Passenger pigeons were once so abundant in the United States that their flocks would stretch a mile wide and blacken the skies. But by the early 1900s, none of the birds were left in the wild.
Now, 100 years after their extinction, some scientists and technology gurus are hoping to bring the bird back.
It's like Jurassic Park, but for pigeons.
The effort is part of a new, growing field of research on "de-extinction." Rapid advances in molecular biology and genome engineering are making it possible to imagine a world where long-gone species -- or hybrid animals very similar to them -- could again take to skies or roam the Earth.
"It may sound like science fiction, but it is very much grounded in the revolution in technology on the ground that is helping human health," said George Church, a pioneering Harvard University geneticist whose gene sequencing technology, developed to help scientists fight human disease, could also open doors to resurrect extinct animals or help severely endangered species.
Scientists around the world are researching how they could bring back passenger pigeons, mammoths and the Tasmanian tiger, among other creatures. Many of them will gather in Washington, D.C., tomorrow for the first public conference on "de-extinction."
"Work that is being done with stem cells to genome editing to sequencing is all converging to actually make de-extinction an increasing possibility," said Ryan Phelan, head of Revive & Restore, the California-based nonprofit hosting the conference. She has support both from the National Geographic Society, a partner for the event, and conference organizer TED.
Revive & Restore is the brainchild of Phelan and her husband, Stewart Brand, who each have a history of supporting unconventional, futuristic and often wildly successful projects.
Brand defined a countercultural ethos of the 1970s as the creator of the Whole Earth Catalog. He now heads the Long Now Foundation, Revive & Restore's parent organization. The quirky San Francisco think tank supports long-term ideas -- looking at problems over 10,000 years.
Now the pair is focusing on finding new uses for molecular technology for "deep ecological enrichment" and working to make de-extinction a real possibility.
Over the past two years, Phelan and Stewart organized two private workshops with established molecular and conservation biologists to gauge interest and explore the possibilities and perils of de-extinction. The success of those workshops led to the work of Revive & Restore and the event this week.
"The technology has advanced quite rapidly just in the last few years, and the cost of genome sequencing has just plummeted," Phelan said. "When you start to apply this to conservation, it means that things that were unthinkable are thinkable."
But the effort has drawn criticism from some prominent scientists. They question whether the work is a waste of money and time and if it could even harm conservation of existing species, if the public knows the animals could be brought back by science.
'The great comeback' for the pigeon?
Forget an actual Jurassic Park. There is no viable DNA from species as old as dinosaurs, and bringing a Tyrannosaurus rex back would pose significant complications.
While Revive & Restore will support and promote revival efforts for other species, like the Tasmanian tiger or woolly mammoth, the group is investing in the passenger pigeon first because of its iconic status and relative practicality. Hundreds of passenger pigeons are preserved, providing a ready DNA source, and trees that nested the birds 100 years ago are still standing.
A team of scientists at the University of California, Santa Cruz, led by ancient-DNA expert Beth Shapiro, has already sequenced the passenger pigeon's DNA.
Revive & Restore dispatched Ben Novak, a young scientist and passenger pigeon enthusiast, to work full-time on the project in Shapiro's lab. He and Shapiro's team are working to refine the passenger pigeon genome over the next two years. Novak will also help create a 10-year plan to "rewild" the passenger pigeon.
The group started a campaign, "The Great Comeback," to raise funds for the effort.
But there are significant barriers to overcome before the graceful bird could again take flight.
Novak and his team will work on the genomes of both the passenger pigeon and the band-tailed pigeon, a very close relative that is still living. They will eventually use technology to go in and, letter by letter, transform the genome of the band-tailed pigeon to that of the passenger pigeon.
That is actually the easy part. New technology from Church's lab at Harvard is making gene replacement more efficient and affordable every year, removing what used to be one of the biggest barriers for such an effort.
But then scientists must also solve the age-old problem of the chicken -- or pigeon -- and the egg. At some point, another bird will have to lay a passenger pigeon egg.
There are also advancements in that field. In research published last year, scientists at Dubai's Central Veterinary Research Laboratory succeeded in having one species essentially create another. They created a "chimeric" duck that produced chicken sperm. It mated with hens and produced four chicken chicks.
"The technology is definitely in this decade; the progress curves definitely point to that," Novak said in an interview this week. "There might be some very difficult challenges facing us, but five years from now we would be having a very, very different conversation."
For Novak, this is a labor of love. The 26-year-old scientist is pursuing his childhood dream before he has even completed a graduate degree.
As a 13-year-old concerned about conservation, Novak did a science fair project on how to bring the extinct dodo bird back to life. He won the fair and "got pumped," as he tells it, to enter a career in science.
The dodo, a relative of the pigeon, opened a new world to Novak. He became a pigeon guy, breeding and raising the birds. He fell in love with tales of the long-lost passenger pigeon, a species he says is a vital part of the forest ecosystem, "the animal counterpart to the trees."
From abundance to extinction
The history of passenger pigeons is a tale of excess.
John James Audubon, one of the fathers of ornithology, wrote in the 1800s about flocks of passenger pigeons that obscured the "light of noon-day ... as by an eclipse." He counted 163 flocks in 21 minutes, then gave up counting as more and more pigeons flew by, a never-ending stream of birds that he says continued for three days straight.
At the same time, people hunted the abundant passenger pigeons for food and shipped them by the thousands to the marketplace. Some were even used as live targets at shooting galleries. Pigeons were killed in large groups, sometimes blasted out of trees with poison. Meanwhile, the birds also lost large swaths of their forest habitat, cleared for construction.
By the early 1900s, passenger pigeons were gone from the wild, according to the Smithsonian Institution. The last living passenger pigeon, Martha, died in the Cincinnati Zoo in 1914.
How the population disappeared from the wild remains a mystery to scientists. Some think the birds may have needed to nest in large colonies to survive.
"It is an astronomical story of extinction and its complete impact," Novak said. "Humans thought the bird was inexhaustible, there were billions of birds, you could never kill them all, but you don't have to kill them all in order for them to quit being able to breed and quit being viable."
Creating a viable population of pigeons -- or any other extinct animal -- is another challenge. There are also habitat and ecosystem concerns, which grow significantly with efforts to revive bigger species like the mammoth or Tasmanian tiger.
Some prominent biologists question if the effort is worth it.
"While it may become technically feasible to do some of the things that are proposed, we should ask whether the technology would really be contributing to 'conservation,' where the generally agreed motivation is to protect species so that they can thrive into the future," said William Holt, a reproductive biologist at the Zoological Society of London.
Stuart Pimm, a renowned conservation biologist from Duke University, takes it a step further. He is concerned that even attempting de-extinction could foster a political environment that could make it harder to protect currently living, imperiled species.
"I think it is at best a colossal waste of money and at worst has potential to do significant harm," Pimm said. "The people working on this are lovely and I like them a lot, but I am afraid they are being politically naive. I am afraid the political consequences of this would be disastrous."
Pimm recounted past arguments, when his foes opposed protections for species in the wild, as long as there was a small population that could rebound somewhere. He is worried hopes of de-extinction could exacerbate that attitude.
But Phelan and other supporters of the de-extinction movement say their efforts are rooted in conservation, and the technology developed could help imperiled species. The scientific push to resurrect a bygone species is coming, like it or not, they say, and they want to work together to make sure it is done well and benefits the environment.
"We do it because we care about diversity, we have the scientific knowledge that diversity is a good thing, and we believe that in order to create healthy environments, we want to have species flourish," Phelan said. "This is potentially the moral thing to do. These species lived here ... so I think that the argument for de-extinction is not that different than for why we protect species. We couldn't really ask that question when we did not have the technology to do it, and now we do."