The History of Animal Evolution
| Classification | Plant                      Evolution | Animal Evolution | Homology |
The History of Animal Evolution
For many people animals are perhaps the most familiar, and most interesting, of living things. This may be because we are animals ourselves. As such, we have a number of features in common with all the organisms placed in the animal kingdom, and these common features indicate that we have 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 as bacteria, plants                      and animals have a relatively recent evolutionary origin.                      DNA evidence suggests that the first eukaryotes evolved from                      prokaryotes, between 2500 and 1000 million years ago. That                      is, eukaryotes as a taxon date from the Proterozoic Era, the final Era of the Precambrian.                      Fossils of both simple unicellular and more complex multicellular                      organisms are found 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 one cell ingests another, but for some reason then              fails to digest it. The evidence for this lies in the way their cells              function. Both plant 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 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 found              in the eukaryote cell's 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 million years. This is because, 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 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 by smooth, wormlike organisms.                Such trace fossils have been found in rocks from China, Canada,                and India, but they tell us little about the animals that made them                apart from their basic shape.
The appearance of modern mammal                groups
The Ediacaran                      animals
Between 620 and 550 million years ago (during the Vendian Period) relatively large, complex, soft-bodied multicellular animals appear in the fossil record for the first time. While found in several localities around the world, this particular group of animals is generally known as the Ediacaran fauna, after the site in 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.e. 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 relationship to animals we know. Although many                of the Ediacaran organisms have been compared to modern-day jellyfish                or worms, they have also been described as resembling a mattress,                with tough outer walls around fluid-filled internal cavities - rather                like a sponge.
As a group, Ediacaran animals had a flat, 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 significant, as it suggests there must have been a lengthy period of evolution prior to their first appearance in the fossil record.
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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 place we find representatives                of almost 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 time scale, and their abrupt appearance                is often described as the "Cambrian explosion" however, bear in                mind that the fossil record of the "explosion" is spread over about                30 million years. I keep taking things out of brackets because it                is interesting relevant and memorable
One of the most famous assemblages of Cambrian fossils comes from                the Burgess                Shale of British Colombia. The rocks of the Burgess Shale were                laid down in the middle Cambrian, when the "explosion" had already                been underway for several million years. They contain familiar animals                such as trilobites, molluscs and echinoderms, but also the first                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, not only for their evidence of early variety among animal forms, but 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 case 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 assemblage, 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 evidence of predatory animals (e.g. Anomalocaris ), and therefore of complex predator-prey relationships. They also                give insights into how evolution might have 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.
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What caused the Cambrian                      "explosion"?
The cause of the proliferation of animal forms in the Cambrian is a matter of considerable debate among scientists. Some point to the increase in atmospheric oxygen levels that began around 2000 million years ago, supporting a higher metabolic rate and allowing 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 hard body parts such as teeth and supporting skeletons based on calcium carbonate (CaCO3), and also supporting higher levels of primary production as a result of increased concentrations of phosphates and nitrates. The mass extinction that marked the end of the Vendian period would have opened up ecological niches that the new animals exploited, as would habitat changes wrought by continental drift.
Genetic factors were also crucial. Recent 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 period of evolutionary                experimentation -- and competition. Many forms seen in the fossil                record of the Cambrian disappeared without trace. Future evolutionary                change was then limited to acting on the body 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 record. Genetic data suggest that multicellular                animals evolved around 1000 million years ago; this is supported                by fossil                embryos from rocks in China that date back 600 million years.                These embryos are more complex than those of simple organisms such                as sponges and jellyfish, which suggests that multicellular animals                must have evolved much further back in time. In addition, trilobites                were a very diverse group even early in the Cambrian, and some scientists                suggest that this indicates that the arthropod group must have had                a much earlier evolutionary origin.
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A foot on the land
Whatever their origins, animals may have 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                about 530 million years ago has changed this view. These animals                were arthropods, and resembled centipedes about the size of crayfish.                They probably didn't live on land, instead coming ashore to mate                or evade predators. At this time the only land plants appear to                have resembled mosses .
The earliest                      vertebrates
Animals continued to diversify in the Ordovician seas (505 - 440                million years ago). They were mostly 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 only by their tiny, but very common, teeth .
In terms of number of species invertebrates were by far the most common Ordovician animals - as they still are today. However, 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. However, 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 only the jawless fish had evolved from a chordate                ancestor. 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. However, neither the sharks                nor the agnathans became common until the Devonian. The other two                living lineages, the ray-finned (e.g. carp and kahawai) and the lobe-finned fish (e.g. lungfish and the coelacanth), evolved during the Devonian                period.
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The jawless fish
Agnathans,                        or jawless fish, were the earliest fish: an excellent                        fossil of Haikouichthys ercaicunensis dates back                        about 530 million years, to the Cambrian. Previously the                        earliest-known agnathans were dated to around 480 million                        years ago. Agnathans have traditionally been placed with                        the vertebrates due to the presence of a skull, although the modern                        forms such as 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. Only the lineage that produced the modern                        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 step as they colonised the land for the first time. We are not sure why this advance occurred, but it 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 million years earlier, were the first animals to become more permanent colonists. Fossil footprints made in the sandy flats surrounding temporary lakes dating back about 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 land 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 too big and too heavy and the walking legs too small to venture                onto land for any length of time and so they lived in marginal marine                (deltaic) environments.
Problems encountered                      in the move to land
These early land animals had to solve the same problems that                      plants faced when they moved to the land: water 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, ways of reproducing that did not involve                      water, and strong support systems ( endoskeletons and exoskeletons)                      that allowed them to move about on land. Remember that not                      all animal taxa were equally successful in solving these problems.
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The evolution                      of amphibians
By the Devonian period two major animal groups dominated                      the land: the tetrapods (4-legged terrestrial vertebrates) and the arthropods, including                      arachnids and wingless insects. The first tetrapods were amphibians ,                      such as Ichthyostega, and were closely related to a group                      of fish known as lobe-finned fish e.g. 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: it 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 last is important as a                      terrestrial predator cannot rely on water current to bring                      food into its mouth, but must move its head to catch prey.                      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 it retained a deep tail with fins. While Ichthyostega had four strong limbs, the form 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. However, careful examination                      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 amphibians were large-bodied animals with strong bodies and prominent ribs - quite different in appearance from modern representatives such as frogs and axolotls.
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What drove amphibian                      evolution?
It was originally believed that the tetrapods evolved during periods of drought, when the ability to move between pools would be an advantage. The animals would also have been able to take advantage 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, so that drought was not an issue. They were unlikely to be feeding on land: arthropods are small 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, avoiding predation by aquatic vertebrates (such as 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                land. However, they must still be in a moist environment, and the                size of the egg is restricted to less than 1.5cm in diameter. This                is because the egg is dependent on diffusion alone for gas exchange,                and means that the embryo must develop rapidly into a food-seeking                larval form rather than undergo prolonged development 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, but their numbers then dwindled rapidly and both groups became extinct by the end of the Carboniferous period. Lobe-finned fish also peaked in numbers during the Devonian.
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Early reptiles and                      the amniotic egg
One of the greatest evolutionary innovations of the Carboniferous                      period (360 - 268 million years ago) was the amniotic                      egg , which allowed 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 land by preventing the embryo                      inside from drying out, so eggs could be laid away from the                      water. It also meant that in contrast 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                      go through a larval food-seeking stage, but undergo direct                      development into a miniature adult form while in the egg,                      and fertilisation is internal.
The earliest date for development of the amniotic egg is                      about 320 million years ago. However, reptiles didn't undergo                      any major adaptive radiation for another 20 million years. Current thinking is                      that these early amniotes were still spending time in the                      water 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 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 rest of the                skeleton was reptilian. Many of these new "reptilian" features are                also seen in little, modern, amphibians (which may also have direct-developing                eggs laid on land e.g. New Zealand's leiopelmid                frogs ,                so perhaps these features were simply associated with the small                body size of the first reptiles.
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The early mammals
A major                transition in the evolution of life occurred when mammals evolved                from one 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 "beast-faced" therapsids. (The other major branching,                the "lizard-faced" sauropsids, gave rise 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 key mammalian                      trait, the presence of only a single bone in the lower jaw                      (compared to several in reptiles) can 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                      found 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 advance that occurred independently in the                      ancestors of the dinosaurs.
The end of the Permian was marked by perhaps the greatest mass extinction ever to occur. Some estimates suggest that up to 90% of the species then living became extinct. (Recent research has suggested that this event, like the better-known end-Cretaceous event, was caused by the impact of an asteroid.) During the subsequent Triassic period (248 - 213 million years ago), the survivors of that event radiated into the large number of now-vacant ecological niches.
However, at the end of the Permian it was the dinosaurs, not 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.
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Developments in the dinosaur lineage
One major change, in the group 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 erect posture, with the limbs held directly under the body. This had major implications for locomotion, as it allowed much more energy-efficient movement.
The dinosaurs , or "terrible lizards", fall into two major groups on the basis                of their hip                structure :                the saurischians (or "lizard-hipped" dinosaurs) and the ornithischians                (misleadingly known as 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 immediate ancestors dominated the world's terrestrial ecosystems during the Triassic, mammals continued to evolve during this time.
Further developments in the early mammals
Mammals are advanced synapsids .                      Synapsida is one 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 eyes. This opening gave the synapsids (and similarly                the diapsids, which have two 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. Later 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-developed canine teeth, the development of a bony secondary palate so that air and food had separate passages to the back of the throat; increased size of the dentary - the main bone in the lower jaw; and holes for nerves and blood vessels in the lower jaw, suggesting the presence of whiskers.
By 125 million years ago the mammals had already become a                      diverse group of organisms. Some of them would have resembled                      today's monotremes (e.g. platypus and echidna), but early marsupials (a group that includes modern kangaroos and possums) were                      also present. Until recently it was thought that placental mammals (the group to which most living mammals belong) had                      a much later evolutionary origin. However, recent                      fossil finds and DNA evidence suggest that the placental                      mammals are much older, perhaps evolving more than 105 million                      years ago. Note that the marsupial and placental mammals provide                      some excellent examples of convergent                      evolution , where organisms that are not particularly                      closely related have evolved similar body forms in response                      to similar environmental pressures.
However, despite the fact that the mammals had what many                      people regard as "advanced" features, they were still only                      minor players on the world stage. As the world entered the                      Jurassic period (213 - 145 million years ago), the dominant                      animals on land, in the sea, and in the air, were the reptiles.                      Dinosaurs, more numerous and more extraordinary than those                      of the Triassic, were the chief land animals; crocodiles,                      ichthyosaurs, and plesiosaurs ruled the sea, while the air                      was inhabited by the pterosaurs .
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Taking wing: 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 but exceptionally 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 clear impression of feathers. This made the find highly                      significant as 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 it was originally described as simply a feathered reptile, Archaeopteryx has long been regarded as a transitional                      form between birds and reptiles, making it one of the most                      important fossils ever discovered. Until relatively recently                      it was also the earliest known bird. Lately, scientists have                      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 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 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  ï¿½experimentalï¿½ forms appearing. Not all achieved powered                        flight, and some looked quite unlike modern birds e.g. 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                        small dromaeosaur. Archaeopteryx itself did not                        belong to the lineage from which modern birds (Neornithes)                        have evolved, but was a member of the now-extinct Enantiornithes. A reconstruction of the avian family                        tree would show a many-branched bush, not 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 million years ago) 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 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 end 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 dominant vertebrate taxon.
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The appearance of modern 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 situation was the starting point for the great evolutionary                diversification of the mammals, which up until then had been                nocturnal animals the size of small rodents. By the end of the epoch,                mammals occupied many of the vacant ecological niches. While mammal                fossils from this period of time are scarce, and often consist largely                of their characteristic teeth, we know that small, rodent-like insectivorous                mammals roamed the forests, the first large herbivorous mammals                were browsing on the abundant vegetation, and carnivorous mammals                were stalking their prey.
The oldest confirmed primate fossils date to about 60 million years ago, in the                      mid-Palaeocene. The early primates evolved from archaic 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 also have 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 period during the early Eocene (55.5                      - 33.7 million years ago). 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 same time as 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 East. These fossils chronicle the change from the                      land-dwelling mesonychids, which are the likely ancestors                      of whales, through animals such as Ambulocetus , which was still a tetrapod but which also has such whale-like                      features as an ear capsule isolated from the rest of its skull,                      to the primitive whales                      called the Archaeocetes .
The trend 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 million years ago). This change 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, first                      appear in the fossil record in the Pliocene (5.3 - 1.8 million                      years ago). The story of human evolution is covered here - Human Evolution material .
  New                      Zealand, by virtue of its isolation and its relatively recent                      geological development, was not the centre of any novel evolutionary                      development. However, many of the species that date back to                      Gondwanaland, or that arrived more recently as migrants, have                      undergone significant adaptive radiation in their new homeland.                      Some of the best examples of this can be related to the major                      ecological changes that accompanied the Pleistocene Ice Ages.
Throughout the Pleistocene there were about twenty 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 South Island was extensively glaciated, and there were small glaciers on the Tararua Ranges and Central Plateau.  Because a lot of water was locked up in ice, the sea levels dropped during the glacials (up to 135m lower than at present). Extensive land bridges joined the main and many offshore islands, allowing the migration of plants and animals.  During the warmer periods large areas became submerged again under water. These repeated episodes of environmental fragmentation drove rapid adaptive radiation in many NZ species, especially (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 2-3 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 selection pressures led to the evolution                of a greater number of separate species.
The distribution of land snails such as Powelliphanta in Marlborough and the southern North Island also 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 water. The fact that no further speciation has occurred                in this case suggests that the land bridge was recently submerged                by rising seas, perhaps only 10,000 years ago.
  New                      Zealand Example