An artist's impression of the Yukon Horse (Equus Lambei), dating back 26,000 years.
© Yukon Beringia Interpretive Centre
The latest discovery is that the leg muscle of a giant sloth had been cut from the bone using a stone tool nearly 14,000 years ago.
It is earliest known evidence of hunting or scavenging by humans in North America - pushing back the date nearly 1000 years - and pre-dates the youngest known horse remains on the continent, which date back as little as 7600 years.
While the latest discovery will add further support to those who believe early hunter-gathers in North America played a part in the extinction of the horse, the weight of scientific evidence still supports climate and vegetation change as the main culprits.
Scientists argue that horses had long been in decline before humans set foot on the continent.
Horses only survived because animals crossed the land bridge which existed at various times between Alaska and Asia.
The cut marks were found on Ice Age bones found in Ohio.
Dr Brian Redmond, curator of archaeology at the Cleveland Museum of Natural History, was lead author in the research, published in the February 22 online issue of the journal, "World Archaeology".
Redmond and researchers analyzed 10 animal bones found in 1998 among the collections of the Firelands Historical Society Museum in Norwalk, Ohio.
They had been found by society member and co-author Matthew Burr, and had been identified as being from a Jefferson's Ground Sloth. The large plant-eating animal became extinct at the end of the Ice Age around 10,000 years ago.
"This research provides the first scientific evidence for hunting or scavenging of Ice Age sloth in North America," Redmond said.
"The significant age of the remains makes them the oldest evidence of prehistoric human activity in Ohio, occurring in the Late Pleistocene period."
A series of 41 incisions appear on the animal's left femur.
Radiocarbon dating of the femur bone estimates its age to be between 13,435 to 13,738 years old. Microscopic analyses of the cut marks revealed that stone tools made the marks.
The pattern and location of the distinct incisions indicate the filleting of leg muscles.
No traces of the use of modern, metal cutting tools were found, so the marks are not the result of damage incurred during their unearthing. Instead, the morphology of the marks reveals that they were made by sharp-edged stone flakes or blades.
The "Firelands Ground Sloth", as the specimen is named, is one of only three specimens of Megalonyx jeffersonii known from Ohio.
Based on measurements of the femur, tibia and other bones, it is one of the largest individuals of this species on record. It had an estimated body mass of 1295 kilograms (2855 pounds).
Top view of the left femur (thigh bone) of an Ice Age Jefferson's Ground Sloth. Microscopic analyses of marks on the bone reveal that they were made by stone tools. The cuts indicate that humans in Ohio hunted or scavenged meat from this species earlier than previously known. This research was published in World Archaeology.
© Cleveland Museum of Natural History
The collection was made known to Hay by Roe Niver, a University of Illinois student who lived in Huron County and died in July 1915. The bones were donated to the Firelands Museum before 1915.
The only documentation with the remains indicates they were found in a swamp in Norwich Township. The exact locality where the bones were first discovered is uncertain.
Until the latest discovery, the earliest evidence of humans in North America eating meat dated back 13,000 years.
Biochemical analysis of stone tools found in a remarkable cache within the city limits of Boulder City, Colorado, in 2008 showed that some of the 13,000-year-old implements were used to butcher ice-age camels and horses.
The University of Colorado study was the first to identify protein residue from extinct camels on North American stone tools and only the second to identify horse protein residue on a Clovis-age tool. A third tool tested positive for sheep and a fourth for bear.
Until 2009, evidence pointed to horse extinction on the continent around 13,000 to 15,000 years ago.
However, in a study published in the Proceedings of the National Academy of Sciences that year, evidence was presented indicating that horses may have survived in North America until 7600 years ago, some 5000 years longer than previously thought.
Researchers who removed ancient DNA of horses and mammoths from permanently frozen soil in central Alaskan permafrost dated the material at between 7600 and 10,500 years old.
It is possible they unearthed the tiny genetic footprint of the last few hundred ancient horses to roam North America.
The findings suggest populations of these now-extinct mammals endured longer in the continental interior of North America, challenging the conventional view that these and other large species disappeared from the continent about 12,000 years ago.
"We don't know how long it takes to pinch out a species," Ross MacPhee, Curator of Mammalogy at the American Museum of Natural History, said.
A close-up view of stone tool cut marks on the lower front surface of the left femur of a Jefferson's Ground Sloth. Research published in World Archaeology reveals the earliest evidence of hunting by prehistoric Ohioans.
© Cleveland Museum of Natural History
At the end of the Pleistocene, the geological period roughly spanning 12,000 to 2.5 million years ago, many of the world's large animals, such as giant sloths, saber-toothed cats, dire wolves, stag-moose, and mammoths, vanished from the geological record.
Because of the apparent sudden disappearance of many megafaunal species in North America, some scientists have proposed cataclysmic explanations such as human overhunting, a meteor impact or the introduction of novel infectious diseases.
The swiftness of the extinctions, however, is not suggested directly by the fossils themselves but is inferred from radiocarbon dating of bones and teeth discovered on the surface or buried in the ground.
Previous evidence had placed the last-known mammoths and wild horses between 13,000 and 15,000 years ago.
But hard remains of animals are rarely preserved, are difficult to find, and are difficult to accurately date because of physical degradation.
Because of this, MacPhee and co-authors Eske Willerslev, of the University of Copenhagen in Denmark, Richard Roberts, of the University of Wollongong in Australia, and Duane Froese of the University of Alberta in Canada, decided to tackle the problem by dating the "last survivors" through dirt.
Frozen sediments from the far north of Siberia and Canada can preserve small fragments of animal and plant DNA exceptionally well, even in the complete absence of any visible organic remains, such as bone or wood.
"In principle, you can take a pinch of dirt collected under favorable circumstances and uncover an amazing amount of forensic evidence regarding what species were on the landscape at the time," says Willerslev, director of the Centre for GeoGenetics at the University of Copenhagen.
"The use of ancient DNA offers the possibility of being able to sample previous life within the last 400,000 years, freeing us from having to rely on skeletal and other macrofossil evidence as the only way to collect information about species that are no longer with us."
In order to prospect for genetic fossils, the team collected soil cores from undisturbed Alaskan permafrost. Wind-blown Stevens Village, situated on the bank of the Yukon River, fitted the bill perfectly.
In this location, sediments were sealed in permafrost soon after deposition.
Two independent methods (radiocarbon and optically stimulated luminescence) were used to date plant remains and individual mineral grains found in the same layers as the DNA.
"With these two techniques, we can be confident that the deposits from which the DNA was recovered haven't been contaminated since these lost giants last passed this way," said Roberts, director of the Centre for Archaeological Science at the University of Wollongong. "It's a genetic graveyard, frozen in time."
Cores collected at Stevens Village offer a clear picture of the local Alaskan fauna at the end of the last ice age. The oldest sediments, dated to about 11,000 years ago, contain remnant DNA of Arctic hare, bison, and moose; all three animals were also found in higher, more recent layers, as would be expected.
But one core, deposited between 7600 and 10,500 years ago, confirmed the presence of both mammoth and horse DNA. To make certain that the integrity of this sample had not been compromised by geologic processes (for example, that ancient DNA had not blown into the surface soils), the team did extensive surface sampling in the vicinity of Stevens Village.
No DNA evidence of mammoth, horse, or other extinct species was found in modern samples, a result that supports previous studies which have shown that DNA degrades rapidly when exposed to sunlight and various chemical reactions.
"The fact that we scored with only one layer is not surprising," says MacPhee. "When you start going extinct, there will be fewer and fewer feet on the ground, and thus less and less source material for ancient DNA such as faeces, shed dermal tissues, and decaying bodies."
A small wetland in Norwalk Township, Huron County, Ohio. Historical research suggests this may be the locality where bones of an Ice Age Jefferson's Ground Sloth were found that included cut marks on the left femur. Research results published in World Archaeology indicate that prehistoric Ohioans hunted earlier than previously known.
© Cleveland Museum of Natural History
"At this point, mammoths and horses were barely holding on. We may actually be working with the DNA of some of the last members of these species in North America," says permafrost expert Froese, associate professor in the Department of Earth and Atmospheric Sciences at the University of Alberta.
"The Yukon Flats includes large shifting river bars with an abundance of high quality forage where large mammals can and could make a living. There may have been a handful of similar sites in Alaska, hosting small remnant populations," says Froese.
"Dirt DNA has lots of exciting potential to contribute to extinction debates in other parts of the world too, as well as a range of archaeological questions," said Willerslev, who also points out that the approach is not restricted to looking back at the past.
"We can also use it to make a list of modern species living in any particular location," he said. "This kind of information is really valuable for studies of animals that are hard to detect, and there are some neat forensic applications, too."