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Neanderthals were our closest relatives. These stocky, heavy-browed humans lived from about 200,000 to 30,000 years ago in Eurasia and the Middle East. They coexisted in time and place with our own species, Homo sapiens. Scientists at Germany's Max Planck Institute and collaborating researchers have spent more than a decade extracting and sequencing fragments of DNA from ancient Neanderthal bones found in caves. The team decoded more than three billion nucleotides--about 60 percent of the Neanderthal genome-and compared it to the genomes of African, European, and Asian people living today. The recently released results are sparking new insights about what made Neanderthals and modern humans so alike-and different.

Unsurprisingly, the two genomes are nearly identical--about 99.8 percent. The scientists calculated that, of these similarities, 1 to 4 percent of the DNA in modern-day Europeans and Asians came from Neanderthals. This, the team suggests, is a sign that the two species mated and produced offspring at some stage, perhaps as Homo sapiens entered Europe from Africa 80,000 years ago.

The interbreeding finding is not yet conclusive, however, and more comparison of the two genomes is needed. It is challenging to discern the origin of DNA that is so alike in two species. Interbreeding is one explanation, but it is also possible that the identical DNA could have been passed down to both species from a common ancestor.

The differences in the genomes of the two species are as intriguing as the similarities. Genes that have a slightly different code in Neanderthals and modern humans suggest traits that evolved in a different way in the two species. These traits may have given each species unique advantages. For example, the gene CADP2 is linked to brain functioning that influences communication and social interaction. This gene is different in the two species. Scientists are eager to study these genome hotspots further to learn how Neanderthals and modern humans evolved.