them were giants: the largest was Slimonia, the size of a man and a relative of the scorpions. This animal was still besides big and as well heavy and the walking legs too small to venture onto land for any length of fourth dimension and and then they lived in marginal marine (deltaic) environments.

Problems encountered in the motility to state

These early on country animals had to solve the same problems that plants faced when they moved to the state: water conservation, gas substitution, 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 substitution systems, ways of reproducing that did not involve water, and potent support systems ( endoskeletons and exoskeletons) that allowed them to move near on land. Remember that not all animate being taxa were equally successful in solving these problems.

The evolution of amphibians

By the Devonian menstruation two major animal groups dominated the land: the tetrapods (4-legged terrestrial vertebrates) and the arthropods, including arachnids and wingless insects. The showtime tetrapods were amphibians , such equally Ichthyostega, and were closely related to a group of fish known equally lobe-finned fish due east.g. Eusthenopteron . In one case thought to exist 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 move around on the bottom of pools, lungs - which meant it could gulp air at the surface, and the beginnings of a neck. This final is important as a terrestrial predator cannot rely on water current to bring food into its oral fissure, but must move its head to grab casualty. And the bones in Eusthenopteron's fins are almost identical to those in the limbs of the earliest amphibians, an example of homology .

Ichthyostega's skull was almost identical to that of the lobe-finned fish Eusthenopteron, a definite neck separated its body from its head, and information technology retained a deep tail with fins. While Ichthyostega had four strong limbs, the course of its hind legs suggests that it did not spend all its fourth dimension on land.

All modern tetrapods have a maximum of v digits on each limb, and are thus said to have a pentadactyl limb. For a long fourth dimension scientists believed that pentadactyly was the bequeathed state for tetrapods. However, conscientious exam of the fossils of early amphibians such as Ichthyostega and Acanthostega has revealed the presence of up to viii toes on each pes!

In addition, these early on amphibians were large-bodied animals with potent bodies and prominent ribs - quite dissimilar in appearance from modernistic representatives such as frogs and axolotls.

What drove amphibian evolution?

It was originally believed that the tetrapods evolved during periods of drought, when the power to move between pools would exist an advantage. The animals would also accept been able to accept reward of terrestrial prey, such every bit arthropods. Juvenile animals could avoid predation past the state-based adults by living in shallow water.

Yet, fossil and geological evidence tells us that the early tetrapods lived in lagoons in tropical regions, so that drought was not an effect. They were unlikely to be feeding on land: arthropods are small-scale and fast-moving, unlikely prey for large, sluggish amphibians. But amphibians that laid their eggs on land, rather than in water, would be at a selective advantage, fugitive predation by aquatic vertebrates (such as other amphibians and fish) on gametes, eggs and hatchlings.

Even today some amphibians e.grand. the Eleutherodactylid frogs of Commonwealth of australia and Republic of indonesia lay their eggs in soil on the state. Yet, they must however be in a moist surroundings, and the size of the egg is restricted to less than 1.5cm in bore. This is because the egg is dependent on diffusion alone for gas commutation, and ways that the embryo must develop chop-chop into a food-seeking larval form rather than undergo prolonged evolution within the egg.

In the Devonian seas, brachiopods had become a dominant invertebrate group, while the fish continued to evolve, with sharks becoming the dominant marine vertebrates. The placoderms and acanthodian fish were quite diverse during the Devonian, simply their numbers then dwindled rapidly and both groups became extinct by the stop of the Carboniferous flow. Lobe-finned fish also peaked in numbers during the Devonian.

Early reptiles and the amniotic egg

1 of the greatest evolutionary innovations of the Carboniferous period (360 - 268 1000000 years ago) was the amniotic egg , which immune early reptiles to move away from waterside habitats and colonise dry regions. The amniotic egg allowed the ancestors of birds, mammals, and reptiles to reproduce on country by preventing the embryo inside from drying out, so eggs could exist laid away from the water. It besides meant that in dissimilarity to the amphibians the reptiles could produce fewer eggs at whatsoever one time, because there was less adventure of predation on the eggs. Reptiles don't go through a larval food-seeking stage, but undergo straight development into a miniature adult grade while in the egg, and fertilisation is internal.

The earliest date for development of the amniotic egg is most 320 1000000 years ago. However, reptiles didn't undergo any major adaptive radiation for some other 20 million years. Current thinking is that these early amniotes were still spending time in the h2o and came ashore mainly to lay their eggs, rather than to feed. It wasn't until the evolution of herbivory that new reptile groups appeared, able to take advantage of the abundant plant life of the Carboniferous.

Early reptiles belonged to a group called the cotylosaurs. Hylonomus and Paleothyris were 2 members of this grouping. They were pocket-size, cadger-sized animals with amphibian-like skulls, shoulders, pelvis and limbs, and intermediate teeth and vertebrae. The residual of the skeleton was reptilian. Many of these new "reptilian" features are also seen in trivial, modern, amphibians (which may also have direct-developing eggs laid on land e.g. New Zealand'southward leiopelmid frogs, so perhaps these features were simply associated with the small trunk size of the showtime reptiles.

The early mammals

A major transition in the development of life occurred when mammals evolved from 1 lineage of reptiles. This transition began during the Permian (286 - 248 million years ago), when the reptile grouping that included Dimetrodon gave rising to the "animate being-faced" therapsids. (The other major branching, the "lizard-faced" sauropsids, gave ascent to birds and modern reptiles). These mammal-like reptiles in turn gave rise to the cynodonts e.g. Thrinaxodon during the Triassic period.

This lineage provides an excellent series of transitional fossils . The development of a central mammalian trait, the presence of merely a single bone in the lower jaw (compared to several in reptiles) tin can exist 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 found in this lineage include the evolution 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 trunk, 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 e'er to occur. Some estimates suggest that up to 90% of the species and so living became extinct. (Contempo research has suggested that this event, similar the better-known end-Cretaceous event, was caused by the affect of an asteroid.) During the subsequent Triassic period (248 - 213 1000000 years ago), the survivors of that event radiated into the large number of at present-vacant ecological niches.

However, at the terminate of the Permian information technology was the dinosaurs, non the mammal-like reptiles, which took advantage of the newly bachelor terrestrial niches to diversify into the ascendant country vertebrates. In the body of water, the ray-finned fish began the major adaptive radiation that would encounter them become the most species-rich of all vertebrate classes.

Developments in the dinosaur lineage

I major change, in the grouping of reptiles that gave rise to the dinosaurs, was in the animals' posture. This changed from the usual "sprawling" mode, where the limbs jut sideways, to an cock posture, with the limbs held direct under the body. This had major implications for locomotion, equally it allowed much more than free energy-efficient movement.

The dinosaurs , or "terrible lizards", autumn into two major groups on the footing of their hip construction: the saurischians (or "cadger-hipped" dinosaurs) and the ornithischians (misleadingly known equally the "bird-hipped" dinosaurs). Ornithischians include Triceratops, Iguanodon, Hadrosaurus, and Stegosaurus). Saurischians are further subdivided into theropods (such equally Coelophysis and Tyrannosaurus male monarch) and sauropods (e.g. Apatosaurus). Well-nigh scientists agree that birds evolved from theropod dinosaurs.

Although the dinosaurs and their immediate ancestors dominated the globe'south terrestrial ecosystems during the Triassic, mammals continued to evolve during this time.

Further developments in the early mammals

Mammals are advanced synapsids . Synapsida is 1 of two nifty branches of the amniote family unit tree. Amniotes are the group of animals that produce an amniotic egg i.eastward. 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 eyes. This opening gave the synapsids (and similarly the diapsids, which accept two pairs of openings) stronger jaw muscles and meliorate 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. Later synapsids include the therapsids and the cynodonts , which lived during the Triassic.

Cynodonts possessed many mammalian features, including the reduction or complete absenteeism of lumbar ribs implying the presence of a diaphragm; well-developed canine teeth, the development of a bony secondary palate so that air and nutrient had separate passages to the back of the throat; increased size of the dentary - the main bone in the lower jaw; and holes for fretfulness and claret vessels in the lower jaw, suggesting the presence of whiskers.

By 125 million years agone 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 group that includes mod kangaroos and possums) were also nowadays. Until recently it was thought that placental mammals (the group to which most living mammals belong) had a much afterward evolutionary origin. Still, recent fossil finds and Deoxyribonucleic acid prove suggest that the placental mammals are much older, maybe evolving more than 105 1000000 years ago. Note that the marsupial and placental mammals provide some splendid examples of convergent evolution , where organisms that are not particularly closely related accept evolved similar body forms in response to similar ecology pressures.

All the same, despite the fact that the mammals had what many people regard as "advanced" features, they were still just minor players on the globe stage. As the world entered the Jurassic menstruation (213 - 145 million years ago), the dominant animals on land, in the sea, and in the air, were the reptiles. Dinosaurs, more than numerous and more extraordinary than those of the Triassic, were the chief state animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited past the pterosaurs .

Taking fly: Archaeopteryx and the origins of the birds

In 1861 an intriguing fossil was constitute in the Jurassic Solnhofen Limestone of southern Germany, a source of rare but uncommonly well-preserved fossils. Given the name Archeopteryx lithographica the fossil appeared to combine features of both birds and reptiles: a reptilian skeleton, accompanied by the articulate impression of feathers. This made the find highly significant as it had the potential to support the Darwinians in the fence that was raging post-obit the 1859 publication of "On the origin of species".

While it was originally described every bit simply a feathered reptile, Archaeopteryx has long been regarded every bit a transitional class between birds and reptiles, making it i of the most of import fossils ever discovered. Until relatively recently it was also the earliest known bird. Lately, scientists have realised that Archaeopteryx bears fifty-fifty 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 potent phylogenetic link betwixt the two groups. Fossil birds have been discovered in China that are even older than Archaeopteryx, and other discoveries of feathered dinosaurs support the theory that theropods evolved feathers for insulation and thermo-regulation before birds used them for flight. This is an example of an exaptation .

Closer exam of the early history of birds provides a good instance of the concept that evolution is neither linear nor progressive. The bird lineage is messy, with a diversity of  �experimental� forms appearing. Not all achieved powered flight, and some looked quite dissimilar modern birds e.g. Microraptor gui, which appears to have been a gliding brute and had disproportionate flying feathers on all four limbs, while its skeleton is essentially that of a minor dromaeosaur. Archaeopteryx itself did not belong to the lineage from which mod birds (Neornithes) take evolved, but was a fellow member of the now-extinct Enantiornithes. A reconstruction of the avian family unit tree would show a many-branched bush-league, non a single straight trunk.

The end of the dinosaur age

Dinosaurs spread throughout the world - including New Zealand, which had its own dinosaur fauna - during the Jurassic, but during the subsequent Cretaceous period (145 - 65 meg years agone) they were declining in species diversity. 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 rise to new species.

The origin of flowering plants (the angiosperms) during the early on Cretaceous triggered a major adaptive radiation among the insects: new groups, such as butterflies, moths, ants and bees arose and flourished. These insects drank the nectar from the flowers and acted as 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 land animal that weighed much more than 25 kg. This cleared the way for the expansion of the mammals on land. In the sea at this time, the fish again became the ascendant vertebrate taxon.

The advent of modernistic mammal groups

At the beginning of the Palaeocene epoch (65 - 55.5 million years ago) the world was without larger-sized terrestrial animals. This unique state of affairs was the starting indicate for the great evolutionary diversification of the mammals, which up until 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 time are scarce, and oftentimes consist largely of their characteristic teeth, we know that modest, rodent-like insectivorous mammals roamed the forests, the get-go large herbivorous mammals were browsing on the arable vegetation, and carnivorous mammals were stalking their prey.

The oldest confirmed primate fossils appointment to about threescore one thousand thousand years agone, in the mid-Palaeocene. The early 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: hands specialised for grasping, rotating shoulder joints, and stereoscopic vision. They besides have a relatively large encephalon size and nails on their digits, instead of claws.

The primeval known fossils of most of the mod orders of mammals appear in a brief period during the early on Eocene (55.5 - 33.7 meg years agone). Both groups of modern hoofed animals, the Artiodactyla ("even-toed" taxa such as 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 book "Hens' teeth and horses' toes".

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

The tendency towards a cooler global climate that occurred during the Oligocene epoch (33.7 - 23.8 million years ago) saw the appearance of the grasses, which were to extend into vast grasslands during the subsequent Miocene (23.8 - 5.3 one thousand thousand years ago). This modify in vegetation drove the evolution of browsing animals, such as more modern 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 Dna evidence suggests that the great apes evolved during the Oligocene, abundant fossils do not appear until the Miocene. Hominids, on the evolutionary line leading to humans, beginning appear in the fossil record in the Pliocene (v.iii - 1.8 meg years ago). The story of human evolution is covered hither - Human Development cloth.

  New Zealand, by virtue of its isolation and its relatively recent geological development, was non the eye of any novel evolutionary evolution. Notwithstanding, many of the species that appointment dorsum to Gondwanaland, or that arrived more recently as migrants, have undergone pregnant adaptive radiations 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 in that location were about twenty cycles of cold glacial ("Ice Age") and warm interglacial periods at intervals of almost 100,000 years. During the Ice Ages glaciers dominated the landscape, snowfall and ice extended into the lowlands, transporting huge quantities of rock with them. During these periods the South Island was extensively glaciated, and at that place were pocket-sized glaciers on the Tararua Ranges and Cardinal Plateau. Because a lot of water was locked up in water ice, the sea levels dropped during the glacials (up to 135m lower than at present). Extensive land bridges joined the chief and many offshore islands, allowing the migration of plants and animals. During the warmer periods large areas became submerged over again under h2o. These repeated episodes of ecology fragmentation drove rapid adaptive radiation in many NZ species, peculiarly (but not exclusively) the alpine plants.

For case, speciation patterns in the native Placostylus flax snails of Northland tin can be related to changes in sea level. Originally two-3 species were widespread at a time of low sea levels. Rising seas at the end of the glacial period isolated these as populations on offshore islands, where differential natural pick pressures led to the evolution of a greater number of dissever species.

The distribution of land snails such equally Powelliphanta in Marlborough and the southern Northward Island as well offers show for the presence of country bridges and the possibility of future speciation. The same varieties are institute both n and south of Cook Strait, implying a continuous land bridge in the by as the animals dice in table salt h2o. The fact that no further speciation has occurred in this example suggests that the state bridge was recently submerged past rising seas, perhaps only ten,000 years ago.

New Zealand Example

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

Reference Books

Chambers, P. (2002) Bones of Contention: the fossil that shook science; John Murray, London

Cowen, R. (1995) History of Life (second edition); Blackwell Scientific Publications

Gould, Stephen Jay ( 1983) Hen's teeth and Horses' toes

Strickberger, Monroe B.  (2000) "Development" (3rd edition), published by Jones & Bartlett

Source: https://sci.waikato.ac.nz/evolution/AnimalEvolution.shtml#:~:text=Genetic%20data%20suggest%20that%20multicellular,date%20back%20600%20million%20years.

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