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De-extinction, also known as resurrection biology, or species revivalism, is the process of generating an organism that is either an extinct species or resembles an extinct species. There are several ways to carry out the process of de-extinction. Cloning is the most widely proposed method, although genome editing and selective breeding have also been considered. Similar techniques have been applied to certain endangered species, in hopes to boost populations. The only method of the three that would provide an animal with the same genetic identity is cloning. There are both pros and cons to the process of de-extinction, ranging from technological advancements to ethical issues.

Cloning
Cloning is a commonly suggested method for possible restoration of an extinct species. This can be done by extracting the nucleus from a preserved cell from the extinct species and swapping it into an egg, without a nucleus, of the nearest living relative. This egg can then be inserted into a relative host. It is important to note that this method can only be used when a preserved cell is available. This means, this would be most feasible for recently extinct species. Cloning has been used in science for years and dates back to the 1950's. One of the most well known, successful clones is Dolly, the sheep. Dolly was born in the mid 1990's and lived a normal life until she had health complications, eventually leading to her death. A sheep is not the only species to undergo cloning. Some common species include dogs, pigs, and horses.

Genome editing
Genome editing has been rapidly advancing with the help of the CRISPR/Cas systems, particularly CRISPR/Cas9. The CRISPR/Cas9 system was originally discovered as part of the bacterial immune system. Viral DNA that was injected into the bacterium became incorporated into the bacterial chromosome at specific regions. These regions are called clustered regularly interspaced short palindromic repeats, otherwise known as CRISPR. Since the viral DNA is within the chromosome, it gets transcribed into RNA. Once this occurs, the Cas9 binds to the RNA. Cas9 can recognize the foreign insert and cleaves it. This discovery was very crucial because now the Cas protein can be viewed as a scissor in the genome editing process.

By using cells from a closely related species to the extinct species, genome editing can play a role in the de-extinction process. Germ cells may be edited directly, so that the egg and sperm produced by the extant parent species will produce offspring of the extinct species, or somatic cells may be edited and transferred via somatic cell nuclear transfer. This results in a hybrid between the two species, since it it not completely one animal. Because it is possible to sequence and assemble the genome of extinct organisms from highly degraded tissues, this technique enables scientists to pursue de-extinction in a wider array of species, including those for which no well-preserved remains exist. However, the more degraded and old the tissue from the extinct species is, the more fragmented the resulting DNA will be, making genome assembly more challenging.

Back breeding
Back breeding is a form of selective breeding. As opposed to breeding animals for a trait to advance the species in selective breeding, back breeding involves breeding animals for an ancestral characteristic that may not be seen throughout the species as frequently. This method can recreate the traits of an extinct species, but the genome will differ from the original species. Back breeding, however, is contingent on the ancestral trait of the species still being in the population in any frequency.

Iterative evolution
A natural process of de-extinction is iterative evolution. This process occurs when a species becomes extint, but then reappears after some amount of time. An example of this process occurred with the White-throated Rail. This flightless bird became extinct approximately 136,000 years ago due to an unknown event that caused sea levels to rise, which resulted in the demise of the species. The species reappeared about 100,000 years ago when sea levels dropped, allowing the bird to evolve once again as a flightless species on the island of Aldabra, where it is found to the present day.

Advantages of De-extinction
It's been argued that revived species can be utilized as a tool to support other conservation initiatives by acting as a "flagship species" - a charismatic organism that generates popular support and funds for conserving entire ecosystems. Along this vein, it is thought that resurrecting the Aurochs would boost the European "rewilding" movement, in turn, transforming abandoned farmland into wildlife corridors. De-extinction would act as a flagship technology where the excitement stirred from the possibility of seeing an extinct species in the wild strengthens the focus on preserving ecosystems. Similarly, the conservationist Josh Donlan claims that if the passenger pigeon were resurrected, there would inevitably be a legal impetus for the protection of its habitat under the Endangered Species Act.

De-extinction would allow for scientists to study organisms they were not able to before. For example, if a plant had gone extinct before any scientific studies could be conducted, then there would be no data to show how beneficial or dangerous this plant could be. There could be major health benefits to this plant that the science community would not know if the de-extinction processes were not possible.

Oppositions to De-extinction
With any controversial issue, there will be concerns. The most pressing opposition to de-extinction is the welfare of the animals that will be raised. There is no way to tell how the animal will react to environmental conditions or how the health of the animal will be. This is particularly true when using the cloning method. Previously cloned animals have shown to be more susceptible to illness.

Woolly Mammoth
The existence of preserved soft tissue remains and DNA of Woolly Mammoths has led to the idea that the species could be recreated by scientific means. Two methods have been proposed to achieve this. The first would be to use the cloning process. This is proposed because even the most intact mammoths have had little usable DNA because of their conditions of preservation. There is not enough to guide the production of an embryo. The second method involves artificially inseminating an elephant egg cell with preserved sperm of the mammoth. The resulting offspring would be an elephant–mammoth hybrid. After several generations of cross-breeding these hybrids, an almost pure woolly mammoth would be produced. However, sperm cells of modern mammals are potent for 15 years at most after deep-freezing, which provides a hindrance to this method. In 2008, a Japanese team found usable DNA in the brains of mice that had been frozen for 16 years. They hope to use similar methods to find usable mammoth DNA. In 2011, Japanese scientists announced plans to clone mammoths within six years. As the woolly mammoth genome has been mapped, complete chromosomal DNA molecules may be synthesized in the future.

It was reported in March 2014 that blood recovered from a frozen mammoth carcass in 2013 now provides a better opportunity for cloning the woolly mammoth, despite previous hindrances. Another way to revive the woolly mammoth would be to migrate genes from the mammoth genome into the genes of its closest living relative, the Asian elephant, to create hybridized animals with the notable adaptations that it had for living in a much colder environment than modern day elephants. This is currently being done by Harvard geneticist George Church. They have already successfully made changes in the elephant genome with the genes that gave the woolly mammoth its cold-resistant blood, longer hair, and extra layer of fat. A revived woolly mammoth or mammoth-elephant hybrid may find suitable habitat in the tundra and taiga forest ecozones.

Harvard geneticist, George Church, gives an example of the positive effects of bringing back the extinct woolly mammoth would have on the environment. He explains that if the newly developed mammoth hybrids were to be placed in areas such as Siberia and Alaska, the outcome may reverse the damage that global warming has caused. He and his fellow researchers predict that mammoths would eat the dead grass allowing the sun to reach the spring grass; their weight would allow them to break through dense, insulating snow in order to let cold air reach the soil; and their characteristic of felling trees would increase the absorption of sunlight. If the theories are proven true, global warming could eventually be lessened in these areas. Scientific American, in an editorial condemning de-extinction, pointed out that the technologies involved could have secondary applications, specifically to help species on the verge of extinction regain their genetic diversity.

Aurochs
The Aurochs was widespread across Eurasia, North Africa, and the Indian subcontinent during the Pleistocene, but only the European aurochs (Bos primigenius primigenius) survived into historic times. This species is heavily featured in European cave paintings, such as Lascaux and Chauvet cave in France, and was still widespread during the Roman era. Following the fall of the Roman empire, overhunting of the Aurochs by nobility and royalty caused its population to dwindle to a single population in the Jaktorów forest in Poland, where the last wild one died in 1627. However, because the Auroch is ancestral to most modern cattle breeds, it is possible for it to be brought back through selective or back breeding. The first attempt at this was by Heinz and Lutz Heck using modern cattle breeds, which resulted in the creation of Heck cattle. This breed has been introduced to nature preserves across Europe; however, it differs strongly from the aurochs in both physical characteristics and behavior, and modern attempts have tried to create an animal that is nearly identical to the aurochs in morphology, behavior, and even genetics. The TaurOs Project aims to recreate the aurochs through selectively breeding primitive cattle breeds over a course of twenty years to create a self-sufficient bovine grazer in herds of at least 150 animals in rewilded nature areas across Europe. This organization is partnered with the organization Rewilding Europe to help restore balance to European nature. A competing project to recreate the aurochs is the Uruz Project by the True Nature Foundation, which aims to recreate the aurochs through a more efficient breeding strategy and through genome editing, in order to decrease the number of generations of breeding needed and the ability to quickly eliminate undesired traits from the new aurochs population. It is hoped that the new aurochs will reinvigorate European nature by restoring its ecological role as a keystone species, and bring back biodiversity that disappeared following the decline of European megafauna, as well as helping to bring new economic opportunities related to European wildlife viewing.

Equus quagga
The Equus quagga is a subspecies of the Plains zebra that was distinct in that it was striped on its face and upper torso, but its rear abdomen was a solid brown. It was native to South Africa, but was wiped out in the wild due to over-hunting for sport, and the last individual died in 1883 in the Amsterdam zoo. However, since it is technically the same species as the surviving plains zebra, it has been argued that the Equus quagga could be revived through artificial selection. The Quagga Project aims to recreate the animal through the selective or back breeding of plains zebras. It also aims to release these animals onto the western cape once an animal that fully resembles the Quagga is achieved, which could have the benefit of eradicating non-native trees.

Thylacine
The thylacine is native to continental Australia, Tasmania and New Guinea. It is believed to have become extinct in the 20th century. The thylacine had become extremely rare or extinct on the Australian mainland before British settlement of the continent. The last known thylacine, named Benjamin, died at the Hobart Zoo, on September 7, 1936. It is believed to have died as the result of neglect—locked out of its sheltered sleeping quarters, it was exposed to a rare occurrence of extreme Tasmanian weather: extreme heat during the day and freezing temperatures at night. Official protection of the species by the Tasmanian government was introduced on July 10, 1936, roughly 59 days before the last known specimen died in captivity.

In December 2017 it was announced in Nature Ecology and Evolution that the full nuclear genome of the thylacine had been successfully sequenced, marking the completion of the critical first step toward de-extinction that began in 2008, with the extraction of the DNA samples from the preserved pouch specimen. The Thylacine genome was reconstructed by using the genome editing method. The Tasmanian devil was used as a reference for the assembly of the full nuclear genome. Andrew J. Pask from the University of Melbourne has stated that the next step toward de-extinction will be to create a functional genome, which will require extensive research and development, estimating that a full attempt to resurrect the species may be possible as early as 2027.