New evidence has bolstered a once popular myth about when the first domesticated horses arrived in North America. Researchers from the Florida Museum of Natural History actually made the discovery partially by accident. The lead researcher, Nicolas Delsol, was initially studying how cattle were domesticated on the continent, but a case of mistaken identity regarding a cow’s tooth led Delsol to offer credence to a widely-held theory of the origins of feral horse herds along the Atlantic coast.

“It was a serendipitous finding,” says Delsol, “I was sequencing mitochondrial DNA from fossil cow teeth for my PhD and realized something was very different with one of the specimens.”

The specimen which drew Delsol’s attention was a fragment of an adult molar, but it was not from a bovine as Delsol originally thought. Instead, the tooth had come from a horse. Delsol found this discovery interesting, because finding horse remnants is a rare occurrence in the region, due to Spanish colonists generally valuing cows over horses.

When Delsol sequenced the DNA and started comparing it to other samples, he was intrigued to find that the horse’s closest biological relatives were a series of horses from the island of Assateague, off the coast of Maryland and Virginia. While scientists have known for years that wild horses roamed those islands, there was never a definitive explanation for their origins. Delsol’s research may answer some of those outstanding questions.

The results of the DNA analysis unequivocally point to Spanish explorers as being the likeliest source of the horses on Assateague, Delsol explained.

"The Spanish were exploring this area of the mid-Atlantic pretty early on in the 16th century,” says Delsol. “The early colonial literature is often patchy and not completely thorough. Just because they don't mention the horses doesn't mean they weren't there."

According to the National Park Service, which manages the northern half of Assateague, the likeliest explanation is that the horses were brought over in the 1600s by English colonists from the mainland in an attempt to evade livestock taxes and fencing laws. Others believe the feral herds descended from horses that survived the shipwreck of a Spanish galleon and swam to shore, a theory popularized in the 1947 children's novel "Misty of Chincoteague."

Until now, there has been little evidence to support either theory.

Proponents of the shipwreck theory claim it would be unlikely that English colonists would lose track of valuable livestock, while those in favor of an English origin of the herds point to the lack of sunken vessels nearby and the omission of feral horses in historical records of the region. To this day, there are roughly 150 feral horses on Assateague.

The horse’s tooth was found in a communal waste basin, commonly referred to as middens. As the Florida Museum of Natural History writes in their announcement of the findings, published on phys.org, “one community’s trash is an archeologist’s treasure.”

Delsol’s findings were published in PLOS ONE, a scientific journal specializing in biology and engineering, among other topics. Delsol’s article illustrates that the DNA sample he found in the horse’s tooth is the oldest specimen of equine DNA ever found in North America, dating back to the 1500s. The tooth was found in the abandoned Spanish colony of Puerto Real, on the island of Hispaniola. Today, Puerto Real is located on the northern coast of Haiti. The port town was abandoned due to piracy and rising illegal trade, so in 1578, Spain’s colonial rulers ordered residents to evacuate the port and the city was destroyed the following year.