House sparrows are everywhere humans are. But despite their suggestive species name, Passer domesticus, they aren’t officially domesticated. The bold, tiny, gray-and-brown birds are found on every continent except Antarctica, hopping around cities, pecking at leftover food on sidewalks, and sometimes chasing away native bird species. A new study suggests how these ubiquitous avians have adapted to living alongside humans: The evolutionary process of natural selection may have favored genetic changes that altered their skull shape and allowed them to digest starch—similar to domesticated animals like dogs.
The house sparrow’s friendly behavior is legendary, with references cropping up in the Bible, early Chinese poetry, and Geoffrey Chaucer’s The Canterbury Tales. But no one really knew what set them apart from the other wild members of the sparrow family, which tend to be skittish around humans.
Looking for a genetic explanation, Mark Ravinet, an evolutionary biologist at the University of Oslo, and colleagues caught dozens of sparrows at sites across Europe and the Middle East. They set up mist nets—long, billowing strands of mesh that harmlessly trapped the birds as they flew inside—measured and tagged the birds, drew blood samples, and then released them. The team collected information on four of the major Eurasian species: 46 house sparrows, 43 Spanish sparrows, 31 Italian sparrows, and 19 Bactrianus sparrows.
Back in the lab, they sequenced the birds’ DNA. When they compared genetic sequences of house sparrows and their most closely related wild cousin, the Bactrianus, the group found that many regions of the house sparrow’s genome appeared to have undergone positive selection since the two species split, meaning certain gene variations within those spots likely helped the birds thrive alongside humans. As soon as he saw the results, Ravinet recalls, he leapt up and down in his office. But he says he didn’t tell anyone until he had “triple-checked all the calculations.”
The most significant sign of positive selection in the birds’ DNA was found in a region with two known genes: one linked to skull development and another that helps create the enzyme amylase, which helps break down starch in humans, dogs, and other animals. Changes to both genes might have helped the house sparrows eat human-cultivated foods, the team proposes this month in the Proceedings of the Royal Society B.
The house sparrow could carry more copies or a different copy of the skull shape gene and the amylase gene, Ravinet says. His team plans to more closely study the variations in both genes, as it’s not clear yet how these genes have altered the birds’ appearances and behavior. Another next step, Ravinet says, is to examine the house sparrows’ diets and see whether any changes to the skull increased bite force, which would have helped the birds chomp into the hardier seeds that littered human farms.
The analysis also suggests the house and Bactrianus sparrows diverged from each other about 11,000 years ago, at the beginning of the Neolithic Revolution, when agriculture was first developed in the Middle East.
Evolutionary biologist Samuel Andrew, who studies sparrows at Macquarie University in Sydney, Australia, says the work is an exciting new step for bird researchers that could answer many questions about how sparrow species diverged to adapt to their different niches. But he and Ravinet agree there may be changes to other genes that were missed in this initial analysis, yet still helped the birds take advantage of humans.
“If you live in a major city, there’s way more animals around you than you realize,” Ravinet says. “They have a history and a story to tell. We’ve changed their histories. I think that’s just something that’s quite profound really.”