When itinerant pigeon Martha died at the Cincinnati Zoo in 1914, the end of a beautiful era began. Once the dominant species in eastern North America, itinerant pigeons inhabited the forests in huge flocks of up to several billion individuals for tens of thousands of years before becoming extinct. The sheer numbers of these birds, combined with their tree seed-based diet, made them one of the most important environmental engineers of the time, shaping the habitat dynamics that these forests relied on. These ecosystems are now losing biodiversity, says Ben Novak, lead researcher at Revive & Restore.
“When this bird went extinct in 1914, there was a grim awakening of the power of industrial mankind to destroy even the most abundant natural resources,” he says. So Novak, working with the California Institute, is creating Revive & Restore, a program aimed at restoring and reclaiming these species and repopulating their natural habitats, using common pigeons and the power of CRISPR.
Can a deceased species be resurrected?
Novak is part of a small group of de-extinction engineers, a relatively large group of scientists who hope to use genetic engineering to protect or revive iconic animal species that have been destroyed by human activity.
For some, de-extinction (or de-extinxtion) is a journey of redemption on an ecological scale. Yes, biodiversity is important, but who can say that an extinct species can adapt and survive in an ecological system that is constantly changing and that has changed since its death? Or, more importantly, what if resurrected animals - essentially "aliens" on Earth - do more harm than good to our fragile ecosystem?
“Why do we need this trouble,” asks Novak's team. In the case of wandering pigeons, the answer is simple: Recently, almost a millennium after the disappearance of humans, we finally understood the crucial role they played in shaping the eastern North American ecosphere.
At the moment, scientists have found that wandering pigeons appeared more than 12 million years ago and were exceptionally adapted for life in dense flocks. The new findings, combined with forest ecology studies, clearly show that these birds were key engineers of forest dynamics.
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This central role suggests that itinerant pigeons are far more important to the ecosystem than simply trying to succeed in resurrecting extinct species. “The return of the wandering pigeon will restore the regenerative cycles of forest dynamics that dozens of species need that are currently shrinking and struggling to survive,” says Novak.
De-Extinction: Drafts
The process is two-step: first, the extinct species must be resurrected from the dead. Second, reintroduction of the species into the environment must be carefully evaluated.
We already have similar examples for the second step, explains Novak. Reintroducing species to areas where they have been eradicated is a long-term science, and scientists have already seen some success: wolves in Yellowstone, moose in Kentucky, beavers in Scotland. The ultimate goal is to weigh the risks of doing or not doing it: what happens to the ecosystem if we stop interfering.
In the case of the wandering pigeon, reintroduction is likely to be either neutral - that is, it will become just another bird in the forest with no measurable impact - or positive for the survival of other species.
In any case, the odds are in Novak's favor.
The first requirement - to bring the dead back to life - is a very new science. But thanks to a little tool called CRISPR, de-extinction can take place.
Here's the catch: Unlike Jurassic Park, scientists aren't trying to animate an animal based solely on its DNA.
The team uses a find-and-replace approach: start with a regular pigeon that is scampering around the city's garbage cans, and include genes that are typical of wandering pigeons. We use the closest relatives - pigeons with a striped tail. By selectively removing genes from the animal, the team hopes to concentrate the newly turned on genes in the offspring and thus propel "living surrogates" to passenger pigeons at the genetic level.
With enough generations, we can get curious artificial species with DNA indistinguishable from extinct animals. But whether a hybrid animal will become a wandering pigeon is a philosophical question.
This is the strategy followed by most groups trying to resurrect animals. But Novak's team had an answer.
Compared to dodo, for example, "there is usable DNA because there are more stuffed pigeons in museums and taxidermist collections than any other bird," says Novak. Add in the fact that we have access to striped pigeons and an understanding of their reproductive patterns, and you have that "the science of resurrection could get all the stimuli and perseverance."
De-extinction in practice
Since 2012, the Novak team has arguably achieved more success than any other de-extinction project.
First, in a series of works and collaborations, the team gradually uncovered the important impact that the traveling pigeons had on their ecosystems and answered the long-standing question of how their numbers dwindled so quickly from billions to zero. Birds have evolved to adapt to life in large populations. While they could probably recover from low genetic diversity, the human hunt has put the final nail in their coffin.
Together, these studies are helping to bring the pigeon back home. Novak explained that according to these results, we first need to provide the striped pigeons with the necessary mutations that will cause them to reproduce in the same way as itinerant pigeons did, and they could live in dense communities. We then need to give the birds the adaptations they need to live effectively in high social density environments, such as traits that will make them more sensitive to social cues, or the quick maturation of offspring so they spend less time raising large numbers of children.
The advent of CRISPR changed everything. For the first time, Novak and his team have access to a relatively simple and cheap gene editing tool that will allow them to work with the DNA of a common pigeon.
So far, they have taken several approaches to make the tool more effective. The best way is to directly edit the pigeon sperm or egg, because these changes will be passed on to the next generation. What is the problem? Scientists have not yet been able to grow these cells in a test tube.
The next important point is the introduction of the DNA editing tool - with the necessary tools - into the embryo. But scientists ran into problems here too: they had to rely on viruses to deliver the CRISPR toolkit. As a result, the packaging has become too large to be inserted safely.
To get around these hurdles, Novak's team focused on creating an entirely new line of pigeons, each containing genetic material to make it easier to edit DNA.
CRISPR-Cas9 is a two-step process, Novak explains. Part of CRISPR is hidden in the target DNA, while Cas9 does the cut. Birds bred with Cas9 in each cell essentially have some functional gene editing tool in their bodies, making the delivery of other essential components much easier.
“In the next three years, the world may see the first genetics of the wandering pigeon reborn in living, breathing birds,” says Novak.
If all goes as planned, we will witness a resurgence of extinct species in the next twenty years. Dodo birds and woolly mammoths will also be resurrected. Sooner or later, extinction will remain a thing of the past - for better or worse.
Ilya Khel