Did homo sapiens and Neanderthals have sex? Yes, say geneticists – a position now reinforced by the painstaking sequencing of genetic material found in a Neanderthal lady’s toe bone.

In fact, it seems our ancestors weren’t that fussy when it comes to a fling. The 50,000-year old toe bone shows that not only were the Neanderthals making merry with other species, including apparently the mysterious Denisovans, but they were heavily inbred as well.

At least the toe lady had been the product of incest. The relationship between the Neanderthal woman’s parents is not clear – they could have been half-siblings, first cousins, uncle and niece, or some other relationship close enough to produce a clan of inbred people.

What’s clear is they were close kin; the question is why they mated. It may have been a cultural norm. Or, the reason may have been absence of choice – sheer scarcity, as the Neanderthals spiraled towards extinction.

Inbred to death?

This finding of inbreeding, published in Nature magazine, may shed new light on the causes of Neanderthal extinction, which remain mysterious to this day. Some postulate that modern man, more socially adept, outwitted the less endowed Neanderthals and wiped them out; others speculate about ancient genocide and even cannibalism.

But inbreeding within a small population will do the trick over time, through the accumulation of maladaptive recessive traits.

The upshot is that each individual, and hence the whole population, become less fit with each passing generation. The results of forced inbreeding can be seen for instance in India’s tiger population, which many believe has fallen below the critical mass of genetic diversity needed to sustain a healthy population.

The toe bone in question has been dated to 50,000 years ago, and was found in 2010 in the very same Siberian cave where remains of Denisovan humans were first found, in 2008. Externally, the toe bone showed mixed features – both Neanderthal and modern. To reduce their error margin, the scientists sequenced the DNA more than 50 times and concluded that the bone indeed belonged to a Neanderthal female.

Multiple miscegenation

Intriguingly, genetic studies of this woman’s DNA and numerous other cases have shown multiple mixing of the species.

It turns out that Denisovan remains have traces of Neanderthal DNA. But the reverse is not so – the Neanderthal woman’s DNA did not have Denisovan genes.

As much as 2% of non-African, modern human DNA is believed to be of Neanderthal origin; it seems that up to 6% of the DNA of Melanesians and Australian Aborigines derives from Denisovan origin. Asians and native Americans – but not other human groups, insofar as is known – have a smidgen of Denisovan DNA, about 0.2%, say scientists. That fits in with what is known about the Denisovans’ range, which included much of Asia and Siberia.

In case you are wondering how genetic material – notoriously delicate – survived 50,000 years. One reason is that the cave where the toe bone was found, is in the Altai Mountain range of Siberia and is very cold, with an average temperature of zero degrees. In short, the bone was refrigerated.

The cave also contains remains showing modern humans lived there too.

None of this intermixing has helped unravel the bewildering crazy twine of human origins. It seems the first split between modern man and the common ancestor to Denisovans and Neanderthals was half a million years ago to 750,000 years ago. The Denisovans and Neanderthals apparently diverged several hundred thousand years later. And, as we now know from archaeological studies, they shared the cave life - and apparently the charms of family as well.