The world of Jurassic Park is no longer a complete fantasy. In 2003, scientists briefly brought a wild goat of an extinct species back to life, although the clone died 10 minutes after birth.
Dinosaurs are off the table as DNA decays and they have been extinct for too long to be cloned, but researchers are trying to revive species like the passenger pigeon, once the most abundant bird in North America.
Here are some projects.
WHAT: The passenger pigeons were so abundant in North America in the 19th century that their flocks were said to darken the sky for hours as they passed.
EXTINCTION: By the late 1800s, however, their numbers had plummeted due to habitat loss and hunting. On Sept 1, 1914, the last-known passenger pigeon, a female named Martha, died in the Cincinnati Zoo at the age of 29.
REVIVAL: In 2012, the US-based Revive and Restore Project, which aims to resurrect extinct animals, chose the passenger pigeon as its flagship effort. It hopes to take passenger pigeon DNA from museum specimens and fill in the blanks with DNA from its close surviving relative, the band-tailed pigeon. The team is still studying how to engineer the birds. Some scientists, however, are sceptical about whether today's forests can support a revived population.
WHAT: Giant cows called aurochs once roamed Europe. They weighed nearly 1,000kg, were about 2m tall, and were dangerous. Some were domesticated about 8,000 years ago, but others remained in the wild. Most were killed by the 15th century due to overhunting and habitat loss.
EXTINCTION: The last survivor died in a nature reserve in Poland in 1627, where it had "the dubious honour of being the first documented case of extinction".
• In the 1930s, the Nazis produced a breed of aggressive cattle that was genetically different from the aurochs.
• Private Dutch organisation Stichting Taurus is leading an effort to scour museums for auroch bone and teeth fragments, in the hope of gleaning enough genetic material to recreate its DNA. The researchers will then compare it with modern cattle to see which breed has the aurochs' genes, and try to reverse thousands of years of evolution through selective breeding. The project is seeking funding.
TASMANIAN TIGER (THYLACINE)
WHAT: The wolf-like marsupial was once found throughout mainland Australia and New Guinea.
EXTINCTION: When humans introduced dingoes to the area about 4,000 years ago, the thylacines were probably out-competed for food. Eventually, the species remained only on the dingo-free island of Tasmania.
It was blamed for killing livestock and was hunted to extinction by the early 1900s. The last thylacine, in Tasmania's Beaumaris Zoo, died on Sept 7, 1936.
REVIVAL: The Australian Museum announced plans in 1999 to reconstruct the animal from the DNA of a pup preserved in ethanol in 1866. But in February 2005, it said it was thwarted by poor DNA samples and a lack of technology. Later that year, the University of New South Wales said it would pick up the work with other partners, but nothing has come out of it so far.
WHAT: Once common in northern Spain and the French Pyrenees, the species was reduced to fewer than 100 in numbers because of extensive hunting in the 19th century. It is the only animal that scientists have brought back from extinction, but only for 10 minutes.
EXTINCTION: In 2000, the last of the wild goats, a female nicknamed Celia, was killed when a tree toppled and crushed it.
REVIVAL: In 2003, French and Spanish scientists delivered a newborn clone after 57 tries but it died after 10 minutes, due to a lung defect. The project was mothballed shortly after that. In November 2013, the scientists received new funding, and are still trying to clone Celia.