Scientists attempting to resurrect the woolly mammoth believe they are just two years away from creating a hybrid mammoth/elephant embryo.
If they succeed, it will mark a turning point in plans to haul the ancient beast back from extinction by programming its key traits into an Asian elephant.
The bundle of cells would have genes for mammoth features such as shaggy long hair, thick layers of fat and cold-adapted blood.
However, years more work lie ahead before any serious attempt can be made to produce a living creature.
The US scientists have ambitious plans to grow the beast within an artificial womb, rather than recruit a female elephant as a surrogate mother.
Since starting the project in 2015, the researchers have increased the number of “edits”, where mammoth DNA has been spliced into the elephant genome, from 15 to 45.
Prof George Church, who heads the Harvard University team, said: "We're working on ways to evaluate the impact of all these edits and basically trying to establish embryogenesis in the lab.
“The list of edits affects things that contribute to the success of elephants in cold environments.
“We already know about ones to do with small ears, subcutaneous fat, hair and blood, but there are others that seem to be positively selected.”
He added: “Our aim is to produce a hybrid elephant/mammoth embryo. Actually, it would be more like an elephant with a number of mammoth traits.
“We’re not there yet, but it could happen in a couple of years.”
Extinction
The woolly mammoth roamed across Europe, Asia, Africa and North America during the last Ice Age and vanished some 4,500 years ago, probably due to a combination of climate change and hunting by humans.
Their closest living relative is the Asian, not the African, elephant.
“De-extincting” the mammoth has become a realistic prospect because of revolutionary gene-editing techniques that allow the precise selection and insertion of DNA from specimens frozen over millennia in Siberian ice.
PA