A century ago, vast flocks of passenger pigeons covered the North American skies. Hundreds of millions, even billions, stretched across the horizon in every direction. A flyover could last an entire day. “It was this beautiful super-organism, like a dragon going through the skies,” says Harvard Medical School geneticist George Church.
And then European settlers arrived. Mass hunting and habitat loss rapidly reduced their numbers until on 1 September 1914, Martha, the last known passenger pigeon, died in Cincinnati Zoo.
But if Church has his way, this majestic sight could one day return to our skies. Armed with new reproductive biology and genome engineering technologies, he and other scientists are dreaming up ambitious plans to resurrect long-dead animals from pigeons to Tasmanian tigers and wooly mammoths. The same technologies could also prevent endangered species from going the way of the dodo – or the passenger pigeon.
A key part of this grand ambition lies in the lines of test tubes frozen in liquid nitrogen in a Californian zoo. In 1972, Oliver Ryder, a geneticist at the San Diego Zoo, had the visionary idea of freezing skin samples from endangered animals in the hope they might help protect these species in the future. Many captive animals suffer from genetic abnormalities and inbreeding, and Ryder imagined that his repository of animal cells could be used long after their donors died to help zoo veterinarians manage captive populations.
He never viewed his “Frozen Zoo” – which now stores cells from 9,000 individual vertebrates belonging to more than 1,000 different species – as a way of producing new animals. As far as he and other scientists knew, only stem cells found in embryos had the ability to transform into the building blocks of any part of the body, whether a cell in the liver or the eye. Ryder’s collection of skin cells were just that, thought scientists - they could not possibly switch identity.
“I believed like the vast majority of the scientific community that cells made commitments and those commitments were irrevocable,” says Ryder. “Then Dolly happened.”
On 22 February 1997, Ian Wilmut and his team at the University of Edinburgh’s Roslin Institute unveiled a 7-month-old baby to a stunned world – Dolly the sheep. The team had found a way of replacing the nucleus of a sheep’s egg cell with a nucleus from an adult ewe’s cell, and yet still trick the egg into dividing as normal. After the hybrid egg cell began dividing, it was implanted into the uterus of another animal, which carried the clone to birth. “Dolly is derived from a mammary gland cell and we couldn't think of a more impressive pair of glands than Dolly Parton's,” said Wilmut at the time.
From dogs to cows, scientists rushed to clone a menagerie of animals using Wilmut’s technique, known as somatic cell nuclear transfer (SCNT). Within a few years, attempts to clone endangered species began to surface. The first, Noah, a threatened species of ox from Southeast Asia called a guar, was born seemingly healthy on 8 January 2001. Scientists at the American biotechnology company Advanced Cell Technologies who had helped pioneer cloning in cattle, shuttled nuclei from gaur skin cells into cow eggs and then implanted the embryos into cows.
However, Noah died two days later of an infection unrelated to the cloning process. And like the guar, other attempts to clone endangered species through somatic cell nuclear transfer tended to be one-offs. Many animals were born unhealthy, and the cloning process was inefficient, with success rates of around 1%.