Environment Eassy

5 APRIL 2013 VOL 340 SCIENCE www.sciencemag.org 32

POLICYFORUM

A 1930s f ilm shows a dog running

and jumping inside a fenced enclo-

sure ( 1)—except that the dog has

a strange-shaped head, odd stripes, and a

rigid tail that can only move side-to-side.

The “dog” is actually one of the last thyla-

cines, a marsupial predator also called the

Tasmanian tiger. The fi lm was taken shortly

before humans extinguished the species for-

ever. Or did we? Recently, new technolo-

gies have made it plausible to try to revive

many recently extinct species. Scientists

around the world are discussing, and work-

ing toward, “de-extinction” ( 2).

Currently, three approaches to de-extinc-

tion seem most likely to succeed: back-

breeding, cloning, and genetic engineer-

ing. If the extinct species left closely related

descendants, it might be possible to use

selective breeding to produce progeny with

the phenotypes of the extinct species, as the

auroch project in Europe has been doing

since 2008 ( 3). With newly cheap genome

sequencing methods, one might guide back-

breeding with genome sequences from sam-

ples of the extinct species. Of course, back-

breeding will only be possible in situations

where the genetic variations of the extinct

species survive in the descendant species.

Cloning provides another possibility.

Using cryopreserved tissue from the last

known Pyrenean ibex, a Spanish group

used somatic cell nuclear transfer (SCNT)

to revive that extinct subspecies. Out of

several hundred efforts, however, only one

fetus survived to term, and it died minutes

after birth from lung abnormalities ( 4).

This example highlights two problems with

SCNT: it is neither very safe nor effi cient

and will only work if viable cell nuclei are

available. This will likely be the case in only

a few very recent extinctions.

Genetic engineering offers a third

approach. Take an extinct species—say,

the passenger pigeon—that left suffi cient

samples to allow high-quality whole-

genome sequencing. DNA in cells from a

similar living species—perhaps the band-

tailed pigeon—could be edited to match

the extinct species’ genomic sequence. The

modifi ed cells could then be used to produce

living birds that, genomically, were mainly

band-tailed pigeon but partially passenger

pigeon ( 5). By using targeted replacement

of genomic sequence ( 6) across several loci,

much of the extinct genome could be recon-

structed within several generations.

Neither the back-breeding nor genetic

engineering approaches would yield an ani-

mal that had exactly the same genome as

any member of the extinct species for many

years, if ever. The cloning approach, in the

few cases where viable nuclei are avail-

able, would produce a genomic twin to one

member of the extinct species—but only

one. Does one individual (or a set of clones)

make a “species”? Even if genomic iden-

tity is necessary, is it suffi cient? The revived

individuals would not have the same epigen-

etic makeup, microbiome, environment, or

even “culture” as their extinct predecessors.

Risks and Objections

Objections to bringing back extinct animals

fall into f ive categories: animal welfare,

health, environment, political, and moral.

Animals created in the de-extinction

process could end up suffering, either as

a result of the processes used or because

of their particular genomic variations. We

know, for example, that SCNT can lead to

high levels of deformity and early death

( 7). The Animal Welfare Act and its institu-

tional animal care and use committees limit

precisely this kind of suffering ( 8). Beyond

physical suffering, some animal advocates

might oppose de-extinction as they oppose

zoos—on the grounds that they exploit ani-

mals for unimportant human purposes, like

entertainment.

Newly de-extinct creatures might prove

excellent vectors for pathogens. An extinct

animal’s genome could also conceivably

harbor unrecognized, harmful endogenous

retroviruses.

If the species either is released or escapes

into the general environment, it might do

substantial damage. Even extinct species

that were not pests in their past environ-

ments could be today. For example, less than

200 years ago, billions of passenger pigeons

migrated each year between the eastern

United States and Canada. Today, those

regions have far more humans, far larger

urban centers, very different agriculture,

and largely transformed ecosystems. The

American chestnut, a main food source for

the passenger pigeon, is now nearly extinct

in the wild. Even in the same location, the

passenger pigeon would today be an alien,

and potentially invasive, species—perhaps

another starling or even an avian kudzu.

The political risks are considerable,

What If Extinction Is Not Forever?

GENOMICS

Jacob S. Sherkow 1 and Henry T. Greely 2

Although new technologies may make it

possible to bring extinct species back to life,

there are ethical, legal, and social ramifi cations

to be addressed

Tasmanian tiger. By the 1930s, settlers, encouraged by government bounties, had hunted the thylacine to

extinction in the wild. Well-preserved specimens could pave a way to bringing it back.

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*Corresponding author. hgreely@stanford.edu

1Center for Law and the Biosciences, Stanford Law School, Stanford CA 94305, USA. 2Stanford University, Stanford CA 94305, USA.

Published by AAAS

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www.sciencemag.org SCIENCE VOL 340 5 APRIL 2013 33

POLICYFORUM

too. Current protection of endangered and

threatened species owes much to the argu-

ment of irreversibility. If extinctions—par-

ticularly extinctions where tissue samples

are readily available—are not forever, pres-

ervation of today’s species may not seem as

important. Also, genetics and, more broadly,

modern bioscience, could face a backlash

if citizens perceive public investments in

bioscience as being used to revive species

rather than cure human disease.

Finally, some people will complain that,

whatever its consequences, de-extinction is

just wrong—it is “playing god,” “reversing

natural selection,” or an act of hubris. Oth-

ers may argue that we cannot know enough

about the consequences to re-introduce a

species. But neither do we know the full

consequences of its extinction or its con-

tinuing nonexistence.

Benefi ts

Like the risks or objections to de-extinc-

tion, we see the benefi ts falling into fi ve cat-

egories: scientifi c knowledge, technological

advancement, concrete environmental ben-

efi ts, justice, and “wonder.” These benefi ts

are quite similar to the arguments made for

preserving currently endangered or threat-

ened species.

De-extinction could allow scientists the

unique opportunity to study living members

of previously extinct species (or, at least,

close approximations to those species), pro-

viding insights into their functioning and

evolution. Some revived species may be

translated into useful products; for exam-

ple, it is conceivable that new drugs may be

derived from extinct plants.

De-extinction could lead to techno-

logical advances. The most likely would

be improvements in genetic engineering,

such as the targeted replacement of large

stretches of genomic DNA ( 6).

Some researchers argue that “re-wild-

ing” with existing species, locally extinct in

particular habitats, can help restore extinct

or threatened ecosystems ( 9). The same can

be argued about the restoration of extinct

species. The revival of the wooly mammoth

as a major grazing animal in the Arctic, for

example, might provide substantial ben-

efi ts by helping restore an arctic steppe in

the place of the less ecologically rich tun-

dra ( 10).

Justice is a viscerally attractive argu-

ment for de-extinction, at least for species

that humans drove to extinction: We killed

them. We have the power to revive them. We

have a duty to do so. But to whom or what

do we owe that duty? Would it apply to all

species in whose extinction humans played

the sole, the leading, or a substantial role?

The last benefi t might be called “wonder,”

or, more colloquially “coolness.” This may be

the biggest attraction, and possibly the big-

gest benefi t, of de-extinction. It would surely

be very cool to see a living wooly mammoth.

And while this is rarely viewed as a substan-

tial benef it, much of what we do as indi-

viduals—even many aspects of science—

we do because it’s “cool.”

Legal Issues

We may also need to consider several legal

issues. First, would a de-extinct species be

“endangered”? The answer is unclear. In the

United States, the Endangered Species Act

provides for listing as “endangered” any

species “over utilized” for scientifi c pur-

poses, inadequately protected by current

regulations, or whose existence is threat-

ened by other “manmade factors” ( 11)—all

considerations that would seem to apply to

a newly revived species. Ironically, inter-

national organizations typically tie endan-

gered status to whether species’ population

has declined—the opposite of the concern

about newly revived species ( 12). Uncer-

tainty about the status of de-extinct species

will affect numerous civil, criminal, and

international laws.

Second, could a revived species be pat-

ented? This answer also seems unclear.

The United States and many other coun-

tries allow patents on living organisms ( 13).

Although “products of nature” cannot be

patented, is a revived species a “product

of nature” in light of the inevitable differ-

ences from its predecessors? Additionally,

the “lost arts doctrine” may allow the pat-

enting of previously existing species if they

have been completely lost to the public ( 14).

Last, would de-extinction be regulated

and if so, how? Again, the answer is unclear.

And even if there were no legal regulation,

the concerns previously discussed could

dampen the enthusiasm for de-extinction by

some research entities, such as universities.

This could drive the efforts toward less con-

trolled, or constrained, enterprises.

What Should Be Done?

The answer to the question—What to do

about de-extinction?—depends in part on

closely def ining the question. Consider

three different “bottom-line” questions.

First, should de-extinction be publicly

funded? This answer seems, to us, “largely

no.” The potential tangible benef its from

de-extinction are too small and the poten-

tial objections are too serious to justify sub-

stantial government expenditure. One might

argue that governments fund science proj-

ects with similarly small practical relevance,

but those “cool” projects, like the Mars rov-

ers, present fewer risks and objections.

Second, should de-extinction be cate-

gorically banned? Here the answer seems a

fairly clear “no.” The risks look fairly small

and probably manageable. If people want to

devote their own time, money, and efforts to

the endeavor, the risks to the world do not

seem to justify complete prohibition.

Third, should de-extinction be regu-

lated? Here, we think the answer is “Yes—

somewhat.” The animal welfare and envi-

ronmental concerns are real. They could be

mitigated by protective action but only if

the law requires it. Bringing all de-extinc-

tion efforts under something like the Ani-

mal Welfare Act and requiring careful envi-

ronmental assessments before any planned

releases (as well as approved precautions

against inadvertent release) do seem appro-

priate. Whether other kinds of regulation

are needed is less clear, although there may

be some cases, like any attempted revival of

extinct hominid species, where special con-

trols, or bans, would be appropriate.

De-extinction is a particularly intrigu-

ing application of our increasing control

over life. We think it will happen. The most

interesting and important question is how

humanity will deal with it.

References and Notes 1. Last Tasmanian tiger, thylacine (1933);

www.youtube.com/watch?v=6vqCCI1ZF7o.

2. C. Zimmer, Natl. Geogr. (2013); http://ngm.

nationalgeographic.com/2013/04/species-revival/

zimmer-text.

3. The Tauros programme, www.taurosproject.com/.

4. J. Folch et al., Theriogenology 71, 1026 (2009).

5. M. Ridley, Wall Street Journal, 2 March 2013, p. C4.

6. H. H. Wang et al., Nat. Methods 9, 591 (2012).

7. P. Chavatte-Palmer et al., Placenta 33, (suppl.), S99

(2012).

8. 7 U.S.C. § 2131 et seq.

9. S. A. Zimov, Science 308, 796 (2005).

10. Pleistocene Park, www.pleistocenepark.ru/en/

background/.

11. 16 U.S.C. § 1533(a)(1)(A).

12. IUCN, IUCN Red List Categories and Criteria (1994);

http://www.iucnredlist.org/technical-documents/

categories-and-criteria/1994-categories-criteria.

13. Diamond v. Chakrabarty. 446 U.S. 303 (1980).

14. M. L. Rohrbaugh, AIPLA Q. J. 25, 371 (1997).

Acknowledgments: The authors participated in two work-

shops on de-extinction organized by Revive and Restore and

the National Geographic Society, one in February 2012 (H.G.)

and one in October 2012 (H.G. and J.S.). (H.G. was on the

planning committee for the second workshop.) We would like

to acknowledge the tremendous contribution the speakers at

those workshops made to our understanding of de-extinction

and the issues it raises.

10.1126/science.1236965

Published by AAAS

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(6128), 32-33. [doi: 10.1126/science.1236965]340Science Jacob S. Sherkow and Henry T. Greely (April 4, 2013) What If Extinction Is Not Forever?

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