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. 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. However, they must still be in a moist environment, and the size of the egg is restricted to less than 1. 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. Early reptiles and the amniotic egg. One of the greatest evolutionary innovations of the Carboniferous period - 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 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. New Zealand's leiopelmid frogs , so perhaps these features were simply associated with the small body size of the first reptiles. A major transition in the evolution of life occurred when mammals evolved from one lineage of reptiles.
This transition began during the Permian - 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. 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. 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 - 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. 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.
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. 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. 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 million years ago the mammals had already become a diverse group of organisms. Some of them would have resembled today's monotremes e. 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 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 - 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. Taking wing: Archaeopteryx and the origins of the birds. In 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.
One key organelle is the nucleus: the control centre of the cell, in which the genes are stored in the form of DNA. The engulfed bacteria eventually become mitochondria , which provide eukaryotic cells with energy. The last common ancestor of all eukaryotic cells had mitochondria — and had also developed sexual reproduction.
Later, eukaryotic cells engulfed photosynthetic bacteria and formed a symbiotic relationship with them. The engulfed bacteria evolved into chloroplasts: the organelles that give green plants their colour and allow them to extract energy from sunlight. Different lineages of eukaryotic cells acquired chloroplasts in this way on at least three separate occasions, and one of the resulting cell lines went on to evolve into all green algae and green plants.
The eukaryotes divide into three groups: the ancestors of modern plants, fungi and animals split into separate lineages , and evolve separately. We do not know in what order the three groups broke with each other. At this time they were probably all still single-celled organisms. The first multicellular life develops around this time.
It is unclear exactly how or why this happens, but one possibility is that single-celled organisms go through a stage similar to that of modern choanoflagellates : single-celled creatures that sometimes form colonies consisting of many individuals.
Of all the single-celled organisms known to exist, choanoflagellates are the most closely related to multicellular animals, lending support to this theory. The early multicellular animals undergo their first splits.
First they divide into, essentially, the sponges and everything else — the latter being more formally known as the Eumetazoa.
Around 20 million years later, a small group called the placozoa breaks away from the rest of the Eumetazoa. Placozoa are thin plate-like creatures about 1 millimetre across, and consist of only three layers of cells.
It has been suggested that they may actually be the last common ancestor of all the animals. The comb jellies ctenophores split from the other multicellular animals. Like the cnidarians that will soon follow, they rely on water flowing through their body cavities to acquire oxygen and food. The ancestor of cnidarians jellyfish and their relatives breaks away from the other animals — though there is as yet no fossil evidence of what it looks like. Around this time, some animals evolve bilateral symmetry for the first time: that is, they now have a defined top and bottom, as well as a front and back.
Little is known about how this happened. However, small worms called Acoela may be the closest surviving relatives of the first ever bilateral animal. It seems likely that the first bilateral animal was a kind of worm.
Vernanimalcula guizhouena , which dates from around million years ago, may be the earliest bilateral animal found in the fossil record. The Bilateria , those animals with bilateral symmetry, undergo a profound evolutionary split. They divide into the protostomes and deuterostomes.
The deuterostomes eventually include all the vertebrates, plus an outlier group called the Ambulacraria. The protostomes become all the arthropods insects, spiders, crabs, shrimp and so forth , various types of worm, and the microscopic rotifers.
The first hole that the embryo acquires, the blastopore, forms the anus in deuterostomes, but in protostomes it forms the mouth. The earliest known fossils of cnidarians , the group that includes jellyfish, sea anemones and corals, date to around this time — though the fossil evidence has been disputed. Strange life forms known as the Ediacarans appear around this time and persist for about 33 million years.
A small group breaks away from the main group of deuterostomes, known as the Ambulacraria. This group eventually becomes the echinoderms starfish, brittle stars and their relatives and two worm-like families called the hemichordates and Xenoturbellida.
Fossilised animal trails suggest that some animals are moving under their own power. As the first chordates — animals that have a backbone, or at least a primitive version of it — emerge among the deuterostomes, a surprising cousin branches off. The sea squirts tunicates begin their history as tadpole-like chordates, but metamorphose partway through their lives into bottom-dwelling filter feeders that look rather like a bag of seawater anchored to a rock.
Their larvae still look like tadpoles today, revealing their close relationship to backboned animals. The Cambrian explosion begins, with many new body layouts appearing on the scene — though the seeming rapidity of the appearance of new life forms may simply be an illusion caused by a lack of older fossils.
The first true vertebrate — an animal with a backbone — appears. It probably evolves from a jawless fish that has a notochord, a stiff rod of cartilage, instead of a true backbone. The first vertebrate is probably quite like a lamprey, hagfish or lancelet.
Around the same time, the first clear fossils of trilobites appear. These invertebrates, which look like oversized woodlice and grow to 70 centimetres in length , proliferate in the oceans for the next million years. They probably look like eels. Fossil evidence shows that animals were exploring the land at this time.
The first animals to do so were probably euthycarcinoids — thought to be the missing link between insects and crustaceans. Nectocaris pteryx , thought to be the oldest known ancestor of the cephalopods — the group that includes squid — lives around this time.
The Great Ordovician Biodiversification Event begins, leading to a great increase in diversity. Within each of the major groups of animals and plants, many new varieties appear. The Cambrian Period million years ago witnessed a wild explosion of new life forms.
Along with new burrowing lifestyles came hard body parts like shells and spines. Hard body parts allowed animals to more drastically engineer their environments, such as digging burrows. A shift also occurred towards more active animals, with defined heads and tails for directional movement to chase prey.
Active feeding by well-armored animals like trilobites may have further disrupted the sea floor that the soft Ediacaran creatures had lived on. Unique feeding styles partitioned the environment, making room for more diversification of life.
While Waptia scoured the ocean bottom, priapulid worms burrowed into the sediment, Wiwaxia attached to sponges, and Anomalocaris cruised above. Many of these odd-looking organisms were evolutionary experiments, such as the 5-eyed Opabinia.
However, some groups, such as the trilobites, thrived and dominated Earth for hundreds of millions of years but eventually went extinct. Stromatolite reef-building bacteria also declined, and reefs made by organisms called brachiopods arose as conditions on Earth continued to change.
However, despite all the changes that were to come, by the end of the Cambrian nearly all existing animal types, or phyla, mollusks, arthropods, annelids, etc. Skip to main content. Smithsonian Institution.
Early Life on Earth — Animal Origins. An Oxygen Atmosphere When cyanobacteria evolved at least 2. Multicellular Life However, other innovations were occurring. The First Animals These clusters of specialized, cooperating cells eventually became the first animals , which DNA evidence suggests evolved around million years ago. Ediacaran Biota By about million years ago the Ediacaran Period there was a proliferation of other organisms, in addition to sponges.
The End-Ediacaran Extinction However, about million years ago, most of the Ediacaran creatures disappeared, signaling a major environmental change that Douglas Erwin and other scientists are still working to understand. The Cambrian Explosion The Cambrian Period million years ago witnessed a wild explosion of new life forms.
Related Resources. Video: The Cambrian Explosion of Life. Science Literacy - What Is Biodiversity? Video: The Oldest Animal Fossils. Science Literacy: Extinction Over Time. The Burgess Shale Fossils. Virtual Tour: What Is Life?
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