How far back can the story of humans and dogs be told? When and where did this ancient relationship begin? New DNA evidence suggests our connection with canines can be traced much further into prehistory than has ever been conclusively shown.
According to scientists, analyses of ancient dog DNA suggests dogs were domesticated from Eurasian wolves as far back as approximately 23,000 years ago. Much later, they spread alongside humans as they migrated throughout the world – including entering the Americas by the way of Beringia, the long-lost land bridge that once connected Russia and Canada.
"The only thing we knew for sure is that dog domestication did not take place in the Americas," says geneticist Laurent Frantz from the Ludwig Maximilian University of Munich in Germany.
"From the genetic signatures of ancient dogs, we now know that they must have been present somewhere in Siberia before people migrated to the Americas."
While dogs are thought to have been the first domesticated animal, emerging during the Pleistocene from an extinct wolf population in Eurasia, much has remained unknown about the particulars of the animal's entry into the world, with some claiming the domesticated dog debuted as far back as 100,000 years ago.
Determining the truth isn't always easy, since it can be hard for scientists to authoritatively differentiate the discovered remains of ancient wolves and early domesticated dogs, whether through archaeological observation, or chemical tests using isotopes.
"The challenge for all claims of late Pleistocene dogs has been to show conclusively, across several lines of evidence, that the specimen(s) in question can be clearly distinguished from contemporaneous wolves," researchers explain in a new study led by archaeologist Angela Perri from Durham University in the UK.
"Here, we take a conservative approach and only include those canids whose taxonomic status is unambiguously domestic."
Disregarding the less substantiated claims of ancient dogs, the researchers say the earliest generally accepted domestic dog remains in the archaeological record appeared about 15,000 years ago in Germany and other contemporaneous sites across Europe and in Israel.
But what about outside the archaeological record? After all, genetic evidence suggests the earliest known dog lineages predate the archaeological remains by several thousand years, including a haplogroup (a genetic population with a single ancestor) estimated to date to about 22.8 thousand years ago.
By comparing that population with successive haplogroup lineages that split off from their common ancestor – including lineages that appeared in the Americas at about the same time as human settlers did about 15,000 years ago – the researchers constructed a timeline charting how dogs and their genes dispersed around the globe.
Ultimately, the analysis suggests human travellers likely brought their domesticated dogs with them as they journeyed into new lands, including the Americas, with the introduced dog lineage – haplogroup A2b – having genetic ties all the way back to Eurasia some 7,000 years earlier.
"We have long known that the first Americans must have possessed well-honed hunting skills, the geological know-how to find stone and other necessary materials and been ready for new challenges," says archaeologist David Meltzer from Southern Methodist University in Dallas, Texas.
"The dogs that accompanied them as they entered this completely new world may have been as much a part of their cultural repertoire as the stone tools they carried."
While the circumstances of dog domestication in Eurasia several thousands of years earlier still aren't entirely clear, the researchers say it's possible the extreme, unforgiving cold of the Last Glacial Maximum in Siberia may have triggered the early beginnings of what, with time, would become a beautiful friendship.
"Climatic conditions may have brought human and wolf populations into close proximity within refugial areas, given their attraction to the same prey species," the researchers write.
"Increasing interactions between the two, perhaps resulting from the mutual scavenging of kills, or from wolves drawn to the detritus of human campsites, may have initiated a shift in the relationship between the species, eventually leading to dog domestication."
The findings are reported in PNAS.