The Mammoth Cometh


Nathaniel Rich in the NYT Magazine:

[Ben] Novak is tall, solemn, polite and stiff in conversation, until the conversation turns to passenger pigeons, which it always does. One of the few times I saw him laugh was when I asked whether de-extinction might turn out to be impossible. He reminded me that it has already happened. More than 10 years ago, a team that included Alberto Fernández-Arias (now a Revive & Restore adviser) resurrected a bucardo, a subspecies of mountain goat also known as the Pyrenean ibex, that went extinct in 2000. The last surviving bucardo was a 13-year-old female named Celia. Before she died — her skull was crushed by a falling tree — Fernández-Arias extracted skin scrapings from one of her ears and froze them in liquid nitrogen. Using the same cloning technology that created Dolly the sheep, the first cloned mammal, the team used Celia’s DNA to create embryos that were implanted in the wombs of 57 goats. One of the does successfully brought her egg to term on July 30, 2003. “To our knowledge,” wrote the scientists, “this is the first animal born from an extinct subspecies.” But it didn’t live long. After struggling to breathe for several minutes, the kid choked to death.

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. How close an approximation, it will be impossible to know. As with any translation, there may be errors of grammar, clumsy phrases and perhaps a few missing passages, but the book will be legible. It should, at least, tell a good story.

More here.