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De-extinction is the process of creating an organism which is or greatly resembles a member of an extinct species. De-extinction requires an in-depth study of the biophysical conditions where the species can live and reproduce in relation to other species, including humans and adapt to environmental changes . There are some pros and cons when it comes to de-extinction and bringing back species. These pros and cons are often debated with many unanswered questions if we can bring these species back. Many people think that it is our responsibility to bring back these species, because humans were the ones to drive them to extinction. The main way in which de-extinction can occur is through cloning. The woolly mammoth is the closest species to be brought back, followed by the passenger pigeon.
Cloning is one of the main methods to bringing back these extinct animals. It takes extensive research, highly trained professionals and the best technology.
Cloning relies heavily on genetic engineering and technology and the use of DNA synthesizing.  Scientists believe that they have the type of technology to bring back species that have been extinct. Cross-species cloning is using a mammal that is closely related to the other animal. The scientists can use donor cells to transfer genetic material from one cell to another. The procedure used to produce cloning animals involves several steps and highly trained professionals. The first step is to identify a suitable nuclear donor cell in which to clone. Skin cells are often used to extract the DNA to put into those donor cells. 
Many scientists and researchers are in a debate on whether to bring back these extinct animals. There are many pros and cons, with unanswered questions if we do bring them back. De-extinction has raised a number of ethical and political questions, that many scientists don't know how to answer. Many people think that it is our responsibility to bring back these animals because we have the technology.
Some researchers and scientists believe that it is our responsibility to bring back these extinct animals, because we are the ones who made them extinct. Many scientists also believe that introducing these new species will help with genetic variation and start to introduce new species all together in the future.
There are many concerns from scientists and researchers when it comes to bringing back these animals. Some of these concerns are resource allocation, species identification and classification, and the relationship between technology and nature preservation. Some fear that the re-created species may become vectors or reservoirs for viruses . There is no evidence or research to show that bringing back these animals will not cause harm to other species or even humans. There could also be pregnancy concerns when it comes to the surrogates carrying these engineered animals. Those animals are called the “mother” animal. There could be abnormalities to the newborn, which could cause some problems seeing how we don’t know what kind of personalities these new animals could bring and how they would react with other animals . There is debate about the constant changing environment that can effect these animals who are used to a cold climate all year round . This could be a problem seeing how our planet keeps getting warmer.
The woolly mammoth is probably the most charismatic de-extinction candidate . Four thousand years after the woolly mammoth vanished from the Earth, scientists have deciphered the genetic blueprint that may offer a key to bringing them back. Scientists have done this by recovering DNA from two long dead individuals and a team of researchers have sequenced the species entire genome. The mammoths’ close relative, the Asian elephant would be the perfect host for this genome to be implanted  . A Harvard team is working to introduce specific DNA variants, like hair, tusks, fat and cold resistance to the Asian elephant. Even though the woolly mammoth is the closest animal to de-extinction, it still may take many years. Making an elephant's genes 9% mammoth might take 20 years to develop .
The passenger pigeon could be resurrected by splicing its genes into the genome of its close cousin, the band-tailed pigeon . The passenger pigeon is one of the mammals who became extinct due to humans. Humans used to hunt and shoot these animals, being afraid that they were carrying special messages and hunted for sport. One of the last passenger pigeons died in a zoo, and is now preserved in a museum. Scientists believe that they can use the DNA from the zoo pigeon and start cloning. The band- tailed pigeon can be implanted with the passenger pigeon DNA and start the cloning process. Once the scientists have created a passenger pigeon like genome, they will insert the altered DNA into reproductive cells in the band- tailed pigeon embryos .
Add any other relevant external links in the section.
- Bringing Them Back to Life - by National Geographic
- Woolly Mammoth Clones Closer Than Ever, Thanks to Genome Sequencing - by Livescience Magazine
- Waking the Dead: Bringing Extinct Species Back to Life - by The Long Now Foundation
- How to Clone a Mammoth: Science of De-extinction - by Voice of America