Today we’re hearing more from scientists about a word that first appeared, as far as I can tell, in science fiction in the 1970s. It’s “de-extinction,” and it raises the question: Can we clone lost species?
It might actually be possible.
But are we clear about what we want to do? Why? And what the implications would be? I’m afraid not.
The technology to bring extinct species back to life is still in its infancy. Right now we can sequence the genome of an extinct species, discover which genes are central to making it look and act the way it did, and even edit the genomes of closely related living species so that the new cells express extinct genes. There are, of course, several steps between creating cells that can survive in a petri dish and releasing mammoths into the countryside. At present, there is no clear path from cell to a living creature.
When I ask people which species they would like to revive and why, I find their motivations nearly as diverse as the species they advocate. Some people are excited; they imagine that de-extinction might promote conservation, or that resurrected species might provide ecological benefits. Others focus on the potential technologies that might be useful for other purposes, or for improving our understanding of the processes that lead to extinction. And some just want to see and touch a living mammoth.
Indeed, the most popular candidates among both scientists and the public are the mammoth and the passenger pigeon. Both are iconic species whose extinction was probably our — human — fault. But their de-extinctions raise decidedly different issues. It is not now possible to “clone” birds, in part because of the way their embryos develop. Nor can we yet reproduce long-dead species like the mammoth, because living cells to clone them do not exist.
Despite the many uncertainties, however, it seems almost inevitable that some extinct species will be resurrected. The first species may not be a 100-percent clone of the species that once existed, but we may soon be (if we are not already) able to create living organisms that have been genetically engineered to look and act a little bit like their extinct evolutionary cousins.
Given the diversity of motivations, how should we decide which species to bring back?
Criteria for selecting a species for de-extinction need to consider both feasibility (whether the species can be brought back to life) and morality (whether it should be). Those issues are not always cleanly separable, for example when a technical solution (like finding a surrogate host) bumps up against an ethical dilemma (the only practical surrogate host is itself an endangered species).
Let’s begin with feasibility, as it seems more straightforward.
It seems likely that some extinct species will be resurrected. But how do we decide which ones?
First, suitably well-preserved specimens must exist. Different technologies require different levels of preservation. For example, if the intent is to clone the extinct species using somatic-cell nuclear transfer — the process that brought us Dolly the Sheep in the mid-1990s — then living cells of the species to be cloned are required. Only those whose tissue samples were taken and preserved before their extinction can be brought back in that way. Resurrecting long-dead species, including mammoths and passenger pigeons, will use different technologies, involving piecing together DNA sequences extracted from preserved bones and other remains. DNA does not preserve well in most environments, and is particularly short-lived in locations that are hot and wet. Tropical species are therefore not likely to be good candidates for de-extinction, at least given today’s technical limitations.
Second, it must be possible to create a species that can live independently, if our goal is not simply to create a single animal to live in a zoo. Numerous obstacles must be overcome. Interspecies surrogate pregnancies are rarely successful, so if a species has no evolutionarily close living relatives, it may be difficult to establish and bring to term a pregnancy. Even then, it may not be possible to find a suitable surrogate — consider a Steller’s sea cow developing within the uterus of a much smaller dugong. Birds and other species that develop outside of the body may have an advantage, but egg similarities still matter.
The newborn must also be capable of developing into an adult. A newly de-extinct species will necessarily be the only one if its kind — a problem if it requires significant parental or social-group investment. In addition, the early phases of de-extinction will most likely take place in captive-breeding facilities, and whether a species’ social and behavioral requirements can be met in captivity is both practically and ethically important. How could an elephant mom teach a baby mammoth to be a mammoth, and what social group would it fit into?
Also, we must be able to make more than one individual if we want to release a cloned species into the wild, presumably the goal of de-extinction, and to be sure it will thrive there, which many species reared in captivity fail to do. For example, we have not been able to re-establish the Arizona thick-billed parrot — the only species of parrot native to the United States — even after careful rearing and meticulously planned strategies for reintroduction.
Third, we need to find somewhere for this species to live. The disappearance of a species does not leave a gaping hole that persists indefinitely. Ecosystems adapt, communities change, and other species move in. Particularly for species extinct for a long time, there may no longer be a viable habitat. Both mammoths and passenger pigeons would significantly alter any habitat into which they were released, and possibly in ways that we have not yet imagined. How might farmers from the North American Midwest — or anyone who recently washed and waxed their car — react to the sound and sight of an incoming flock of a billion passenger pigeons? If there is nowhere for a species to go, then it should not be revived.
Fourth, whatever led to the initial extinction has to be identified and corrected. Otherwise we have no means to protect it from re-extinction. If we do know why a species went extinct, the cause must be correctable. Many species that disappeared within the last few centuries were the victims of the expansion of human populations or the invasion of their habitat by species whose populations expand with ours. If it is not possible to create a cat- and rat-free zone on Mauritius, for example, then the dodo, which laid a single egg in a nest on the ground, is not an ideal candidate for de-extinction.
Fifth, de-extinction should not affect other species in ways that we deem unacceptable. Unfortunately, the ethics of such decisions are not as cut and dried as the technicalities. Everything in the ecosystem will be affected, from interactions between predators and their prey to the nutrient cycle. What is acceptable and what is not? And who is the “we” that gets to decide? If the process of de-extinction, from harvesting eggs to surrogacy to captive breeding or the release of resurrected species into the wild, harms other species in a way that threatens the others’ survival, then de-extinction should not proceed.
Less obvious to me is where to draw the line when the effects of de-extinction are not lethal. Life gets harder when more species are competing for the same resources, but is that an unacceptable consequence? Perhaps not, when weighed against the potential benefits of reintroduction. Unfortunately, it may not be possible to measure the costs and benefits without performing the experiment.
Which brings us to the last criterion: There must be a compelling reason to bring a species back to life. That is both the most important and the most ambiguous standard. Compelling to whom? Again, those who favor de-extinction don’t do so for all the same reasons. To me, the goal is ecological — specifically to restore species whose continuing absence threatens the survival of other species.
One of my favorite candidates is the Morro Bay kangaroo rat. You’ve probably heard of a mammoth, but do you know about kangaroo rats? They are small burrowing rodents native to desert plains in Western North America. Today expanding agriculture and urban development have driven six of 22 species to near extinction, and one of them, the Morro Bay kangaroo rat, has not been seen for many years and is probably extinct. Kangaroo rats are tiny environmental engineers whose complex burrow systems maintain the distribution and diversity of other species in their habitat. The domino effects of their disappearance include a rapid decline in plant diversity, which leads to a decline in seed-eating birds. The decrease in foraging and burrowing slows plant decomposition, and the lack of burrows leaves many smaller animal and insect species without shelter. In the absence of kangaroo rats, the desert plain transforms into arid grassland in less than a decade.
Could we bring back the Morro Bay kangaroo rat? Well-preserved specimens exist in museums in California. We know quite a lot about the genomics and reproductive biology of rodents, thanks to their longstanding use in scientific research. They can be raised in captivity, and there is a place to release them if we were successful in bringing them back to life. Of course, we could skip all of that hard work and simply introduce a different species of kangaroo rat to Morro Bay, with what would probably be the same ecological result.
There is no wrong reason to favor — or fear — de-extinction, particularly at this point in history, when no species is close to being resurrected. However, it is time to start considering that, if de-extinction is to become a useful strategy for conservation, the first de-extinctions cannot be ecologically or ethically disastrous. The cost of failure is potentially greater than any gain that might come with success. So, do you want to choose a mammoth, just because it’s big and hairy and, well, a mammoth?
Beth Shapiro is an associate professor of ecology and evolutionary biology at the University of California at Santa Cruz. Her latest book, How to Clone a Mammoth: The Science of De-Extinction, is just out from Princeton University Press.