Neanderthals may have been alive much later in human history than scientists have previously believed, because they lasted in Spain for thousands of years after they had already died out in other places around Europe and the world. And they have our ancestors’ actions to thank.

According to researchers, our species’ cousin may have faded into the evolutionary darkness so slowly because of their sexual and social encounters with Homo sapiens. They explain in the journal Heliyon that the mixture of humans and Neanderthals did not happen very steadily because of challenges like land barriers. As a result, the interbreeding and cultural exchanges between our prehistoric ancestors and Neanderthals was less of a drumroll that led to modern humans and more of a staccato. 

“Modern human emergence is best seen as an uneven, punctuated process during which long-lasting barriers to gene flow and cultural diffusion could have existed across rather short distances,” the study says.

The idea comes after scientists found evidence of Neanderthals in southern and western Iberia, like stone tools, that date to about 37,000 years ago. The artifacts they found from this part of the Stone Age were “distinctly Neanderthal,” journal publisher Elsevier explained.

During the Stone Age, modern humans were migrating out of Africa and into Eurasia, where they mixed with other human species both culturally and genetically — through interbreeding. But the finds that are 37,000 years old contradict previous knowledge on the matter: The general timeline for “transition and the associated process of Neandertal/modern human admixture took place between 40,000 and 42,000 years ago,” according to the study. There could have been different forces at play in this delay, like geographical and environmental barriers that hindered migration to some corners of the continent.

If those natural barriers existed in that one area to slow down the absorption of Neanderthals into our human collective, similar patterns may have sprung up in other areas throughout the prehistoric time that our two species overlapped on this Earth.

According to the study, the “pan-continental networks of gene flow and cultural exchange” could have included “extended periods of significant geographical isolation” in which nothing was exchanged at all.

Humans today still contain genetic evidence of the “punctuated” interbreeding with Neanderthals, in the form of genetic mutations.

“There is still a lot we do not know about human evolution and, especially, about the Neanderthals,” lead study author João Zilhão, from the University of Barcelona, said in the publisher statement. “Our textbook ideas about Neanderthals and modern humans have been mostly derived from finds in France, Germany and Central Europe, but during the Ice Ages these were peripheral areas: probably as much as half of the Paleolithic people who ever lived in Europe were Iberians. Ongoing research has begun to bear fruit, and I have no doubt that there is more to come.”