In 1914, the last Passenger Pidgin died in captivity; its population numbered in the billions fifty years earlier but was hunted to extinction. But scientists believe they can bring the bird back using cloning techniques used on Dolly.

This cloning method, called somatic cell nuclear transfer, can be used only on species for which we have cellular material. For species like the passenger pigeon that had the misfortune of going extinct before the advent of cryopreservation, a more complicated process is required. The first step is to reconstruct the species’ genome. This is difficult, because DNA begins to decay as soon as an organism dies. The DNA also mixes with the DNA of other organisms with which it comes into contact, like fungus, bacteria and other animals. If you imagine a strand of DNA as a book, then the DNA of a long-dead animal is a shuffled pile of torn pages, some of the scraps as long as a paragraph, others a single sentence or just a few words. The scraps are not in the right order, and many of them belong to other books. And the book is an epic: The passenger pigeon’s genome is about 1.2 billion base pairs long. If you imagine each base pair as a word, then the book of the passenger pigeon would be four million pages long.

There is a shortcut. The genome of a closely related species will have a high proportion of identical DNA, so it can serve as a blueprint, or “scaffold.” The passenger pigeon’s closest genetic relative is the band-tailed pigeon, which Shapiro is now sequencing. By comparing the fragments of passenger-pigeon DNA with the genomes of similar species, researchers can assemble an approximation of an actual passenger-pigeon genome.

They hope to have recreated the species by 2025 but there are a host of issues that need solving before any repopulation can take place.