The concept of restoring species lost to extinction, often called resurrection biology or de-extinction, has evolved from a theoretical idea into a serious scientific pursuit. This endeavor involves harnessing advanced genetic technologies to reverse the finality of species loss. Researchers are motivated by repairing human-caused damage to global biodiversity and restoring lost ecological functions. The focus is on species that died out recently or those whose remains offer viable genetic material. This work is attracting significant investment and innovation, pushing the boundaries of conservation science.
Defining What “Brought Back” Means
Achieving true de-extinction—restoring a species to a self-sustaining, stable, and genetically diverse population in the wild—has not yet occurred. The term “brought back” must therefore be understood through distinct scientific lenses. The most immediate achievement is a temporary revival, demonstrated by the Pyrenean ibex, a wild goat subspecies that went extinct in 2000. In 2003, a cloned ibex was successfully born, marking the first revival of an extinct animal, though the clone died minutes later due to a lung defect.
Most current projects, particularly those targeting species extinct for thousands of years, aim instead for a “proxy species.” This involves using genetic modification to create a hybrid animal that closely resembles the extinct species and can fulfill its former ecological role. For example, the Woolly Mammoth project aims for a cold-resistant Asian elephant engineered with specific mammoth traits, not a pure mammoth. Since no perfectly preserved, complete ancient DNA exists, a 100% genetically identical copy of any long-extinct animal is currently unattainable. This approach shifts the goal from genetic purity to functional equivalence.
The Science of Revival
The ability to pursue de-extinction relies on two primary technologies for manipulating genetic material. The first is Somatic Cell Nuclear Transfer (SCNT), a cloning technique famously used to create Dolly the sheep. SCNT involves taking the nucleus from a preserved cell of the extinct animal and transferring it into an egg cell from a living relative whose own nucleus has been removed.
The reconstructed egg is stimulated to begin dividing, creating an embryo that is implanted into a surrogate mother of the related species. SCNT is best suited for species that went extinct relatively recently, as it requires intact cells that have not been degraded over long periods.
The second, more advanced method is gene editing, primarily using the CRISPR-Cas9 system. This tool allows scientists to compare the sequenced DNA of an extinct species with that of its closest living relative, identifying genetic differences. Researchers then use the CRISPR-Cas9 “molecular scissors” to precisely cut the living relative’s DNA and paste in the extinct species’ genes that code for defining traits, such as cold resistance or unique coloration. This gene editing approach is necessary for older species where the recovered DNA is too fragmented for traditional cloning, allowing the creation of a hybrid embryo.
Case Files: Animals Targeted for Revival
The Pyrenean ibex is the only animal to have been successfully revived past birth, albeit briefly, through SCNT. The last natural ibex, a female named Celia, died in 2000, but scientists had collected and cryopreserved skin cells. Researchers implanted hundreds of cloned embryos into domestic goat surrogates, resulting in the birth of a female ibex clone in 2003. The newborn died within minutes due to severe lung defects, likely a consequence of the cloning process itself.
Another prominent project focuses on the Woolly Mammoth, a species that went extinct about 4,000 years ago. The goal is to create a cold-adapted elephant hybrid by editing the genome of the Asian elephant, the mammoth’s closest living relative. Scientists are using CRISPR technology to introduce traits like a thick layer of insulating fat, shaggy hair, and smaller ears, which allowed the mammoth to thrive in the Arctic tundra. The company spearheading the effort, Colossal Biosciences, is working toward producing the first calf, with the ultimate goal of reintroducing the hybrid animal to the Arctic environment.
The Passenger Pigeon, once the most abundant bird in North America, was driven to extinction by unsustainable commercial hunting and habitat loss, with the last individual dying in 1914. The project involves using CRISPR to modify the DNA of its nearest living relative, the Band-tailed Pigeon. Researchers sequence DNA fragments from museum specimens to identify the genes responsible for the pigeon’s unique traits, such as its massive flocking behavior that created forest disturbances. The current phase involves creating a breeding flock of engineered pigeons to make the gene-editing process more efficient, aiming to hatch the first genetically modified birds for captive breeding between 2029 and 2032.
Ecological and Logistical Challenges
The reintroduction of a revived species faces practical hurdles that extend beyond the laboratory. A primary concern is finding suitable habitat, as the environments the extinct species once occupied have changed significantly. The eastern forests the Passenger Pigeon once inhabited, for instance, are ecologically different now, and the Arctic steppe that sustained the Woolly Mammoth is also greatly altered.
The small number of individuals created at the start of any de-extinction project poses a major problem for genetic diversity. Beginning with a limited gene pool, especially one derived from degraded ancient DNA, creates a population susceptible to inbreeding depression and disease. This lack of genetic variation limits the population’s ability to adapt to new pathogens or a changing climate.
The responsibility of managing a revived species requires a detailed plan for its long-term persistence in the wild. Since a restored animal is a novel entity in the modern ecosystem, there is uncertainty about its ecological effects on other organisms. Scientists must commit to ongoing conservation efforts and management until the population is robust enough to sustain itself without human intervention.