Since the discovery of the first recognised Neanderthal remains in the Neander Valley in Germany in 1856, scientists have debated the relationship between Neanderthals and humans.
At first thought to be an early human ancestor, Neanderthals lived from 370,000 to 25,000 years ago. They inhabited a vast geographical area from Portugal to western Siberia and from northern Europe to the Middle East.
Studies of the remains of Neanderthals indicate that they were not as tall as humans, had shorter limbs, thicker bones, pale skin, red hair and (surprisingly) high-pitched voices.
Despite their short stature, they were more robust than humans. Fossil Neanderthal bones exhibit the kinds of injuries sustained by American rodeo riders, suggesting that they probably hunted in close proximity to large prey.
From DNA research published in Science in 2010, we know that Neanderthals contributed to the human gene pool, with 4% of human nuclear DNA tracing back to them.
This contribution represents the equivalent of one great-great-great grandparent – an incredible finding considering how long ago Neanderthals became extinct.
While this and other recent studies of nuclear DNA reveal an infiltration of Neanderthal genetic material into the human lineage, Neanderthals and humans do not share one another’s mitochondrial DNA.
Since mitochrondrial DNA is only inherited from female ancestors, this is a vital clue that only Neanderthal men contributed to the human gene pool, and key to understanding patterns of interbreeding.
Mitochondria are tiny organelles involved in the energy regulation of each cell of the body. They evolved from symbiotic bacteria vital to cellular function.
Mitochondria reproduce asexually – without mixing of genes – so in the long run, they may build up deleterious mutations that are potentially lethal to their ongoing reproduction.
Mitochondrial DNA in sperm is a candidate for mutation. Nuclear DNA is also a potential site of mutation in sperm. So the constant production of male germ cells from puberty onwards increases the likelihood of alterations in genetic sequences.
Female germ cells, however, are held in a prolonged phase of dormant suspension from the moment of formation in late foetal life until just prior to ovulation.
Nuclear and mitochondrial DNA in oocytes – female germs cells involved in reproduction – is protected from mutations. For this reason, female parents pass on their mitochondrial DNA but male parents do not.
So unlike nuclear DNA, the DNA in mitochondria is exclusively inherited from the mother.
Sperm are dependent on maternal mitochondrial DNA for their ability to move towards the egg, but these mitochondria are destroyed by the oocyte immediately after fertilisation. Once fertilised, the egg contains only maternal mitochondrial DNA.
Since Neanderthal mitochondrial DNA is absent in living humans, it suggests that any hybrid offspring who carried the mitochondrial DNA from Neanderthals did not produce a lineage that survived until today.
One possibility is that mitochondrial DNA from Neanderthals contained detrimental mutations that led to the eventual extinction of carriers.
Another is that hybrid children of Neanderthal mothers may have been raised in Neanderthal groups, and like the rest of the Neanderthals, eventually became extinct.
A third scenario, published this week in the journal Hypothesis, is that female Neanderthals did not produce fertile offspring with male humans. Gene flow from Neanderthals to humans was the product of male Neanderthals mating with female humans.
Surprising it turns out the most common offspring of Neanderthal men and human women would have been female hybrids.
In 1922, the British geneticist and evolutionary biologist Jack Haldane wrote a key paper on the sex ratio of the offspring of hybrid animals.
Studying interspecific hybrids, Haldane showed that fertile XY progeny are likely to be absent, rare or sterile. The high mutation rate of genes on the Y chromosome may preclude a large number of fertile male offspring.
In our research into Neanderthal men mating with human women, applying Haldane’s Law would suggest that the offspring were predominantly female.
Neanderthals and humans did not always live in the same geographical region, but they did coexist in to various extents. We know anatomically modern humans left Africa 100,000 to 60,000 years ago and interbred with Neanderthals in the Middle East 80,000 to 50,000 years ago.
While traces of Neanderthal DNA have not been found in African populations, once modern humans migrated out of Africa, Neanderthal genetic material spread across many lines of the genealogies of humans.
The significant amount of nuclear DNA – 4% – from Neanderthals in the human gene pool is evidence of substantial interbreeding between Neanderthals and humans.
Indeed, the two groups interacted at various culture sites, and researchers have uncovered fossil remains of Neanderthal-like human ancestors.
Nineteenth century European scientists uncovering these fascinating glimpses into the interactions of humans and Neanderthals perhaps never imagined that it was the men, not the women, who were part of the ancient past of their family tree.
Neanderthal-human hybrids, Hypothesis
A draft sequence of the Neandertal genome, Science