[Time] Romulus and Remus are doing what puppies do: chasing, tussling, nipping, nuzzling. But there’s something very un-puppylike about the snowy white 6-month olds—their size, for starters. At their young age they already measure nearly 4 ft. long, tip the scales at 80 lb., and could grow to 6 ft. and 150 lb. Then there’s their behavior: the angelic exuberance puppies exhibit in the presence of humans—trotting up for hugs, belly rubs, kisses—is completely absent. They keep their distance, retreating if a person approaches. Even one of the handlers who raised them from birth can get only so close before Romulus and Remus flinch and retreat. This isn’t domestic canine behavior, this is wild lupine behavior: the pups are wolves. Not only that, they’re dire wolves—which means they have cause to be lonely.
The dire wolf once roamed an American range that extended as far south as Venezuela and as far north as Canada, but not a single one has been seen in over 10,000 years, when the species went extinct. Plenty of dire wolf remains have been discovered across the Americas, however, and that presented an opportunity for a company named Colossal Biosciences.
Relying on deft genetic engineering and ancient, preserved DNA, Colossal scientists deciphered the dire wolf genome, rewrote the genetic code of the common gray wolf to match it, and, using domestic dogs as surrogate mothers, brought Romulus, Remus, and their sister, 2-month-old Khaleesi, into the world during three separate births last fall and this winter—effectively for the first time de-extincting a line of beasts whose live gene pool long ago vanished. TIME met the males (Khaleesi was not present due to her young age) at a fenced field in a U.S. wildlife facility on March 24, on the condition that their location remain a secret to protect the animals from prying eyes.
The dire wolf isn’t the only animal that Colossal, which was founded in 2021 and currently employs 130 scientists, wants to bring back. Also on their de-extinction wish list is the woolly mammoth, the dodo, and the thylacine, or Tasmanian tiger. Already, in March, the company surprised the science community with the news that it had copied mammoth DNA to create a woolly mouse, a chimeric critter with the long, golden coat and the accelerated fat metabolism of the mammoth.
If all this seems to smack of a P.T. Barnum, the company has a reply. Colossal claims that the same techniques it uses to summon back species from the dead could prevent existing but endangered animals from slipping into extinction themselves. What they learn restoring the mammoth, they say, could help them engineer more robust elephants that can better survive the climatic ravages of a warming world. Bring back the thylacine and you might help preserve the related marsupial known as the quoll. Techniques learned restoring the dire wolf can similarly be used to support the endangered red wolf.
"We are an evolutionary force at this point," says Beth Shapiro, Colossal’s chief science officer, speaking of humanity as a whole. "We are deciding what the future of these species will be." The Center for Biological Diversity suggests that 30% of the planet’s genetic diversity will be lost by 2050, and Shapiro and Colossal CEO Ben Lamm insist that genetic engineering is a vital tool to reverse this. Company executives often frame the technology not just as a moral good, but a moral imperative—a way for humans, who have driven so many species to the brink of extinction, to get square with nature. "If we want a future that is both bionumerous and filled with people," Shapiro says, "we should be giving ourselves the opportunity to see what our big brains can do to reverse some of the bad things that we’ve done to the world already."
The woolly mouse, to a minor extent, and the dire wolves, to a scientifically seismic one, are first steps in that direction. But not everyone agrees. Scientific history is rife with examples of newly introduced species becoming invasive species—the doctrine of unintended consequences biting humans when we played too cute with other animals. An exotic pet escapes and multiplies, decimating native species. A toad brought in to kill off beetles ends up killing off the marsupials that eat the toads. And genetic engineering is still a nascent field. Nearly 30 years after Dolly the sheep was cloned, the technology still produces problems in cloned animals, such as large birth size, organ defects, premature aging, and immune-system problems. What’s more, cloning can be hard on the surrogate mother that gestates the cloned embryo.
"There’s a risk of death. There’s a risk of side effects that are severe," says Robert Klitzman, professor of psychiatry and director of the bioethics master's program at Columbia University. "There’s a lot of suffering involved in that. There are going to be miscarriages."
Still, Colossal’s scientists believe they are on to something powerful. Matt James, the company’s chief animal officer—who once worked as senior director of animal care at the Dallas Zoo and Zoo Miami, where he managed the welfare of 7,000 animals representing 500 species—felt the significance of the science when Romulus and Remus were just 5 or 6 weeks old. The staff was weighing the little pups, and one of the veterinary techs began singing a song from The Little Mermaid. When she reached a point at which she vocalized first up, then down, Romulus and Remus turned her way and began howling in response.
"For me," James says, "it was sort of a shocking, chilling moment." These pups were the first to produce a howl that hadn’t been heard on earth in over 10,000 years.
It takes surprisingly few genetic changes to spell the difference between a living species and an extinct one. Like other canids, a wolf has about 19,000 genes. (Humans and mice have about 30,000.) Creating the dire wolves called for making just 20 edits in 14 genes in the common gray wolf, but those tweaks gave rise to a host of differences, including Romulus’ and Remus’ white coat, larger size, more powerful shoulders, wider head, larger teeth and jaws, more-muscular legs, and characteristic vocalizations, especially howling and whining.
The dire wolf genome analyzed to determine what those changes were was extracted from two ancient samples—one a 13,000-year-old tooth found in Sheridan Pit, Ohio, the other a 72,000-year-old ear bone unearthed in American Falls, Idaho. The samples were lent by the museums that house them. The lab work that happened next was painstaking.
Cloning typically requires snipping a tissue sample from a donor animal and then isolating a single cell. The nucleus of that cell—which contains all of the animal’s DNA—is then extracted and inserted into an ovum whose own nucleus has been removed. That ovum is allowed to develop into an embryo and then implanted in a surrogate mother’s womb. The baby that results from that is an exact genetic duplicate of the original donor animal. This is the way the first cloned animal, Dolly, was created in 1996. Since then, pigs, cats, deer, horses, mice, goats, gray wolves, and more than 1,500 dogs have been cloned using the same technology.
Colossal’s dire wolf work took a less invasive approach, isolating cells not from a tissue sample of a donor gray wolf, but from its blood. The cells they selected are known as endothelial progenitor cells (EPCs), which form the lining of blood vessels.
The scientists then rewrote the 14 key genes in the cell’s nucleus to match those of the dire wolf; no ancient dire wolf DNA was actually spliced into the gray wolf’s genome. The edited nucleus was then transferred into a denucleated ovum. The scientists produced 45 engineered ova, which were allowed to develop into embryos in the lab. Those embryos were inserted into the wombs of two surrogate hound mixes, chosen mostly for their overall health and, not insignificantly, their size, since they’d be giving birth to large pups. In each mother, one embryo took hold and proceeded to a full-term pregnancy. (No dogs experienced a miscarriage or stillbirth.) On Oct. 1, 2024, the surrogates birthed Romulus and Remus. A few months later, Colossal repeated the procedure with another clutch of embryos and another surrogate mother. On Jan. 30, 2025, that dog gave birth to Khaleesi.
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