Neanderthals may have survived at least 3,000 years longer than thought in what is now Spain – much after the species had died out everywhere else, a study has found.

The findings suggest that the process of modern human populations absorbing Neanderthal populations through interbreeding was not a regular, gradual wave-of-advance but a “stop-and-go, punctuated, geographically uneven history.”

Over more than ten years of fieldwork, researchers excavated three new sites in southern Spain, where they discovered evidence of distinctly Neanderthal materials dating until 37,000 years ago.

“Technology from the Middle Paleolithic in Europe is exclusively associated with the Neanderthals,” said Joao Zilhao, from the University of Barcelona in Spain.

“In three new excavation sites, we found Neanderthal artefacts dated to thousands of years later than anywhere else in Western Europe,” said Zilhao, lead author of the study published in the journal Heliyon.

“Even in the adjacent regions of northern Spain and southern France the latest Neanderthal sites are all significantly older,” he said.

The Middle Paleolithic was a part of the Stone Age, and it spanned from 300,000 to 30,000 years ago. It is widely acknowledged that during this time, anatomically modern humans started to move out of Africa and assimilate coeval Eurasian populations, including Neanderthals, through interbreeding.

According to the research, the process was not a straightforward, smooth one. Instead, it seems to have been punctuated, with different evolutionary patterns in different geographical regions.

In 2010, researchers found unambiguous evidence for symbolism among Neanderthals from the site of Cueva Anton in Spain.

Putting that evidence in context and using the latest radiometric techniques to date the site, they showed that Cueva Anton is the most recent known Neanderthal site.

“We believe that the stop-and-go, punctuated, uneven mechanism we propose must have been the rule in human evolution, which helps explaining why Paleolithic material culture tends to form patterns of geographically extensive similarity while Paleolithic genomes tend to show complex ancestry patchworks,” said Zilhao.

The key to understanding this pattern lies in discovering and analysing new sites, not in revisiting old ones.

Although finding and excavating new sites with the latest techniques is time-consuming, it is the approach that pays off, he said.