That's not an entirely new idea. A recent study pointed out striking physiological similarities between these Sima hominins and Neanderthals, our close cousins. Neanderthals only emerged about 400,000 years ago, but researchers suggested that the 28 hominins found in the pit of bones might belong to the species Homo heidelbergensis, which lived in the right place and time and was thought to be an ancestor of Neanderthals.
Then the plot thickened: DNA analysis published in 2013 suggested that the Sima hominins were actually more closely related to Denisovans – a less understood lineage of human.
But this isn't just another he-said-she-said DNA tale. The 2013 study analyzed mitochondrial DNA (mtDNA), which is easier to recover from ancient bones. But it doesn't tell the whole story. It's a small molecule of DNA found in the mitochondria of each cell – the cell's power plant. When an egg and a sperm meet, only the egg cell holds onto its mtDNA – so while a person's nuclear DNA will paint a unique portrait of their genetic lineage, mtDNA extracted from the same individual will show an identical copy made straight down the maternal line, save for any mutations that have been picked up along the way.
While the new study confirms that the Sima hominins really do have mtDNA that loops them in with the Denisovans, it also shows that the nuclear DNA is consistent with that you'd find in a Neanderthal ancestor.
The results suggest that Neanderthals and Denisovans diverged a good 400,000 years back, giving the Sima lineage time to pick up a new set of mtDNA as the generations went on – one that would match Neanderthals as we know them.
And based on the DNA analysis, the researchers believe that Neanderthals and Homo Sapiens may have split from their last common ancestor more than 600,000 years ago, which is a few hundred thousand years earlier than previously assumed.
“Research must now refocus on fossils from 400,000 to 800,000 years ago to determine which ones might actually lie on the respective ancestral lineages of Neanderthals, Denisovans and modern humans,” Chris Stringer of the Natural History Museum in London, who wasn't involved in the study, told New Scientist.
In other words, the new study poses more questions than it answers. Theories on the early days and interwoven family ties of humanity – already poorly understood – might be in need of a major overhaul. But while these proto-Neanderthals may have scientists scratching their heads for some time, the analysis of their long-sought DNA shows how far scientific techniques have come in just a few decades.