De-extinction is the effort to bring back species that have gone extinct, or to create close functional equivalents of them, using modern biotechnology. It sits at the intersection of genetics, conservation biology, and reproductive science, and it has moved from theoretical concept to active, well-funded research in the past decade. No extinct species has been fully revived yet, but several projects are underway targeting animals like the woolly mammoth, the Tasmanian tiger, and the passenger pigeon.
Three Scientific Approaches
There are three main pathways scientists use to attempt de-extinction, each with different strengths and limitations.
Back-breeding is the oldest and simplest method. It uses selective breeding to concentrate ancestral traits that still exist in a living population into a single individual. If an extinct animal’s descendants still carry some of its genes, breeders can try to “reverse engineer” the original traits over many generations. This approach works only when the extinct species has close living relatives that retain enough of its characteristics.
Cloning uses a technique called somatic cell nuclear transfer. Scientists take a preserved cell from the extinct animal, extract its DNA, and fuse it with an egg cell from a closely related living species. That egg, now carrying the extinct animal’s genetic instructions, is implanted in a surrogate mother. The catch: you need intact, well-preserved cells from the extinct species, which limits this method to very recently extinct animals.
Genome editing is considered the most promising route for species that went extinct long ago. Scientists sequence the genome of the extinct species from preserved remains, compare it to the genome of its closest living relative, and then use tools like CRISPR-Cas9 to edit the living species’ DNA so it more closely matches the extinct one. The result isn’t an exact replica. It’s a hybrid or “proxy” species engineered to resemble and function like the original.
The Only Clone of an Extinct Animal
The closest anyone has come to true de-extinction happened on July 30, 2003, when scientists in Spain delivered a clone of the bucardo, a Pyrenean ibex that had gone extinct three years earlier. A surrogate mother gave birth via Caesarean section to a cloned kid carrying the bucardo’s DNA. It survived for seven minutes before dying of a lung malformation, a common birth defect in cloned hoofed animals. For those seven minutes, extinction was technically reversed for the first time in history.
The bucardo attempt demonstrated that cloning an extinct species is biologically possible but extraordinarily difficult. Lung and organ defects plague cloned animals at high rates, and the process requires preserved cells of exceptional quality. No one has repeated the feat since.
Why Dinosaurs Are Off the Table
DNA degrades over time, and the rate depends heavily on environmental conditions. Cold, dry climates preserve genetic material far longer than warm or humid ones. Even under ideal preservation conditions, genome-scale DNA recovery is restricted to relatively recent extinctions. Dinosaurs went extinct 66 million years ago, and their DNA has long since broken down beyond any possibility of reconstruction.
Even for more recently extinct species, DNA degradation creates serious obstacles. A 2024 study using the Christmas Island rat (extinct for about a century) found that roughly 5% of its genome couldn’t be recovered, despite having a close living relative as a reference. The problem is that as species diverge over evolutionary time, their genomes accumulate insertions, deletions, and rearrangements that make it impossible to map the extinct species’ fragmented DNA back onto the living relative’s genome. The further back in time, the worse this gets.
The Woolly Mammoth Project
The highest-profile de-extinction effort targets the woolly mammoth, led by the biotechnology company Colossal Biosciences. The plan is to edit the genome of the Asian elephant, the mammoth’s closest living relative, to produce a cold-adapted hybrid. The two species share about 99.6% of their DNA, and Colossal is using CRISPR to close that 0.4% gap by editing 65 genes responsible for traits like dense hair, fat deposits, a dome-shaped skull, curved tusks, and cold-tolerant hemoglobin.
The process involves nine steps, from recovering ancient mammoth DNA to sequencing both species’ genomes, designing and testing the edits, creating a viable embryo through nuclear transfer, and implanting it in a surrogate elephant for a 22-month gestation. Colossal has not publicly announced a specific birth date, but the project is described as “well underway.” The company has raised $435 million in total funding since launching in September 2021, with its most recent $200 million round placing it at a $10.2 billion valuation.
What Colossal would produce isn’t a woolly mammoth. It’s a cold-adapted Asian elephant with mammoth-like traits. Whether that distinction matters depends on what you think de-extinction is supposed to accomplish.
Other Active Projects
At the University of Melbourne, the TIGRR Lab (Thylacine Integrated Genomic Restoration Research) is working to bring back the Tasmanian tiger, a marsupial predator that went extinct in 1936. The project focuses on three technical challenges: marsupial genomics, developing and maintaining marsupial stem cells, and assisted reproductive techniques for marsupials. Marsupial reproduction is fundamentally different from that of placental mammals, which means many of the tools developed for other cloning and gene-editing work don’t directly apply.
The passenger pigeon, once the most abundant bird in North America before being hunted to extinction by 1914, is the target of a project by the nonprofit Revive & Restore. The approach involves recovering genetic codes from museum specimens and inscribing them into the genome of the band-tailed pigeon, the passenger pigeon’s closest living relative. The goal is to identify which genes and gene variants were lost with the passenger pigeon and restore them in a living bird.
Ecological Risks of Revived Species
Bringing back an extinct species, or something close to it, raises serious questions about what happens when you release it into a world that has moved on. Ecosystems don’t hold a vacancy open. The niches once filled by extinct species have been colonized by other organisms, and food webs have reorganized.
Engineered proxy species don’t fit traditional definitions of “native” wildlife. Their ecological roles are uncertain, and researchers have flagged several specific risks. One is the enemy release hypothesis: a species introduced without its natural predators, parasites, or competitors may spread more aggressively than expected, potentially at the expense of existing species. A revived dire wolf proxy, for example, could impose significant predation pressure on native prey species and disrupt ecological relationships that have developed over thousands of years without it.
Disease is another concern. Even if a proxy species is vaccinated against known pathogens, it could serve as a novel vector for diseases it wasn’t screened for, spreading them to wildlife that has no evolved resistance. And in landscapes that have been heavily modified by agriculture and urbanization, the ecological niche an extinct species once occupied may simply no longer exist. Releasing a proxy into such an environment could lead to territorial conflicts with established species, failed establishment, or cascading effects across multiple levels of the food chain.
Legal and Conservation Status
Proxy species exist in a legal gray zone. Current endangered species laws are built around measurable metrics like population decline and range contraction, and a newly created proxy species would have none of those. It wouldn’t have a current range, a population history, or a clear place in existing legal frameworks.
Many jurisdictions prohibit the release of non-native species, and proxy species could fall under that definition, meaning any release into the wild would require special exemptions. Because genome-edited organisms are classified as genetically modified organisms in most countries, they face additional layers of regulation that could restrict movement across borders and release into natural environments.
International agreements like CITES and the Convention on Biological Diversity don’t have clear provisions for proxy species either. The IUCN has noted that a proxy species might qualify for listing as “Extinct in the Wild” on the Red List while all individuals remain in captivity, but it’s still unclear how genetically similar a proxy would need to be to the original species to take its place on that list. Each proxy species will need to be evaluated individually against each piece of relevant legislation, a process that could take years and produce different outcomes in different countries.