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Will Dinosaurs Come Back?

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Linsey Gong

on 5 March 2014

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Transcript of Will Dinosaurs Come Back?

Will Dinosaurs Exist Again?

Will Dinosaurs Exist Again?
Mesozoic Era: Permian - Triassic Period
During the late Permian Period and early Triassic Period, which was approximately 250 million years ago, the archosaurs, a family of primitive reptiles, roamed the Earth. Known as the direct ancestors of dinosaurs, the archosaurs had fairly similar characteristics. The major difference was that archosaurs were a lot smaller in size compared to the well-known dinosaurs. After the Permian mass extinction, the early archosaurs started dominating over the habitats left behind by the organisms that died out.

Among the early archosaurs, evolutionary changes occurred that became a strong resemblance to the dinosaurs that descended after it. The archosaurs were originally very lizard-like and their bodies were more sprawled out, but they slowly evolved into organism that walked with their legs directly under their bodies. Secondly, their cold-blooded metabolism genes changed to warm-blooded. Although this did not occur in all archosaurs, the change was present in all dinosaurs.

With the many extinctions occurring near the end of the Triassic Period, this began in the rise in the population of dinosaurs as environmental changes greatly affected reptiles.

Mesozoic Era: Triassic Period
As the Jurassic Period came about, the supercontinent, Pangaea, began to break apart, leading to the formation of separated continents. This change led to the distribution of the then existing dinosaurs and allowed for diversity to take place as evolution occurred. Through Darwin’s theory of natural selection, dinosaurs with the most suited genes to accommodate the changing environment were able to survive, grow and reproduce as others died off.

The individuals in the population that had the traits better suited in their environment survived and produced more viable, fertile offspring compared to other individuals. This leads to the accumulation of favourable traits over time as the less beneficial genes are not passed on, thereby resulting in the extinction of certain species. As the environment keeps changing and the dinosaurs moved to different locations, natural selection resulted in new adaptations to accommodate the new conditions, thus giving rise to the evolution of new species.

Many reptiles, including the early archosaurs, had more difficulty surviving through the changes in the environment, ultimately leading to their extinction. The mild/warm climate of the Jurassic period that came about benefited the larger dinosaurs, as it allowed them to evolve. Some dinosaurs began showing changes in skeletal structure such as the Vulcanodon, to match up to the increase in size and weight. This evolution led to more diversity as dinosaurs in different areas changed differently in structure. For some herbivorous dinosaurs, they acquired the genes for longer body parts such as the neck and forelimbs to reach higher vegetation. This adaption is an advantage over other herbivorous dinosaurs as they are able to acquire more food sources while the others could only reach vegetation in areas closer to the ground.
Mesozoic Era: Cretaceous Period
By the Cretaceous Period, around 150 million years ago, the nature and appearance of the continents became more like present day. With this separation of the original supercontinent, groups of dinosaurs that were originally grouped together were isolated from each other. This led to more distinct evolutionary changes, such as adaptations for different food sources that became more abundant, as they couldn’t interbreed and exchange similar genes.

Herbivores developed many changes to adapt to the vegetation available. The larger sauropods, which were large herbivorous dinosaurs, were slowly replaced by smaller dinosaurs called ornithopods. This was most likely due to the fact to the environment change that led to a diet of new flowering plants during this period.

Other dinosaurs evolved differently to adapt to the new vegetation such as the nodosaurs, which like some earlier dinosaurs, developed a horny, beak like structure for grazing on plants near the ground.

Evolution mainly occurs through series of random events over time. In nature, events could randomly occur and alter the frequencies of species in different populations. One of the most important aspects for evolution to take place is genetic variation and this occurs through several processes. Some events include occurrences such as genetic drift, gene flow, mutations and sexual reproduction.

In genetic drift, which occurs most prominently in smaller populations, chance events cause the allele frequencies to fluctuate over generations. Alleles could be overrepresented, underrepresented or not appear at all in the following generation after the event. For example, a volcano erupted and killed off many of the dinosaurs that lived in that area. As time goes on, the species of dinosaurs that was not as affected by the event would continue to reproduce and flourish in numbers while the population of the other species continued to decrease.

When gene flow occurs, genes are moved from one population to another due to the movement of an individual and its gametes. A species of dinosaur that migrates to a new habitat can carry specific genes that were not present or were previously present in the new population which adds to genetic variability. However, with this movement of genes, the presence of gene flow can also cause two populations to become genetically similar, thus decreasing the chances of new species being formed.

Altering Allele Frequencies
Mutations and Sexual Reproduction
Mutations can randomly occur in individuals and as a result of the change in the sequence of DNA, new functions can form which can benefit or harm the organism. These changes, depending on whether it has a positive or negative impact on the organism can lead to genetic variation. If the mutation is negative, then the individual will have a disadvantage in its environment and will end up not being able to produce offspring so the trait will eventually die out. If the mutation is positive, then the individual has a better chance of survival in its environment, thus passing on its beneficial traits to the next generation. The inheritance of these traits leads to the evolution of species as they better adapt to their environment and continue reproducing.

Along with mutations, the process of sexual reproduction results in the most genetic variation that leads to evolution. The unique genotypes of each organism is produced from three mechanisms of sexual reproduction; random fertilization, recombination between homologous chromosomes during meiosis and independent assortment. When two organisms mate, alleles are exchanged, randomly distributed into gametes and then brought together to form offspring with individual genotypes. Depending on how beneficial these genes were in the environment of the parent, the offspring will either have a better or worse chance of survival due to natural selection.

Evidence in the Paleontology
Dinosaurs Found

Osteocyte cells
There are about 700 species of dinosaurs that have been found, however, only about half of them have been confirmed to have complete, or mostly complete fossils. There are about 300 valid dinosaur genera concluded, but there are about 500 different unconfirmed genera that have been given names to. In each genus, there is an average of one dinosaur species, but there are still approximately 100 different genera of unknown fossils that have been discovered. These 100 unknown fossils have been deemed too incomplete to be confirmed as dinosaur remains, possibly actually fossils of a different, already recognized group. Scientists are unsure of the actual number and groups of dinosaurs that existed, and scientists will never be certain of the number of dinosaurs that existed. This is because many dinosaur fossils never fossilized, causing them to be lost in history forever. However, paleontologists do expect to discover at least 700 to 900 more dinosaur genera.
The Kinds of Dinosaurs
The two most basic classifications of dinosaurs are as either lizard-hipped (Order Ornithischi) dinosaurs or bird-hipped (Order Saurischia) dinosaurs. Lizard-hipped dinosaurs had the ilia in the sacrum that stuck to their spine and they had left and right pubes that extended below the ilia. Also, their left and right Ischia extended down the back, behind the pubes and ilia, while in some dinosaurs, the pubes extended down forwards, similar to what we see in lizards. Lizard-hipped dinosaurs included dinosaurs such as the theropods and sauropods. The theropods were carnivorous dinosaurs that moved on two legs. Early birds and modern day birds are thought to have evolved from their theropod ancestors. A few examples of theropods include the tyrannosaurus rex, velociraptor, spinosaurus, allosaurus and the carnotaurus. The sauropods were enormous dinosaurs that evolved to walk on four legs, but were only plant eaters (herbivores.) A few examples include the apatosaurus (Brontosaurus), diplodocus and the brachiosaurus.

The second main classification of dinosaurs are the bird-hipped dinosaurs which were dinosaurs with pelves, a bone at the front of the lower jaw and a bone rimmed around the upper part of the eye socket. Bird-hipped dinosaurs included the herbivore, thyreophora dinosaurs, which were armored dinosaurs that survived during the early Jurassic period straight into the late Cretaceous period. Two examples of threophora dinosaurs are the stegosaurus, and the ankylosaurus. Another bird-hipped dinosaur is the cerapods, a variety of interesting groups of dinosaurs that included the triceratops, iguanodon, and parasaurolophus.

Some of the earliest existing dinosaurs were neither bird-hipped nor lizard-hipped, and were considered too specialized to be direct ancestors of dinosaurs. Some of these specialized organisms included the staurikosaurus and the herrerasaurus.
Reason They Died Out
The dinosaurs existed during the Triassic period, Jurassic period and Cretaceous period which were all during the Mesazoic era (248 to 65 million years ago.) Most of these interesting creatures survived the Triassic and Jurassic period mass extinctions, but unfortunately, all of them disappeared during the Cretaceous period mass extinction.

The Cretaceous period was a time period 145.5 to 65.5 million years ago. The lower half of the Cretaceous period was 146 to 98 million years ago, where there was a minor extinction that impacted a few organisms. The upper Cretaceous period was 98-65 million years ago, and by the end of this upper Cretaceous period, a mass extinction occurred. This mass extinction occurred at the K-T Extinction or the Cretaceous-Tertiary Boundary. The K-T Extinction is known to mankind to be the second largest extinction ever, which managed to kill all of the dinosaurs that existed. Although this was such a massive event, scientists are unsure about what major event could have caused such an extreme extinction.

There are a few beliefs for the cause of the K-T Extinction, one being that a comet or an asteroid hit our Earth 65.5 million years ago. Evidence for this extinction comes from the high iridium concentrations found at many boundary sites in marine and terrestrial sediments around the world. All of these iridium concentrations were dated to have come from the same time as the K-T Extinction. Iridium is an element only found in our Earth’s mantle, meteors, and comets from outer space, so it could have been deposited to Earth by a large meteor or comet. Also, scientists discovered evidence of a 10 kilometer in diameter meteor that hit the Yucatan Peninsula in Mexico. The meteor probably encouraged other disastrous events to occur, such as large forest fires. Colder weather occurred due to the disappearance of the sunlight, and hotter climates grew from the CO2 released into the atmosphere. All of these factors probably had a role to play in the fast disappearance of the dinosaurs during the Cretaceous period mass Extinction.

A second belief for the cause of the Cretaceous extinction is major volcanic eruptions. Evidence for volcanic eruptions is found along the K-T Boundary of India and Africa. It is believed that the Deccan areas erupted with an eruption rate 30 times greater than the Hawaiian volcanoes today. The Deccan eruptions erupted as lava flows, contrary to gigantic explosive flows over the course of a million years; shaking the K-T Boundary.

Some scientists even believe that the contribution of a comet or asteroid hitting the Earth along with multiple volcanic eruptions resulted in the K-T Boundary Extinction, the extinction that was able to kill all of the dinosaur kinds.

The top two extinction theories are common beliefs however, they are not guaranteed to be correct. Other theories include competition between mammals, as well as diseases. It is very difficult for scientists to estimate the amount of time such drastic extinctions could have taken, since these ideas are still mainly a theory. Though, this extinction is predicted to have taken as long as 200,000 years.

Evolution of Dinosaurs
A Triassic Period Scene
A Cretaceous Period Scene
The KT Boundary
Deccan Volcanic Eruptions
Two Types of Dinosaur Hips
Evidence in Physiology
Gene Flow Between Two Populations
Genetic Drift
A Mutation
Dinosaurs Closely Related to Birds: Anatomy and Genetics
One of the pieces of evidence supporting evolution is anatomy. Scientists compared the anatomy of different species, and believed that species with similar structures must have acquired them from a common ancestor. By comparing the anatomy, scientists can also determine how related a species is to another. Structures, from two more organisms that came from a common ancestor, although perhaps serving different purposes in each respective organism are called homologous structures. Vestigial structures are anatomical features that serve no purpose in their present organism, but were important in their ancestors.

Birds are widely believed to be the direct descendants of dinosaurs. Scientists have discovered that birds share many of the same skeletal features as some species of dinosaurs, including hollow bones, hips that allow walking upright. Other evidence includes the fact that only dinosaurs had the same ankle joint and ilium as birds. Some dinosaurs also had the same rear foot with a rear-pointing fourth toe, and both dinosaurs and birds share the same torso, long and flexible neck, and long rear limbs. Using proper terminology, birds are avian dinosaurs.
Sinosauropteryx: First found in China in 1996, the Sinosauropteryx is the first dinosaur to show evidence of having primitive feathers.
Archaeopteryx: Considered to have been the first bird, the Archaeopteryx is accepted to be the transitional form between dinosaurs and birds. It had a full set of teeth, a flat sternum, a long, bony tail, and three claws on its wings, bearing resemblance to a dinosaur, but had feathers, wings, and a wishbone, the characteristics of a modern bird.
Confuciusornis: The first prehistoric bird with a true break, the Confuciusornis lived during the Cretaceous period. The bird retained several features found in dinosaurs, including clawed fingers on its wings, but unlike other flying creatures found in this era, the Confuciusornis had no teeth, long flight feathers, and a short tail.

Continues on next slide...
The Sinosauropteryx
The Confuciusornis
The Archaeopteryx
Feathers in Dinosaurs
The use of feathers in feathered dinosaurs is still a questionable debate regarding the relationship between birds and dinosaurs.

Feathers could act as temperature regulators. Birds have high body temperature so they need to keep in heat, requiring some form of insulation. It is known that some dinosaurs had a bird-like metabolism; therefore feathers could have also helped dinosaurs with insulation, similar to a bird. Those who did not benefit from the insulation could have used the feathers to insulate their offspring, or eggs. Feathers could have also provided shade for the dinosaurs, and helped cool the dinosaurs.

Feathers have a large variety of sizes, patterns and colours compared to scales, therefore, they were probably a better alternative to scales on dinosaurs. Also, lightweight feathers along the body would have been more advantageous over heavy scales on the body. Related to this idea, feathers could also be easily lost or gained, depending on the amount needed, and they could also change colour or shape, easily camouflaging the dinosaur when camouflage was needed.
It is also believed that feathers could have been used for defense, where the still bristles were difficult to eat and attack. The feathers could have protected them against certain parasites, however, could have made them vulnerable to fleas.

There are various benefits that feathers could have had for dinosaurs, similar to their current bird relatives. The feathers could have provided balance to dinosaurs when they ran, and climbed, similar to ostriches today.
All of these ideas, and several more could have been a non-avian dinosaurs’ use for feathers. Therefore, believing that feathers are used for the sole purpose of flight is incorrect, but that feathers have evolved to aid with flight through its response to evolutionary novelties and selective pressures.

Evidence also suggests that dinosaurs like the Maniraptora- a group of theropods that many believed birds descended from. The semilunate carpal is a bone unique to this group, and along with other modifications of the forelimb, it makes flight stroke in birds possible. Other characteristics found in maniraptorans include a fused clavicle and sternum. Research shows that prehistoric birds had primitive wings with rigid layers of feathers for gliding, and natural selection could modify the forelimbs into feathered wings. Study of the Archaeopteryx shows that their wings composed of layers of long feathers, suit for gliding or flying since they produced a strong air foil.

Though the evidence is vague since there is no well-preserved fossil that could help the comprehension of the use of feathers in early dinosaurs, the theories could be combined to form a hypothesis that the dinosaurs used their feathers for warmth, then eventually for flight.

The Microraptor
With feathers on 4 limbs, thought to be link between dinosaurs and birds.
First found in China, 1996. Shows primitive feathers
Considered to be the first bird. Shows elements of dinosaurs such as the long, bony tail, flat sternum and teeth, as well as feathers and wings, resembling a bird.
Lived during the Cretaceous period. first bird with a true beak. It had no teech, and long feathers.
Flight of Dinosaurs vs Birds
The questionable origin of flight of the birds is also widely debated between birds and dinosaurs. There are 2 theories of flight, one being the arboreal, or “trees-down” theory, and the other being the cursorial, or “ground-up” theory. The arboreal theory suggests that early birds developed to fly from the ground, up. And the cursorial suggest that the early birds jumped from a tree and glided down. It is arguable if feathered dinosaurs used their feathers for flight or for other uses.

Some claim that flight is most likely originated from an arboreal ancestor, though the Theropods were ground dwellers who hunted on the ground. It seems unreasonable that ground dwellers would have ever taken to the air. A much more plausible proposal would be that birds had evolved from creatures that already lived in the trees, and perhaps glided. Also, the rib cage of the Archaeoteryx showed a rather flat sternum, compressed laterally while the bird ribcage is compressed more dorsoventrally. Alan Feduccia noted that the three types of vertebraes that developed true powered flight- birds, bates and pterosaurs. These mammals all have a common arboreal origin. None of these began to fly from a ground dwelling habitat. Gravity provides free energy for take-off, whereas running takes energy.

Contrary to the arboreal theory, that gliding is a simple process, gliding requires the development of a membrane called the “patagium” that animals stretch out. Flapping, however,is an anatomically demanding adaptation. Flapping requires many distinct skeletal specializations, such as forearm and shoulder anatomy and fusion of clavicles into a furcula. If evolving from a glider into a flapper involves so many anatomical changes, it seems unlikely, and that a strong advantage of the arboreal theory would be neglected. Also, the Archaeopteryx, the “first bird”, if it had learnt flight the “tree-down” way, there had to be trees in the landscape but it lived next to a lagoon. Since the theropods, believed to be the ancestors of the birds, were ground dwellers, the birds had to have learnt flight from the ground up.

Cold Blooded or Warm Blooded? : Cold Blooded
Were dinosaurs warm or cold blooded? The debate of this topic has been quite controversial. Some might argue that they were cold blooded like their reptile ancestors, but some argue that dinosaurs were active warm blooded mammals, related to birds and therefore endothermic. Though there are theories that dinosaurs evolved from cold blooded animals, reptiles, they evolved into warm blooded animals, the birds. There were differences in dinosaurs; the size of the dinosaur may have altered their regulation methods. An example in modern times would be monotremes such as the duck billed platypus, whose metabolisms were cold blooded.

Evidence that dinosaurs were cold blooded includes that of a cretaceous theropod, the Scipionyx. It is known as an extremely detailed specimen that included fossilized soft tissues such as muscles and internal organs. Its fossilized internal organs reveal that its colons and livers were similar to modern day crocodilians, cold blooded animals. The position of the liver gives information of its lungs, since a muscle in its liver helps regulate the function of the lungs in crocodilians. Therefore, the Scipionyx most likely had lungs resembling a reptilian, unlike that of a bird’s. Extremely large dinosaurs could also maintain their body heat from their lack of activity, so they would not need internal body processes to regulate their temperature. Another argument would be that the temperature the dinosaurs lived in is warmer than it is today, making the ability to provide body heat unnecessary. There are also no signs of dinosaurs having respiratory turbinates, which are folded bones in sinuses important in conserving water loss and are common in warm blooded animals. Spinosaurus and Ouranosaurus had large sails on their backs, and the Stegosaurus had numerous plates, these structures had been related to the use of heat dispersion and collection of heat. If these structures were used to regulate temperature, then it suggests that dinosaurs were indeed cold blooded. The dinosaurs earliest avian relatives had also be suggest to that they also had not obtained endothermy, since seasonal variation of their bone deposition indicates variable body temperature. However, there are controversies surrounding the evidence in the way that not all dinosaurs were large, and that even large dinosaurs hand small offspring. The temperature of the climate they lived in also varied in the Mesozoic era, some fossils were found in cooler regions with less mild climates. The issue of the respiratory turbinates also have critical evidence that argue that so far, there has been no evidence, but many birds do have them, therefore birds must have evolved them sometime during the Mesozoic.
The Scipionyx
The Large Sail of an Ouranosaurus
Cold Blooded or Warm Blooded? : Warm Blooded
There is also evidence that dinosaurs were warm blooded animals. Endothermic animals such as birds evolved from dinosaurs, so there is logic that they must have inherited their warm blooded nature from dinosaurs. Research also shows that the dinosaur’s limbs are arranged quite like a mammal’s limbs. Computer study also shows that the dinosaurs could actually move rather quickly, corresponding to that the faster the animal’s metabolism tends to be, the faster an animal moves. A fossilized dinosaur heart had also been recently discovered with four chambers, in contrary to the 3 chambered hearts of cold blooded creatures. It belonged to the Thescelosuarus, a late Cretaceous period ornitpod. It had been analyzed to have four chambers and a single aorta, similar to modern warm blooded animals. If the evidence is true, it suggests that other dinosaurs may also be warm blooded.

From the University of Adelaide, researchers experimented to see if the fossil bones of dinosaurs could indicate the bone metabolic rate and be used to determine the metabolic rate of the whole body. Our bones have tiny holes known as “nutrient foramen” that supply blood to inner bone cells and the size of these holes can indicate the maximum rate of activeness of an individual during exercise. Bone cells have a high metabolic rate, requiring large amounts of blood to deliver oxygen and there is the possibility that the size of the nutrient foramen could indicate how much blood was needed to maintain the bones in good condition.

To test this out, the researchers compared the size of the nutrient foramen in thigh bones and metabolic rates of living mammals and reptiles. Their results were ambiguous, with the size of the holes related only by the maximum metabolic rates during the most active movement of the individuals. The results were then compared to the fossils of ten different species of dinosaurs. The comparisons showed that even with the difference in body sizes of all the different species, all dinosaurs had holes that were bigger than mammals and reptiles. This indicates that dinosaurs were actually more active than present day organisms and were in fact warm-blooded creatures.

The Femur of Centrosaurus Apertus
The Dinosaurs Are Not Coming Back
Reason 1: Oxygen
The dinosaurs lived in the Triassic or Cretaceous periods, a time when oxygen levels were greatly different from the oxygen levels today. In the atmosphere today, there is said to be approximately 21% of oxygen. Though, during the Cretaceous period, it is said that oxygen levels were at approximately 30%. The Triassic period oxygen level is believed to be about 10 to 15%. Dinosaurs from both these periods, even if they had came back, would not be able to survive due to the excess, or lack of oxygen in the atmosphere. Lacking oxygen, respiratory system fail, and the oxygen would not be able to support the muscles of the dinosaurs; and if there was an excess of oxygen, the dinosaurs will be deprived of other gases. This excess of oxygen also increases the chances of a spark, leading to a disaster.
Reason 2: Human Influence on Their Niche
Due to the mass amounts of human resource exploitation and industrial development, there is a small amount of physical space for the dinosaurs. The rapid development of new infrastructure would also disturb the dinosaurs niches. Also, artificial selection of crops as well as general natural selection would deprive the dinosaurs of food and prey. The earth would also not be likely to be able to supply the dinosaurs with the food they need, therefore the dinosaurs would likely have to compete with humans for food. This competition makes sustaining a healthy population difficult, risking the healthy gene pool established.

CO2 pollution and other chemical / nuclear influences would also be extremely harmful to dinosaurs, since if they were to evolve back from birds, the birds who do not have resistance to these harmful chemicals would not give rise to dinosaurs who have the resistance.
Reason 3: Natural Selection
Natural selection, as coined by Darwin, ensures the "survival of the fittest", meaning that those best adapted to their environment will go on and reproduce. Since the dinosaurs have gone extinct, and their traits were lost due to the changing environment, it is safe to say that the dinosaurs will not be best fit for the current environment anymore. Also, due to the overwhelming amount of mutations accumulated overtime, natural selection is limited to act on only those existing genotypes or phenotypes in a population, organisms cannot grow an extra body part due to will.
Unless an earth changing disaster hits the earth, and the environment changes drastically, similar to the time of the dinosaurs, they may come back as a result of natural selection. Though this scenario is unlikely, and due to the large amount of mutations, the process of evolving back to dinosaurs would also be slow, and mutations are random, which could lead to another organism instead.
Reason 4: Global Warming
Global warming raises this interesting possibility of the dinosaurs evolving back, but also diminishes the possibility. NASA confirms that the trend of global warming will continue, and this continuous change in temperature will soon make the temperature no longer suitable for the dinosaurs. Also, as global warming progresses, there will be more species going extinct due to the change in their environment and food. With the increased extinction rate, birds would also suffer, thus making the dinosaurs less likely to come back due to the shrink in the gene pool. This selection due to the rise in temperature would select the birds suited for the climate as well, so, since the birds are the best suited for their environment at that time, it is unreasonable for dinosaurs to evolve back.
Cladogram of Dinosaur Relationships
Cladogram of Dinosaurs in Relation to Other Species
project by: Jasmine chan, linsey gong, xuedan xu
thank you for reading!
After a study completed at the Oxford University, it is believed that the shortening of a bird’s bony tails over 100 million years ago allowed the birds to develop legs that help would help them with more than one task, an evolutionary improvement. The researchers studied bird fossils from the Cretaceous period in China, North America and South America which have already had the power of flight and differences at the forelimbs, but their main focus was on the hind limbs of the fossils. The researchers discovered that the birds’ long, bony tails that disappeared after they were able to fly led to a diversity of the hind legs of the early birds. When the researchers compared measurements of the early birds’ legs to their dinosaur relatives, they determined whether or not bird evolution was exceptional. This research shows that although birds may have just been another type of dinosaur, they quickly evolved to develop special advantages over their dinosaur relatives, to survive even to modern days. This evolutionary advantage was losing the long, bony dinosaur tail, which allowed them to show fast evolutionary diversification, compared to the dinosaurs.
Though there are controversies with this link as well. The interpretation of the relationship may be false, and there is evidence to support that birds are not descendants of dinosaurs. The birds have a demand of powered flight, therefore, requiring very difference physiology and breathing. Their system does not permit oxygenated air to mix with deoxygenated air, making it much more efficient than cold-blooded reptiles such as crocodiles. Birds also have an immobile thigh bone, which aids flight. The relationship between this locked thigh bone and the air-sac structure of the lungs is strong. The immobile femur is unique to birds and not seen in vertebra, making it unlikely to be found in dinosaurs.

Another method of determining relatedness of two species is by looking at their proteins. Similarities in the amino acid sequences of proteins would show that two organisms might've originated from the same common ancestor. Although species would have evolved due to natural selection, the protein sequences would still retain elements from the original sequence, hence why similarities would be found. The nucleic acid sequences in species that are closely related would be very similar, while sequences in distantly related species would have different bases at many spots along the sequence, due to mutations accumulating over a longer period of time.

The link between birds and dinosaurs are again supported with the discovery of links between dinosaur genes and bird genes. The fossil feathers of one specimen, Shuvuuia deserti, have been tested positive for beta keratin, the main protein in bird feathers. The T. Rex proteins has also been recovered and been partially sequenced from a 68 million year old fossil first described in 2005. The researchers then compared the protein to 21 living animals such as chickens, ostriches and alligators. Investigation of this protein had produced a link between dinosaurs and birds.
Evolution and Natural Selection
Evolution, as defined by Darwin, is "descent with modification. It refers to the changes seen over time in a population. The changes in the population are often because of natural selection, or other mechanisms explained later. These heritable traits are passed on through generations, from parent to offspring through sexual production, which also contributes to genetic variation of organisms. Evolution happens on 2 scales: macroevolution and microevolution. Macroevolution is seen on a grand scale, it looks at the arching history of life in terms of stability, change, and extinction. Microevolution is referring to a change in gene frequency within a population. Evolution at this small scale can be observed over short periods of time, between generations.
Examining beyond individual species
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Natural selection occurs as a process of evolution, with some gene frequencies decreasing and increasing as a result of its disadvantage or advantage in an environment. This idea originated from Charles Darwin who published his findings in the mid-1800s. His theory states that:
1. The individuals best suited to their environment have a higher chance of surviving and passing on their beneficial traits to their offspring
2. All species tend to “overreproduce”, with the environment only being able to support a certain amount of individuals, resulting in competition for resources among members of populations
3. Over time, environments continuously change and with new conditions comes new adaptations, giving rise to new species
The organisms with the more beneficial gene will survive and successfully reproduce, leading to the second generation full of individuals with the more advantageous gene. With this continuously occurring, it results in the domination of one gene over another in a population, tipping the genotypic ratio for the gene that increases the chance of survival.
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