Researchers now have a better idea of when exactly humans and Neanderthals first started hooking up, potentially revealing details about the disappearance and migration patterns of these long-gone relatives.

The fossil record indicates that Neanderthals, relatives of the only surviving ancestral human lineage, could be found in Europe and Asia up to 40,000 years ago. Then they disappeared, and the cause of this has long been a mystery. Some expert suspect that Neanderthals simply died out because their brutish strength could not make up for inferior intelligence when competing for resources with early humans. However, new studies have found that Neanderthals could have been just as smart as humans, even proving crafty enough to catch small and elusive prey like pigeons.

Another theory was that they simply bred into the ancestral human population so completely, that they simply became "part of their club," no longer recognizable as their own sub-species. This seems more probable, as recent estimates have revealed that Neanderthal DNA makes up one to four percent of the modern Eurasian genome.

Now, an analysis of DNA extracted from a 45,000-year-old bone of an ancient Siberian human is helping scientists pinpoint exactly when humans and Neanderthals first started to interbreed.

"This is the earliest directly dated modern human outside of Africa and the Middle East, and the oldest modern human [genome] to have been sequenced," Janet Kelso, a computational biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, told Live Science(Scroll to read on...)

Kelso co-authored a study recently published in the journal Nature that details how this ancient thigh bone indicates that early modern human migrations into Eurasia were not solely via a southern route as has been previously suggested - otherwise this Siberian would not have been boasting Neanderthal DNA.

The analysis and genetic variety of the Siberian suggests Neanderthal genes flowed into the ancestors of this man 7,000 to 13,000 years before he lived - indicating that humans were interbreeding with Neanderthals 50,000 to 60,000 years ago.

Lead investigator Svante Paabo added that the ancient DNA also suggests a rate of mutation that is different than what has been suggested by fossil evidence alone, and more similar to what we see in family lineages.

"We have caught evolution red handed!" he gleefully told BBC News.

He added that this could mean that the very first species of the human line separated from apes 10 or 11 million years ago - rather than the five or six million years ago that the textbooks currently indicate.

However, he's quick to add that mutation rates can change unexpectedly over time, making this purely speculation. Much more analysis will be needed before we can really say "humans started here."