Researchers at Cornell University in Ithaca studied 3,000 dogs from 143 breeds and found small dogs all shared a mutation in a gene that influences size in other animals. This form of the gene was almost absent in large dog breeds. Their findings were reported in the scientific journal Science.
The 14 small dog breeds included the Portuguese water dogs, chihuahuas, fox terriers and pomeranians and the study found that they all share a specific sequence of DNA that includes the gene for making a hormone called insulin-like growth factor 1 (IGF-1). The scientists also looked at DNA from nine large breeds including Irish wolfhounds, St Bernards and Great Danes. The IGF-1 gene has been known to influence size in other organisms, including mice and humans.
The new research suggests that a mutation in this gene led to the appearance of small dogs more than 10,000 years ago. Over the several thousand years that ensued, selective dog-breeding ensured the dog gene was kept around and spread through human migration and trade. All domestic dogs are descended from wolves. They were the first animals to be domesticated more than 15,000 years ago, although early humans and wolves may have co-existed for about 400,000 years.