(Others weigh in here (Nature, ScienceDaily) and here (BBC).)
From The Scientist:
Sima de los Huesos has yielded the skeletons of at least 28 individuals from the middle Pleistocene period. These hominins have been traditionally classified as Homo heidelbergensis, and their features suggest that they were very early ancestors of Neanderthals.
But rather than clustering with Neanderthals, their mitochondrial genome was most closely related to the Denisovans. This enigmatic group of people are a sister group to Neanderthals and are known only from a finger bone and molar. Both were found 4,000 miles to the east, in Siberia. “It’s not what we expected,” said Meyer. “It’s hard to make sense of it.”
It is unlikely that the Sima de los Huesos hominins were actually Denisovans or even related to their immediate ancestors—for one, their teeth have a different shape. They might pre-date the split between Denisovans and Neanderthals.
Or they could be related to the ancestors who gave rise to both Neanderthals and Denisovans. Alternatively, another archaic group of hominins could have brought their mitochondrial DNA into both the Denisovans and the Sima de los Huesos people. …
We must be getting somewhere. It sounds like a soap opera now. In short, maybe we’ll never know but at least we’ll have a really detailed explanation of why we don’t.
From New Scientist:
The Sima de los Huesos genome is particularly exciting because it is from a time that is very close to the origin of our human line. The archaeological evidence suggests these early humans were developing significant new behaviours. On the one hand, they were still using fairly primitive stone tools like a crafted hand axe – nicknamed Excalibur – that was found in the pit. But the bones also suggest more modern traits.
For instance, some believe the pit might have been an early burial site, part of a simple funeral rite. Excalibur could be a tribute to the dead, suggests Stringer.
And the deformed skull of a girl who lived to be around 12 years old, also found in the pit, suggests that the tribe cared for her. “There’s a hint of something human – caring for the disabled,” says Stringer.
There is other evidence that Neanderthals did that as well.