Henry Nicholls examines a clear appraisal of what it would really take to resurrect extinct species.
How to Clone a Mammoth: The Science of De-Extinction
- Beth Shapiro
A hazard of studying an extinct charismatic species such as the woolly mammoth is that you spend a lot of time answering the same question: “Is it possible to clone it?” For evolutionary molecular biologist Beth Shapiro, who has a long-standing interest in both ancient DNA and mammoths, the solution was to write a book.
In How to Clone a Mammoth, Shapiro has an alluringly simple goal: “to provide a road-map for de-extinction” in a single volume “that separates science from science fiction”. She begins by considering which species might be targeted for de-extinction, anticipating many of the difficulties to come. The poor preservation of dodo remains, for instance, means that any DNA fragments recovered are unlikely to give a clear idea of its genome. The moa, a giant flightless bird from New Zealand, is probably out on the basis that its closest living relative (the tinamou) is not close enough to assist in assembling its DNA. Steller's sea cow (Hydrodamalis gigas) has no living surrogate that could accommodate a cloned fetus, and the natural habitat of the Yangzte River dolphin (Lipotes vexillifer; widely thought to be extinct) is currently too polluted to receive the cetaceans.
Shapiro contends that the focus on individual species is misguided. “De-extinction has a place in our scientific future, but not as an antidote to extinctions that have already occurred,” she writes. Engineering extinct traits into living organisms, however, could help those organisms to adapt to environmental shifts, and could re-establish ecological interactions that disappear when a species goes extinct. In Shapiro's view, this is the real value of de-extinction technology. Yet the little that we currently understand about the operation of bygone genes means that her argument is necessarily vague on detail.
Targeting the woolly mammoth (Mammuthus primigenius), a keystone species of the steppe-tundra during the last glacial period (from roughly 110,000 to around 12,000 years ago), Shapiro begins the journey towards de-extinction, and hunts for a well-preserved specimen. One bizarre expedition to Siberia ends with the juxtaposition of armed nomadic reindeer herders and a French couple with a cooler box filled with cheese. But cells and DNA degrade rapidly after death, so it is unlikely that anyone will ever discover an intact mammoth nucleus that will allow the species to be rebooted with the cloning technique of somatic-cell nuclear transfer.
Breeding the mammoth back into existence through carefully arranged elephant back-crosses is also a non-starter, Shapiro shows. Elephants are tropical beasts with none of the adaptations to low temperatures that would need to be concentrated through artificial selection. The only realistic route, she concludes, is to engineer mammoth-like traits into an elephant stem cell. As sci-fi as this sounds, she predicts that mammoth revivalists will have achieved it within a few years.
Several hurdles remain, and Shapiro presents these clearly and entertainingly as a brilliant thought experiment at the boundaries of biological plausibility. The engineered stem cell would need to be cloned, the cloned embryo transferred to a surrogate or artificial uterus, mammoth-like elephants reared in sufficient numbers in captivity, a suitable habitat made available, and the public convinced of the benefits of releasing these genetically modified organisms. The beasts would then need to be set free.
Given the astronomical odds against pulling off all these steps in succession, it is remarkable to discover that a respected scientist such as Shapiro is actively involved in projects to bring the woolly mammoth and passenger pigeon (Ectopistes migratorius) back from the dead. The idea of enriching biological diversity by engineering long-lost genetic innovations into extant species is certainly worth considering and not so very different from how we already engineer crops. Unfortunately, the ecological argument for mammoth de-extinction is distinctly woolly, based more on the appeal of rewilding than on clear experimental evidence of the role of large herbivores on the Arctic tundra.
In the case of the passenger pigeon, the argument in favour of de-extinction is thinner still, and involves some seriously bird-brained ideas, such as the proposal to paint thousands of homing pigeons to look like passenger pigeons so that they can train up the resurrected birds for release.
Shapiro acknowledges that there are probably better cases for de-extinction, though she does not explore any in detail. This is a shame. The mammoth and passenger pigeon might be the perfect species to illustrate the fraught, often contradictory logic of de-extinction, but it is in simpler, more tractable settings, where there are fewer mammoth leaps of faith, that de-extinction might become the “powerful new tool” for conservation that Shapiro predicts.
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Nicholls, H. De-extinction: A behemoth revived. Nature 521, 30–31 (2015). https://doi.org/10.1038/521030a