This Neanderthal skeleton was found in 1856 in the Neander Valley in Mettmann (Image: Action Press / Rex Features)
Neanderthals may have gone extinct because their cells couldn't cope with climate change, according to a new hypothesis presented at a genetics conference this month.
Metabolic adaptations to Ice Age Europe may have proved costly to Neanderthals after the continent's climate started to change, says Patrick Chinnery, a molecular biologist at Newcastle University, UK.
He and colleague Gavin Hudson identified potentially harmful mutations in the newly sequenced Neanderthal mitochondrial genome. In particular, the researchers found genes that are associated with neurodegenerative diseases and deafness. "If they were found in modern humans they would be bad news," Chinnery says.
The extinction of Neanderthals, close relatives of modern humans, some 25,000 years ago remains unexplained.
One theory holds that they were physically outcompeted by modern humans , another that they were economically eclipsed by us.. Yet another theory suggests that Neanderthals couldn't adapt to climate change.
The discovery of harmful mutations in the Neanderthal mitochondrial genome supports the climate-change idea, with a twist.
Chinnery and Hudson suggest that mutations in mitochondria helped Neanderthals cope with the cold weather, but that when the climate started fluctuating between warm and cold periods, they were at a disadvantage.
In all cells, from yeast to human, a mitochondrion's main job is to produce the energy that powers cells - this takes the form of a chemical called ATP. Our mitochondria do this quite efficiently under ideal conditions, making 36 ATP molecules with the energy stored in a single molecule of glucose sugar.
Mutations that sap this efficiency would generate heat instead - a potentially useful trick for Neanderthals who are known to have had adaptations to cold weather, Chinnery says. However, a warmer and less climatically stable habitat could have spelled trouble for Neanderthals with such mutations.
Perhaps the Neanderthals' mitochondrial DNA adapted them to the cold, and they couldn't cope when the climate started to change, he says.
However, with only a single Neanderthal DNA sequence decoded so far, that hypothesis remains provisional.
"This 'n of 1' experiment raises a question which needs to be tested on a large number of cases," Chinnery says.
They might not have to wait long. "We hope to be able to provide [Neanderthal] subjects for doing that kind of analysis really soon," says Edward Green, a researcher at Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Green and colleague Svante Pääbo published the first complete Neanderthal mitochondrial genome earlier this year.
However, Green cautions against reading too much into a Neanderthal's - or a human's - mitochondria.
Unlike DNA in the cell's nucleus, mitochondrial DNA does not reflect a healthy mix of maternal and paternal genes. We inherit all mitochondrial genes from our mothers, so a potentially advantageous gene has no way to spread through a population, without bringing along the rest of the genome.
Most scientists contend that changes to mitochondrial genes instead occur by chance, making them a good marker for human and Neanderthal ancestry.