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De-extinction, also known as species revivalism or resurrection biology, is the process of re-generating extinct species. The idea of de-extinction has been there for some time and has even received 海角社区wide attention through science fiction movies such as Jurassic Park. Bringing back such animals is both thrilling and terrifying. In real life, de-extinction is, however, incredibly challenging, and it remains uncertain whether dinosaur DNA is recoverable. Current technology can only utilize DNA samples of 1 million years old. Theoretically, scientists can bring back a Neanderthal but not a Triceratops as they last roamed the surface of the earth 65 million years ago. Approaches to de-extinction include cloning, back-breeding, and genome editing. Species that have been considered for de-extinction are the woolly mammoth, Pyrenean ibex, aurochs, thylacine, quagga, and the passage pigeon.

Cloning

Cloning is a term used to refer to somatic cell nuclear transfer, a technique that is used to create a genetic copy of an organism. It is among the most common methods suggested for the restoration of extinct species. The somatic cell nuclear transfer (SCNT) is a multistage procedure. Somatic cells are first harvested and cultured in vitro before nuclei are extracted from the cultured cells. Egg cells are concurrently harvested from a species that is closely related to the target animal. The enucleated egg is then fused with the extracted nucleus from the somatic cell. After cell division, the embryo is then implanted into a surrogate maternal host, which later gives birth to the genetic copy of the animal. Dolly, a sheep, born in the mid-1990s, is perhaps the most well-known example of a cloned animal. Cloning is an attractive de-extinction approach as it results in an organism that is identical at the nuclear genome level to the donor of the somatic cell. Intact living cells are, however, required for cloning. Unfortunately, most extinct species do not have intact living cells. That limitation means that only recently extinct species can undergo de-extinction through cloning from cultured and frozen cells that have been collected before death. Cloning of extinct species has been tried before with considerable success. In 2003, a cloned bucardo, an extinct subspecies of the Iberian Ibex, was born to a surrogate mother nine years after the death of the last individual. Years earlier, French, Spanish, and Belgian scientists had managed to harvest a skin sample of the last member of the subspecies, a female called Celia from which cells were cultured and then frozen. A majority of the reconstructed embryos failed to develop to term. Out of this process, a single bucardo was born but unfortunately died after birth due to a lung deformity.

Back-breeding

Back-breeding is the use of selective breeding to enhance certain ancestral traits in existing populations of living organisms. The technique aims to concentrate the desired ancestral traits that persist in a population into one individual. Mating pairs are selected if they display a desired behavioral or morphological quality. Gradual back-breeding eventually leads to the resurrection of traits that had previously been lost or diluted over time. The technique, however, has several limitations as a de-extinction approach. For example, the method requires that target ancestral traits persist in living species. Back-breeding can also lead to high levels of inbreeding or the creation of undesired combinations of alleles, which can lead to an overall decline in the population鈥檚 fitness. Back-breeding in the past has been used to try and bring back aurochs, an extinct species that is related to modern-day domestic cattle. Experts believe that the species had forward-facing horns, an aggressive temperament, and was also larger than the present-day cow. The traits can be found today dispersed across various breeds of cattle. The first attempt of de-extinction of aurochs began in the 1920s. Lutz and Heinz Heck, both directors of a German zoo, started the back breeding project that led to the creation of a Heck cattle. The cattle had a primitive appearance and were more aggressive. They were, however, not considered successfully back-bred. Experts note that they lacked the auroch鈥檚 distinctive morphological traits. Today, scientists are trying to bring back aurochs using genetics to guide the process to ensure a higher success rate.聽

Genetic Engineering

Genetic engineering takes advantage of advances made in the field of genome editing and DNA technologies. Advances in DNA sequencing and ancient DNA extraction technologies are increasingly making it feasible to reconstruct complete genome sequences of extinct species. With the help of such technology, scientists can align genomes to the genome sequences of closely related living species. Differences between the living relative and extinct animal鈥檚 genome are then edited through genome engineering resulting in living cells with the extinct genes. The living cells can then be used for cloning. In species that have been extinct over a long time, the reconstruction of the genome sequence is recoverable from various tissue types since DNA survives despite increasingly getting fragmented over time. DNA of extinct species in cold environments decays at a slower rate compared to those found in hot environments. Scientists, therefore, mainly concentrate on animals that died in Arctic regions. The oldest specimen that has undergone genome sequencing is a horse bone unearthed from frozen soil in the Canadian Arctic that is estimated to be 700,000 years old. The technique has made the resurrection of genome sequences belonging to samples of various extinct species possible, including mammoths. The method, however, has several limitations, including difficulty in recovering DNA from remains found in hot and wet environments. Genome from extinct species and those lacking close living relatives may also be challenging to assemble.

How Exact Is De-extinction?

聽It is important to note that none of the approaches listed lead to the 鈥渞esurrection鈥� of an organism that is precisely identical to the extinct species. For example, organisms that are cloned from frozen cells collected from extinct species share a nucleus genome but also have mitochondria from the enucleated egg, which belongs to a different animal. Genome editing leads to similar genes responsible for a phenotype, but the genes are expressed as part of a different genomic background. Precise replication of extinct species is, however, not necessary when de-extinction is aimed at conservation. The majority of de-extinction procedures currently being carried out are aimed at creating functional equivalents of the extinct species, ecological proxies capable of occupying the ecological niche left by the extinct species.

Ethics

Several critics of de-extinction have raised ethical issues as a reason as to why such projects should be discouraged. Motives of de-extinction have been questioned. Are we bringing back extinct species because we feel guilty for driving some of them to extinction? Are we seeking restoration justice? In that case, who is the justice for? Critics believe that justice is not for the individuals of species that have undergone de-extinction because they might suffer from malformation and maladjustment. Atoning for our ancestor鈥檚 actions by potentially causing further suffering to 鈥渞esurrected鈥� individuals animals, therefore, poses an ethical dilemma.

Ethology

Various animals, including mammals and birds, learn behavior by watching and interacting with other members of the species. De-extinction presents the challenge of having organisms that do not have other older individuals of the same species to interact with and learn from. Captive condor breeding programs have, in the past, demonstrated the dangers of not having parents from the same species. Despite humans using puppets as parents to ensure that the birds did not imprint on humans, the captive birds, once released into the wild, displayed an unhealthy curiosity for humans. The birds were also less social with other members of the species.聽

Ecology

The reintroduction of extinct species that have undergone de-extinction into the wild could also have unintended consequences on the overall ecology as we know it. For example, similar attempts on living species such as the Gray wolf鈥檚 reintroduction into Yellowstone National Park led to wide-ranging ripple effects, including the reduction of deer and elk populations, which led to the thriving of aspens.

Economics

Experts believe that apart from costs related to de-extinction, maintaining individuals that have been brought back could also be a very costly affair. For example, maintenance costs associated with a healthy elephant are $70,000 per year. Maintenance of animals gets more expensive as they get older. One must, therefore, consider long-term maintenance costs of the individuals that have undergone de-extinction; otherwise, we risk having a situation whereby individuals are euthanized once the excitement wears off and funding runs low.

De-extinction, A Method Of Conservation

According to Professor Xiuchun (Cindy) Tian of the University of Connecticut, with sufficient funding and political will, it might take only a decade to have zoos with rare and endangered animals. In 2000, plans to display a cloned gaur named Noah at the San Diego Zoo were not realized after the animal got an infection and died after two days. The Zoo, however, managed to house a cloned banteng for seven years before it was euthanized. Advocates believe that de-extinction is the next step in the conservation. Extinct species can be brought back to fill ecological functions that have been left vacant. Experts believe that organizations such as the IUCN are best suited to regulate de-extinction in the future.