For decades the notion of “de-extinction” hovered on the scientific fringes, Science‘s David Schultz stated in his recent article. Now new advances in genetic engineering, especially the CRISPR-Cas9 revolution, have researchers believing that it’s time to start thinking seriously about which animals we might be able to bring back, and which ones would do the most good for the ecosystems they left behind.
In “Should we bring extinct species back from the dead?,” David Schultz explains what is de-extinction, how does a species become extinct, and how close are we are bringing extinct animals back into the ecosystem. In his piece, Schultz focuses on two specific extinct animals – the woolly mammoth and the passenger pigeon. A brief excerpt of his piece can be found here:
Why bring back extinct animals?
As cool as it might be to visit a zoo filled with woolly mammoths, saber-toothed tigers, and giant tortoises, the best reasons for bringing back extinct animals have more to do with ecology than tourism. “If this is always going to be a zoo animal, then stop,” says ecologist Ben Novak, the lead researcher on the passenger pigeon project at Revive & Restore—a foundation devoted to genetically rescuing endangered and extinct species in San Francisco, California. “The goals have to be about ecological restoration and function.”
…Both the passenger pigeon and woolly mammoth were functionally unique species, and when they went extinct, their habitats changed dramatically. Harvard University’s George Church, the lead researcher working to de-extinct the mammoth, says that bringing back the giants could help convert the Arctic tundra back to grasslands that existed during the last ice age… Reviving the mammoth, Church says, could help slow climate change by shifting the landscape back toward the grasslands. “There’s twice as much carbon at risk in the tundra than in all the forests of the world put together.”
Likewise, the passenger pigeons, whose numbers are estimated to have reached nearly 5 billion at the start of the 19th century, played a dramatic role in shaping the forests they inhabited. Their numbers were so great and their droppings so prevalent and flammable that they destroyed trees and increased forest fires…
How do you de-extinct an animal?
There are three main approaches to de-extinction scientists talk about. The first, called backbreeding, involves finding living species that have traits similar to the extinct species…This isn’t really a true de-extinction, but it might still let us fill in missing ecological functions. In the case of mammoths, scientists might try to mate Asian elephants with more body hair than usual, for example.
A second option is cloning. Scientists would take a preserved cell from a recently extinct animal (ideally before the last of its kind died) and extract the nucleus. They would then swap this nucleus into an egg cell from the animal’s closest living relative and implant the egg into a surrogate host…Cloning may eventually give us basically identical genetic copies of extinct species, but we’ll be restricted to animals that went extinct more recently and have well-preserved cells with intact nuclei. The mammoth and the passenger pigeon may never be cloned.
The newest option is genetic engineering. Here, researchers would line up the genome of an extinct animal with that of its closest living relative. They would then use CRISPR and other gene-editing tools to swap relevant genes from the extinct animal into the living species and implant the hybrid genome into a surrogate (or grow it in an artificial womb). This approach doesn’t produce genetically identical copies of extinct animals, but rather modern versions of an animal engineered to look and behave like its extinct relatives. This is the technology being used by the mammoth and passenger pigeon groups.
How close are we?
That depends on what you count as a true de-extinction, which is sort of a gray area. If scientists engineer an Asian elephant to have small ears, extra fur, and more body fat by swapping in mammoth DNA, is it still an Asian elephant?
“If you’re willing to accept something that is an elephant that has a few mammoth genes inserted into its genome and therefore is able to make some proteins that mammoths might, we’re probably closer to that,” says Beth Shapiro, author of How to Clone a Mammoth: The Science of De-Extinction and an evolutionary biologist at UC Santa Cruz specializing in ancient DNA.
The passenger pigeons project faces similar questions. Novak wants to resurrect the bird using its closest living relative, the band-tailed pigeon, but how many genes need to be swapped to constitute success is somewhat arbitrary. “The two genomes are 97% the same. That 3% has built up over many millions of years and the majority of it is noise,” he says. “So the actual differences are much likely a smaller portion—probably within the realm of several thousand mutations. What we want to find is the key 20 or 100 mutations that affect the traits that are most important.”
There’s also a divide as to what constitutes a de-extinction success for the scientists versus the public. Genetically coaxing to behave like their extinct relatives might restore the ecosystem’s lost function, Novak says. But is that good enough to count? “I don’t think anyone in the world is really going to call it de-extinction unless the bird looks right.”
Even if researchers can pinpoint and transfer those key mutations (a daunting task), DNA is only half the battle. From there it’s a matter of getting the hybrid cell to grow in a surrogate, hoping all the genes work harmoniously together, bringing the hybrid to term, and hoping it acts like the extinct species even though it was raised by a modern relative. If all that goes right, you still need the hybrids to mate and give birth to fertile offspring….
How do we choose which animals to de-extinct?
In their recent publication, Douglas McCauley, an ecologist at UCSB, and his colleagues argue for three criteria to consider when choosing de-extinction candidates: Select target species with unique functions, concentrate on species that went extinct recently, and only work with species that can be restored to levels of abundance that meaningfully restore ecological function.
Although the mammoth and the passenger pigeon might pass McCauley’s first criterion, experts are skeptical about whether they’re truly the best animals to focus on. Shapiro points out that ecosystems are not static and have continued to change since these animals went extinct. “I worry about the dramatic changes to the forest in the eastern part of the North American continent,” she says. “I think there’s a lot we need to understand better about the passenger pigeon’s ecology and the effect that the passenger pigeon would have on that habitat before we can make a sufficiently educated decision.”
McCauley has similar worries: “Forests have fragmented, forests have expanded and contracted. A passenger pigeon that hits that forest again is going to be like a middle-aged guy who really wants to go back to high school and then he gets back there and he’s like, ‘Whoa I don’t fit in anymore.’”
He thinks that de-extinction efforts should instead be focused on recently extinct animals like the Christmas Island pipistrelle bat (Pipistrellus murrayi), the Réunion giant tortoise (Cylindraspis indica), and the lesser stick-nest rat (Leporillus apicalis). Although not as charismatic as a woolly mammoth, he says these creatures still have habitats to return to and would restore a unique function in their ecosystems. The lesser stick-nest rat, for instance, did what its name implies and built large stick nests in central Australia that became hubs of biodiversity.
What are the risks?
The spread of genes can be difficult to control. We probably won’t lose track of mammoths in Siberia, but what about rats? “It becomes hard to control those sorts of populations,” Seddon says. “And there are the same fears one might have about genetically modified crops—the idea that a modification may move into relatives, may jump in and out, or may not be expressed in the way that you expect.” Scientists are confident that there’s a safe way to proceed, but mistakes may come at a high cost if we can’t put the genie back into the bottle if something goes wrong. “If we lose sight of the true gravity of extinction and overzealously embrace de-extinction as a mitigation tool, it would be really easy to manufacture forests, savannas, and oceans full of Franken-species and Eco-zombies,” McCauley says.
But in spite of any danger, McCauley says his biggest concern isn’t a runaway genetic experiment wreaking havoc on a fragile ecosystem. “Honestly, the thing that scares me most is that the public absorbs the misimpression that extinction is no longer scary,” he says. “That the mindset becomes: Deforest, no biggie, we can reforest. If we drive something extinct, no biggie, we can de-extinct it.”…
“De-extinction is just the next step in a progression that conservation has already been on,” Novak says. “If you want to restore the ecological function of an extinct species, and you don’t have any living species that will do that, you take the closest living species you can get and adapt it based on the genome of the extinct species.”
…Still, there are no laws requiring that researchers take its advice. The only legal structures governing de-extinction are borrowed from genetically modified organism and cloning research—fields regulated by the U.S. Food and Drug Administration, Department of Agriculture, and Environmental Protection Agency.
Regardless, de-extinction is speeding closer to reality, and now is the time to start thinking about it, McCauley says. “For a long time it was easy to just put it aside because the technology wasn’t there,” he says. “But I don’t think we can do that anymore.”