To revive an extinct bird, you first need an artificial egg

Snyder and his colleagues developed this 3D-printed plastic egg to advance one of Colossal's big goals: resurrecting the dodo and another extinct flightless bird called the giant moa, which looked like a giant ostrich when it roamed New Zealand hundreds of years ago.

The eggs of the dodo, which lived on an Indian Ocean island before disappearing, were slightly larger than typical chicken eggs, and those of the dodo's closest living relative, the Nicobar pigeon, Snyder says.

The moa's eggs were about the size of a football, which is far larger than the eggs of the moa's closest living relatives, such as the emu.

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"There's no bird on Earth today that could grow a moa embryo inside of one of their eggs," Snyder tells NPR during a recent tour of the company's lab. "So we have to come up with artificial eggs to be able to support those embryos. But to understand all the things that the egg needs to do, we're starting with chicken eggs."

On Tuesday, Colossal announced that it had hatched healthy chicken chicks that were incubated in the company's artificial eggs, providing a proof of concept that they work.

"This is the coolest thing I've ever worked on," Snyder says.

Colossal claims it has already brought the dire wolf back from extinction and hopes to resurrect other species, including the woolly mammoth and the Tasmanian tiger.

But the company's efforts have sparked controversy. Critics question whether it would be ethical and safe to revive extinct species, or even really possible. Colossal dismisses those criticisms and doubts.

More than two dozen chickens have been born so far from Colossal's artificial eggs and more chicks will follow soon, the company says. Colossal is already working on artificial eggs that would be big enough for the dodo and moa embryos.

The company's plan is to try to recreate dodos and moas from embryos made from gene-edited cells from the Nicobar pigeon for the dodo and possibly the emu for the moa.

The genetically modified embryos would be placed inside artificial eggs, which are engineered to replicate the functions of a natural egg. The honeycomb structure, for example, is designed to let oxygen in while keeping the contents from leaking out.

"This is really an incredible feat to be able to do this," says Andrew Pask, Colossal's chief biology officer. "It's really fantastic."

Some other scientists are thrilled because the artificial eggs could also help save birds and reptiles on the brink of extinction.

"I'm genuinely blown away by it. This is brilliant. I just think it's fantastic," says Neil Gostling, a paleobiologist at the University of Southampton who isn't involved in Colossal's work. "It's the sort of thing of science fiction. It's remarkable honestly."

While others agree the advance could be exciting for conservation, some question trying to bring back extinct species.

The animals Colossal creates could be doomed to suffer and become extinct again because their habitats have been destroyed or changed too much, they argue. Some even worry reintroducing these long-gone animals could cause unpredictable, possibly catastrophic, damage to the environment.

"There are a whole lot of ecological and ethical concerns around bringing back extinct species," says Nic Rawlence, associate professor of ancient ecology at the University of Otago.

Rawlence also questions whether what Colossal is doing is really reviving extinct species. Instead, he argues, Colossal is genetically engineering existing species to resemble extinct animals.

"They're not what I would actually call de-extinct species," Rawlence says. "They're poor facsimiles of extinct species. Extinction is still forever."

Colossal defends its work.

"In the past, extinction was permanent," Snyder says. "We're changing that. Extinction is no longer permanent. We can literally wind back the clock and bring back things that humans caused to disappear off the face of the Earth. And the fact that we can undo that damage is just on the verge of miraculous."

Colossal is also working on creating artificial wombs to gestate mammals the company wants to bring back from extinction, such as the mammoth.

"We're undoing the sins of the past," says Brian Lamm, Colossal's co-founder and CEO. "There's nothing more ethical than what we're doing."