Extinction is forever … or is it? New advances in DNA technology and cell biology are enabling scientists to help species on the brink of extinction, and will soon allow recently extinct species to be brought back to life.
How close are we to resurrecting extinct species? That all depends on the DNA.
DNA Is the Key
The process of bringing a species back to life is called de-extinction or resurrection biology. This cutting-edge research typically requires nearly complete DNA sequence information from the extinct species. With current technology, scientists can easily obtain this information from living organisms, frozen tissue samples and sometimes even preserved museum specimens.
A bigger challenge is “ancient DNA” from archaeological sites, samples frozen in permafrost and even some fossils. Nonetheless, scientists have successfully sequenced DNA that is more than half a million years old, as explained in Nature Reviews. Even with new collection technologies, under the best possible conditions, the limit of DNA survival is perhaps 1 million years. The last of the dinosaurs went extinct 65 million years ago, so Jurassic Park likely won’t become a reality anytime soon.
The species seriously being considered for de-extinction include woolly mammoths (which went extinct 4,000 years ago), passenger pigeons (last seen around the year 1900), dodo birds, Carolina parakeets, saber-toothed tigers, gastric-brooding frogs, great auks, quaggas and giant tortoises.
Why Attempt De-Extinction?
As the journal Genes notes, a major goal of de-extinction is to bring back “keystone species” that played essential roles in shaping their ecosystems and allowed many other species to thrive.
Woolly mammoths once roamed through what we know as Europe, across Asia and into North America, according to Revive & Restore. Mammoths knocked down trees, ate grass and spread seeds with their dung. When they disappeared, biodiversity declined as the lush mammoth steppe was replaced with coniferous taiga forests and mossy tundra, which is now thawing due to climate change and releasing carbon into the atmosphere. This is having a negative impact on the biodiversity that the mammoths unknowingly helped to cultivate. Passenger pigeons played a similarly important role in shaping the deciduous forests of eastern North America.
By returning recently extinct animals to their natural habitats, scientists hope to repair some of the damage that humans have caused and restore entire ecosystems. Comparable efforts from recent times include the successful reintroduction of the California condor to the American west, wolves to Yellowstone National Park, black-footed ferrets to the US high plains, and beavers to much of Europe.
How Does De-Extinction Work?
If there are well-preserved cells with intact nuclei, an animal can be cloned. The cells can be grown in a Petri dish under conditions that cause them to behave like embryonic cells instead of mature cells. A cell’s nucleus contains the genomic DNA, so an intact nucleus can be transferred into a donor egg that’s had its nucleus removed. The egg can then be implanted into a surrogate mother and hopefully give rise to a healthy baby that can grow and reproduce naturally. The donor egg and surrogate mother would come from a closely related living species.
The first mammal was cloned from a non-extinct female sheep in 1997 at the University of Edinburgh. Finding the right conditions for cell growth can be a challenge, but the FDA now considers cloning a standard technique for livestock production.
The first extinct species to be cloned was the Pyrean ibex, according to National Geographic. Derived from a frozen skin sample, the cloned goat was born in 2003 and unfortunately died within a few minutes. Later that year, a healthy Javan banteng calf was cloned from a frozen skin sample, as reported by the Washington Post. While the banteng is endangered and not extinct, this success shows that de-extinction through cloning is absolutely possible.
The frozen banteng cells were obtained from the Frozen Zoo, which was started in 1972 by the San Diego Zoo. The Frozen Zoo now contains cryopreserved oocytes, sperm, embryos and other cell types from nearly 1,000 endangered or extinct species. The Frozen Zoo is just one of many frozen cell repositories around the world.
So, how close are we to resurrecting extinct species? Really, really close — for species that have frozen cells.
What If There Isn’t DNA to Work With?
For woolly mammoths and passenger pigeons, there are no well-preserved cells with intact nuclei. However, scientists do have nearly complete DNA sequence information that was acquired by sequencing many small pieces of DNA. Woolly mammoth DNA is 99.4% identical to the DNA of the living Asian elephant, according to the Mammoth Genome Project. Passenger pigeon DNA is 97% identical to the DNA of the living band-tailed pigeon, according to News from Science.
In cases where two species are closely related, many of the DNA sequence differences are inconsequential and don’t affect the proteins produced. Accordingly, researchers aren’t trying to produce an animal that’s 100% mammoth; they’re working to modify the DNA of Asian elephant cells to produce hybrid animals that have mammoth-like traits. These traits would include long shaggy fur, thick rolls of insulating body fat, and hemoglobin that can carry oxygen in sub-zero environments. However, the DNA differences that might contribute to mammoth-specific behaviors may be more difficult to identify.
Major advances in gene editing technology, including the CRISPR-Cas9 system, are allowing scientists to make targeted changes to the DNA inside cells. Once a cell is successfully modified, the nucleus would need to be transferred to a donor egg and implanted into a surrogate mother to develop into a healthy baby. Elephants have a two-year gestation period and don’t reach sexual maturity for 15 years. Pigeons hatch after 18 days and reach sexual maturity in seven months.
With current technology and research, it could take 5-10 years to bring back a hybrid passenger pigeon, and at least 10 for a hybrid woolly mammoth.
What About Endangered Species?
Every day, an estimated 30 to 150 species disappear from the face of our planet. Many species on the brink of extinction could be helped by the same technologies being developed for de-extinction. Cloning can increase numbers, while gene editing technology can be used to reintroduce some of the genetic diversity that was present in museum specimens but lost when natural populations declined.
De-extinction is another example of how technology — in this case biotechnology — is being used to restore nature and counteract some of the harmful effects of human activity. For these efforts to be successful, humans must also work to prevent and reverse the causes of extinction: habitat destruction, climate change, pollution, overharvesting and more.
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