The transformation from fearsome wolf to tiny chihuahua is thought to have been driven entirely by the human desire for pets doubling as fashion accessories – but new research indicates that the genetic mutation that gives rise to toy dog breeds was, in fact, present in ancient wolves.

The study, published in the journal Current Biology, casts doubt on the notion that humans painstakingly reduced these mighty canids to pugs and Pomeranians, suggesting instead that the genes required to produce lap-sized dogs were already in existence long before domestication began.

Modern dogs vary in size more than any other species on Earth, with hefty breeds being forty times larger than the smallest handbag dogs.

According to the researchers, these differences in stature are controlled by around 20 different body size genes, with one in particular – called insulin-like growth factor 1 (IGF1) – accounting for 15 percent of the variation in size between breeds.

After analyzing 1,431 genome sequences from 13 species of modern and ancient canids, the researchers reported two different versions – or alleles – of the IGF1 gene that consistently correlate to body size.

When they then looked at the genomes of several hundred modern dog types, they found that one of these alleles was predominant in breeds with an average body mass of less than 15 kilograms (33 pounds), while the other appeared in breeds that weigh more than 25 kilograms (55 punds).

“We looked at 200 breeds, and it held up beautifully,” explained study author Elaine Ostrander in a statement. She and her colleagues, therefore, labeled these two variants the “small allele” and the “large allele.”

After searching for these genetic variants in ancient canids, they found that the small allele existed in a species of Pleistocene wolf that lived 53,000 years ago, thus confirming that the capacity for toy dogs was written into the canid genome long before humans started messing about.

However, because this particular wolf was heterozygous, meaning it had one of each allele, the researchers could not determine which of the two variants emerged first.

To identify the ancestral allele, they analyzed the genomes of 24 additional canid species including coyotes, jackals, and foxes, finding that the majority possessed two copies of the small allele. Based on this observation, they conclude that the small allele most likely existed before the large allele appeared, and therefore represents the “ancestral state” of all dogs.

“This is tying together so much about canine domestication and body size, and the things that we think are very modern are actually very ancient,” says Ostrander.

The large allele increased in prevalence due to natural selection during the ice age, when bulkier bodies conferred significant survival advantages, the researchers say. During this period, the small allele almost disappeared entirely and was preserved only in a few diminutive species that lived in warmer climates close to the equator.

Yet the small allele was brought back from the brink when humans began selectively breeding dogs, increasing in prevalence significantly over the past two centuries thanks to the rising popularity of smaller breeds.

Overall, these findings imply that the creation of toy dogs was probably much more straightforward than the traditional narrative suggests. Rather than altering the wolf genome to create miniature dogs, human breeders merely reactivated ancient alleles, thereby shrinking their pets by tapping into a pre-existing genetic heritage.