soon after svante paabo's team revealed that neanderthals and modern humans outside africa had interbred, anthropologist john hawks reopened the case files. he wanted to know if this interbreeding had happened a little or a lot. hawks needed more than just the five modern human genomes that paabo had analyzed. and he got a lucky break. a team of scientists published a huge new database of individual human genomes from around the world. the 1000 genomes project data began to become available. and so we were able to expand the comparison to literally more than a thousand individuals from different populations. narrator: if interbreeding had been a relatively rare event, then all non-african humans across the world would have inherited the same small dosage of neanderthal dna. but that's not what hawks found. hawks: now, my lab, we were able to look at more people in china, more people in tuscany in the uk. and as we're doing this, we're discovering that there are some differences among these populations. narrator: hawks uses the jelly beans to illustrate the relative percentages of neander

when svante paabo started his work the human genome had already been decoded. one had attempted to map the neanderthal genome. paabo faced a seemingly impossible task in attempting to map the dna in the nucleus of a 30,000-year-old cell. ed green is a geneticist on paabo's team. as soon as it was obvious that this was possible in theory, we started to think about how do we do this in practice, if we can get nuclear data if we can get some amount. and we did some back-of-the-envelope calculations and it looked like yes, this is feasible. paabo: so we spent a lot of time looking at many archaeological sites and many different bones... ...and eventually identified this site in croatia. narrator: the vindija cave in northern croatia contained genetic gold dust-- the 30,000-year-old leg bone fragments of three female neanderthals. the exceptionally well-preserved bones offered paabo's team the best chance of extracting neanderthal dna. in sterile conditions, the team took samples of bone, carefully dissolving them in solution before spinning them at high speed in a