Paleo-DNA and De-Extinction

When prehistoric humans arrived in America they found a continent populated by mammoths, wooly rhinos, giant sloths, sabertooths, horses, and camels. By the time Columbus arrived, all of these species were extinct, due mainly to human hunting. But today, paleobiologists are sequencing the genomes of these extinct species. Furthermore, genetic engineering is approaching the point where genetically engineered versions of these extinct species may walk the Earth again. This year's big news—cloning of a dead pet dog—is thus merely a small beginning.

In the near future, the news will concern what paleo-DNA specialist Beth Shapiro dubs "de-extinction": generating living organisms bearing genes recovered from extinct species. Paleo-DNA (the somewhat degraded DNA recovered from bones or hair of extinct species) can be extracted from recently extinct species like mammoths and those exterminated by humans during historical times including passenger pigeons, dodos or thylacines (Jurassic Park fans note: the truly ancient DNA from dinosaurs is too degraded to currently allow sequencing). Trace amounts of DNA can be recovered, amplified and sequenced. Key genes could then be engineered into cells of the closest living relatives (Asian elephants for mammoths) to produce shaggy, cold tolerant elephants. This is nearly within technological reach today. Although birds pose unique challenges (being unclonable with current technologies), the de-extinction of passenger pigeons or moas (3.5 m tall flightless birds exterminated by hunting when the Maori arrived in New Zealand) appears within our technical grasp, and major de-extinction projects for these species are already underway.

So should we do it? Shapiro's recent book How to Clone a Mammoth provides an excellent introduction to the arguments. Given the polarized opinions generated by reintroducing wolves to Yellowstone, one can easily imagine diversity of public reactions to reintroducing sabertooth tigers or giant cave bears. I think that the best "pro" arguments are ecological—by reviving lost species we can restore habitats damaged or destroyed as our species spread over the planet. Mammoth-like elephants stomping through the tundra of Siberia's Paleozoic Park would benefit the environment by slowing the process of permafrost melting and the attendant carbon release. From a purely scientific, curiosity-driven viewpoint, de-extinction will offer biological and ecological insights available in no other way. Economically, tourists would pay top dollar to watch moas wandering the beech forests of New Zealand, or mammoths roaming Siberia. The con arguments are mostly practical (why spend money reviving extinct species rather than saving living endangered species?) or techno-fearful (humans shouldn't play god), but nonetheless passionately advocated.

By far the most significant issues will concern extinct hominids like Neanderthals, and society needs to prepare for the challenging ethical questions raised by such research. In 1997 researchers at Svante Pääbo's paleo-DNA lab in Leipzig made the news by sequencing mitochondrial DNA from Neanderthals. Today, after breathtaking technological progress, a full Neanderthal genome is available online. Even more exciting, in 2010 Pääbo's group discovered Denisovans, a previously-unknown Asian hominid species, based on DNA extracted from a tiny finger bone. The discovery of Denisovans from paleo-DNA makes it crystal clear that when modern humans emerged from Africa, they encountered a world inhabited by multiple near-human species—all of them now extinct.

Recovering the genome sequence of an extinct hominid species is exciting because it provides answers to a host of biological questions concerning which stones and bones (the previous mainstay of paleoanthropology) will remain forever mute. For example, it seems likely based on pigmentation genes that Neanderthals had light skin and some had red hair. Paleo-DNA has clarified that some interbreeding probably occurred when the first modern humans migrated out of Africa and encountered Neanderthals. As a result, all non-African human populations bear traces of Neanderthal DNA in their genomes (and many Asians bear additional Denisovan DNA). Similarly, the issue of whether Neanderthals had spoken language has divided scholars for decades. Although the case remains far from closed, we now know that Neanderthals shared the derived human version of the FOXP2 gene which enhances our speech motor control. This suggests that Neanderthals were able at least to produce complex vocalizations, even if they lacked modern, syntactic language. Such findings have fueled an ongoing sea-change in contemporary interpretations of Neanderthals—from oafish thugs to smart, resourceful near-humans. 

Paleo-DNA sequencing has changed not only our understanding of Neanderthals, but of ourselves. Neanderthals were not modern humans: they lacked the rapid cultural progress characterizing our species, and thus presumably some of our cognitive capacities. But what precisely were these differences? Do these differences make us, the survivors, "human?" Or were Neanderthals human, but "differently abled?" Certainly, with the bodies of Olympic wrestlers and brains slightly larger than modern humans, they'd be first picks for your rugby scrum; perhaps they had unique cognitive abilities as well. Progress will be rapid in addressing these issues, because each new insight into the genetic basis of the human brain yields parallel insights into our understanding of Neanderthal brains—and of the cognitive differences between the two.

But the truly deep ethical issues concern the possible de-extinction of Neanderthals or other extinct hominids. From a scientific viewpoint this would promise insights into hominid evolution and human nature unimaginable a decade ago. But from a legal viewpoint it would involve creating humans expressing Neanderthal genes, and thus require human cloning already forbidden in many countries. But few doubt that, within this century, genetic engineering of our own species will be both technologically possible and ethically acceptable in at least some sub-cultures. Clearly, a human expressing Neanderthal genes (as many of us already do!) would retain all basic human rights, but the moral and ethical implications raised by Neanderthals in the workplace (or on college football teams) might easily eclipse those raised by racism or slavery.

Clearly paleo-DNA will remain in the news for the foreseeable future, offering scientific insights and posing unprecedented ethical quandaries. It will thus behoove all thinking people (especially politicians drafting legislation) to become acquainted with the technology and the biological facts before forming an opinion.