From The New Yorker:
At any given moment, Pääbo has at least half a dozen research efforts in progress. When I visited him in May, he had one team analyzing DNA that had been obtained from a forty- or fifty-thousand-year-old finger bone found in Siberia, and another trying to extract DNA from a cache of equally ancient bones from China. A third team was slicing open the brains of mice that had been genetically engineered to produce a human protein. In Pääbo’s mind, at least, these research efforts all hang together. They are attempts to solve a single problem in evolutionary genetics, which might, rather dizzyingly, be posed as: What made us the sort of animal that could create a transgenic mouse?
The question of what defines the human has, of course, been kicking around since Socrates, and probably a lot longer. If it has yet to be satisfactorily resolved, then this, Pääbo suspects, is because it has never been properly framed. “The challenge is to address the questions that are answerable,” he told me. Pääbo’s most ambitious project to date, which he has assembled an international consortium to assist him with, is an attempt to sequence the entire genome of the Neanderthal. The project is about halfway complete and has already yielded some unsettling results, including the news, announced by Pääbo last year, that modern humans, before doing in the Neanderthals, must have interbred with them. Once the Neanderthal genome is complete, scientists will be able to lay it gene by gene—indeed, base by base—against the human, and see where they diverge. At that point, Pääbo believes, an answer to the age-old question will finally be at hand. Neanderthals were very closely related to modern humans—so closely that we shared our prehistoric beds with them—and yet clearly they were not humans. Somewhere among the genetic disparities must lie the mutation or, more probably, mutations that define us. Pääbo already has a team scanning the two genomes, drawing up lists of likely candidates.