HOW WAS THE INTERACTION BETWEEN THE FIRST PEOPLE IN THE AMERICAS AND THE MEGAFAUNA?
Findings like those of Monteverde in Chile or Pedra Furada in Brazil suggest the arrival of human beings into the continent several millennia before the time proposed in the most widely accepted models, opening a debate to which Arroyo del Vizcaíno contributes.
Those large mammals are extinct. One of the hypothesis to explain it, apart from climatic change, is the impact caused by human presence. According to the most accepted model, about 12 to 15 thousand years ago (that is to say, little before the extincion) our species Homo sapiens crossed from Siberia to North America through Beringia, the then emerged territory that today lies under the cold waters of the Bering Strait in the North Pacific.
When the remains collected during the first effort in 1997 were still in the local high school,the Spanish palaeontologist Alfonso Arribas observed that a clavicle of the ground sloth Lestodon showed marks that could be interpreted as made by human tools. This was corroborated in a preliminary study published in 2001, in which the features of those marks were studied, as well as its association with areas of muscle insertion and their clearly bimodal orientation. However, Alfonso’s keen and trained eye hadn’t been the first in finding such an interesting evidence: already in 1997, the enthusiastic students had identified those signs in a rib.
Even so, the greatest surprise had yet to show up: some years later, a rib from this site (also belonging to the sloth Lestodon) and the marked clavicle itself were radiocarbon dated.
The ages obtained turned out to be much older than expected: between 28 and 29 thousand years before present (twice as much the date widely accepted for the human presence in the whole of the American continent.
As usual when dealing with unexpected findings, scepticism should be overcome with compelling evidence. We have recently published the finding with the outcome of our 2011 and 2012 fieldwork. From the numerous collected bones (over 1,000 catalogue entries), a few dozen showed marks, whose features were studied with 3D reconstructions built up from microphotographs (photo) to tell whether they had been made by the careless trampling by other animals when they becoame exposed before burial or if they could be assigned to human tools. Also, we found some pieces of fossil wood and some associated lithic elements, one of which has the form of a scraper as well as a typical micropolish congruen with its usage on dry hide.
Other five dates recently obtained after remains from the outcrop, four of them on bone and another on wood, corroborated the dates previously obtained and make the fossiliferous locality of Arroyo del Vizcaíno into the oldest site in the Americas with interesting evidence of human presence.
Rib of Lestodon collected in the site. It bears some marks whose features resemble those yielded by human activity (ie, the usage of tools for processing an animal for food) and different from those with different agency. The most important characteristics for correct assignent are that they are straight, that their section is V-shaped and that they show microstriae as a result of the secondary edges of the tool.
TRAMPLING OR HUMAN ACTIVITY? One of the most important challenges of the research was to identify and assign the human-made marks from those made by other agents, like rodents, carnivorans, roots and, particularly, trampling by other animals. Since trampling implies impacts between the sediment and the bones it may yield marks that sometimes are similar to those made by human tools. To study them, 3D models were built up and then measured with precision.
