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Gilbert’s work speaks to the difficulties of de-extinction through genetic engineering, a popular approach favored by researchers such as George Church, a professor of genetics at Harvard University who is leading a project aimed at bringing back the woolly mammoth of prehistory. With a large recent funding boost from the startup Colossal, Church is hopeful that they’ll make headway in the next decade or so by genetically editing mammoth genes into Asian elephants, a closely related living pachyderm.

But the de-extinction field encompasses more than genetic engineering. Using an approach called selective back-breeding, some groups are restoring ancient traits from extinct species by selectively breeding individuals that still carry the genes for them. For instance, the Tauros Program aims to back-breed modern cattle to make them more like their pre-domesticated ancestors, the aurochs, and the Quagga Project in South Africa is selecting for zebras that still have genes from the quagga, a subspecies hunted to extinction in the 19th century.

Still, even if these genetic engineering and selective breeding efforts succeed, they can only create a kind of hybrid rather than a purely resurrected species.

The closest you can get to an exact genetic replica of an extinct species is a clone created from a living or preserved cell from that species. Scientists don’t have useable cells from woolly mammoths, dodos, the Tasmanian tiger or most other species that are hyped up in the realm of de-extinction, but they do from some more recently extinct species. In 2003, researchers used cloning to bring back the bucardo, a species of wild goat, using a modern goat as a surrogate parent and egg donor. The baby bucardo, the only extinct species to ever be cloned, died after only seven minutes because of a lung malformation.

But even if cloning is someday more successful, according to the International Union for Conservation of Nature (IUCN), it could also lead to proxies “that differ in unknown and unpredictable ways from the extinct form.” For example, researchers may not know everything about potential epigenomic differences affecting DNA activity or the microbiome needed to support the species’ health. They also may not be able to recreate the exact learning environment in which the original species was reared, which could cause the behavior of the de-extinct species to deviate from that of the original.

Despite these differences, Novak said, “from an evolutionary standpoint, a clone is an authentic, or ‘true’, de-extinct organism.” In fact, although cloning is officially included in the IUCN guidelines and some other researchers would disagree, Novak doesn’t think cloning should even be considered de-extinction but rather a “true recovery.”

Functional Equivalents

The problems that riddle the field don’t dissuade de-extinction researchers. For them, a good proxy or functional equivalent of a lost species may be good enough. “I don’t actually know anyone who said we have to get a perfect copy of anything,” said Church. The practical goal of the woolly mammoth project he’s leading is to help endangered Asian elephants adapt to the frigid environments of the Arctic tundra.

“Make sure people don’t think they’re going to get a mammoth, because they’re not,” said Gilbert, who is not involved in that research. They will instead get a “hairy elephant” that can live in the cold.

Mammoth-elephant hybrids could be relocated to places such as Pleistocene Park, a large area of tundra in Russia where scientists are trying to restore the much more biodiverse and climate-friendly grasslands ecosystem it once was, when large grazers including mammoths populated the area. By trampling the soil and allowing cold air to seep in, the mammoth hybrids could in theory slow the melting of permafrost and the release of greenhouse gases that are warming the globe. The team also hopes that in the process, they can rescue the endangered elephant species by placing them in a large open area free from human conflict.

Similarly, Novak is working to resurrect the extinct passenger pigeon and the heath hen as genetically engineered hybrids of modern species, in the hope that they might help to restore their respective ailing ecosystems and motivate restoration efforts. The San Diego Zoo is trying to save the northern white rhino, a species that is functionally extinct because two females are the only ones left in the world. The zoo’s scientists are developing stem cells that could differentiate into northern white rhino sperm and eggs, and any resulting embryos might be carried to term by surrogate southern white rhinos.

“I’m excited about [de-extinction] and keep talking about it and keep doing interviews about it, not because I think we really are going to get a mammoth — I don’t think we will,” Shapiro said. “But because the path to getting us there is so important for conservation of living species.”

And if resurrected species are introduced into the wild, some of de-extinction’s successes may go even further in the long run. “If we get our proxies close enough,” Novak said, “evolution itself is probably going to converge them even closer to the original form than we can actually succeed in doing.” That is, if the forces that felled the original species don’t render their replacements extinct too.

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