In a significant discovery, a "pit of bones" found at a possible burial site in northern Spain has offered a rare glimpse into the early evolution of the Neanderthals.
The distinctive facial features of the big-bodied hunter-gatherers, who dominated Europe long before modern humans arrived there, evolved stepwise, the discovery reveals.
A team led by Juan Luis Arsuaga, a palaeoanthropologist from Complutense University of Madrid, Spain, recovered the remains of around 28 individuals in the 13-metre pit.
They analysed 17 of those skulls.
"The 430,000-year-old skulls display key Neanderthal features, such as the beginning of a prominent brow ridge, a distinctive jaw shape and patterns of cusps on the teeth," said Arsuaga.
Yet, the skulls lack other traits that define the species - notably a large cranium - holding a brain bigger than that of Homo sapiens, on average.
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According to Arsuaga, Neanderthals evolved in a piecemeal fashion after their common ancestor with Homo sapiens left Africa more than half a million years ago.
The emergence of Neanderthals is just as mysterious as their disappearance about 30,000 years ago.
Sima de los Huesos - Spanish for "pit of bones" - is one of the richest human-fossil sites in the world.
The study appeared in the journal Science.