It was the last day of the expedition in the Gruta da Aroeira cave in Portugal. The archeologists were getting ready to close up the site, but one of their final scans of the thick rock revealed something interesting: an outline of a human skull.

What they discovered was the oldest skull ever found in Portugal, dating back 400,000 years, and the westernmost fossil of a human skull ever found in Europe.

The discovery was made in 2014, but it took 2½ years to remove the cranium from the sediment. The slow and arduous work paid off, leaving the cranium in "remarkably good condition," according to Rolf Quam, co-author of the study published in Proceedings of the National Academy of Sciences.

Also discovered at the site was a set of Acheulean stone tools, teardrop-shaped hand-axe tools believed to have originated in the Middle East. The tools first appeared in Europe about 500,000 years ago.

Acheulean stone tools were found at the cave site where the 400,000-year-old cranium was discovered. (Rolf Quam)

The discovery was something of a surprise and illustrates how swiftly the invention of tools spread among early humans.

"This site being at the extreme western end of Europe … suggests the spread of this tool technology was relatively rapid," Quam said.

Not a Neanderthal

The researchers were unable to determine whether the cranium belonged to a male or female, though they determined it belonged to an adult.

And while the cranium revealed some clues, what's not yet clear is where this early ancestor of ours belongs in the evolutionary tree.

"There are some features that are telling us that these guys are ancestors of the Neanderthals — they're not fully Neanderthal yet, but they're on the way," Quam said.

One difference is the brain size, which is smaller for the newly discovered specimen than a Neanderthal's.

However, some findings suggest that although the skull is not purely Neanderthal, it's pretty close. In Neanderthals, the mastoid process — a projection of bone behind the ear opening — doesn't project much, nor does it in this cranium. Earlier fossilized craniums show a protruding mastoid process.

As well, the shape of the brow ridge resembles that of the Neanderthals, though there is an anatomical difference.

"What species name is it?" Quam asks. "It is broadly similar to other ones from the Middle Pleistocene in Europe, but shows a unique combination of features, and these are all broadly ancestral to the later Neanderthals."

Quam said that the specimen will be further studied with the hope it will yield more information about the human evolutionary puzzle.