Humans’ gift of gab probably wasn’t the evolutionary boon that scientists once thought.
There’s no evidence that FOXP2, sometimes called “the language gene,” gave humans such a big evolutionary advantage that it was quickly adopted across the species, what scientists call a selective sweep. That finding, reported online August 2 in Cell, follows years of debate about the role of FOXP2 in human evolution.
In 2002, the gene became famous when researchers thought they had found evidence that a tweak in FOXP2 spread quickly to all humans — and only humans — about 200,000 years ago. That tweak swapped two amino acids in the human version of the gene for ones different than in other animals’ versions of the gene. FOXP2 is involved in vocal learning in songbirds, and people with mutations in the gene have speech and language problems. Many researchers initially thought that the amino acid swap was what enabled humans to speak. Speech would have given humans a leg up on competition from Neandertals and other ancient hominids.
That view helped make FOXP2 a textbook example of selective sweeps. Some researchers even suggested that FOXP2 was the gene that defines humans, until it became clear that the gene did not allow humans to settle the world and replace other hominids, says archeaogeneticist Johannes Krause at the Max Planck Institute for the Science of Human History in Jena, Germany, who was not involved in the study. “It was not the one gene to rule them all.”
The FOXP2 sweep theory first ran into trouble in 2008, when researchers discovered that Neandertals also had the two amino acid tweaks (SN Online: 11/14/08). That meant the change happened at least 700,000 years ago, before humans and Neandertal became separate branches of the hominid family tree. Then in 2009, some members of the 2002 team that originally reported the sweep presented new evidence showing that the two amino acid change wasn’t what swept to evolutionary prominence after all.
“That was sad, but that’s how it is,” says Wolfgang Enard, an evolutionary geneticist at Ludwig-Maximilians-Universität in Munich, who was involved in both the 2002 and 2009 studies. Still, there were hints that other genetic variants in and around FOXP2 might have been involved in a sweep, so the debate continued.Evolutionary and population geneticist Elizabeth Atkinson of Massachusetts General Hospital in Boston and colleagues decided to revisit the gene’s evolution “to see if FOXP2 ’s story held up using modern techniques,” she says. The researchers conducted a similar statistical analysis of patterns of genetic variation in FOXP2 as was done in the 2002 study. But this time the team studied more people, especially more people of African descent, and used data from the entire genome.
In a selective sweep, one pattern of genetic variants around a gene becomes much more common than other versions of the gene until nearly everyone has the popular version. When considering all the people in their study together, the researchers picked up the same statistical signal for a selective sweep as Enard’s group had. But when Atkinson’s team examined Africans separately from Europeans and Asians, the signs of a sweep were erased.
That result reflects what happened in human history, Krause says.
When humans migrated out of Africa, certain versions of genes were carried with the migrants and other forms were left behind in Africa. The version of FOXP2 that left with the migrants became more common as the migrant population grew. Atkinson’s team identified the statistical signal as being from population growth, rather than a selective sweep, by looking at changes elsewhere in the genome. If FOXP2 were getting swept, it would be the only gene sending the statistical signal. Instead, other parts of the genome scored similarly to FOXP2 on the statistical test.
The finding doesn’t mean that changes in FOXP2 weren’t important for language evolution, says Kirk Lohmueller, a population geneticist at UCLA. But geneticists may have to rethink some assumptions about how the evolution of species works. Selective sweeps were thought to be a major way that natural selection — the process that drives evolution — altered species. But these and other results suggest that selective sweeps were not very common in human evolution.
Many of the traits associated with being human, including speech and language, are controlled by multiple genes, so no one gene may have given a sweep-worthy boost. Or perhaps a speech and language sweep happened, but so long ago that its signal is too weak to pick up now, Lohmueller says.