Proyecto Arquelogico Hoyo Negro, Quintana Roo, Mexico

Three questions at the foundation of Americanist Archaeology are “When did Homo sapiens first arrive in the Western Hemisphere?” “By what route or routes did they migrate south of glacial ice?” and “what impact did they have on the region’s diverse megafauna?”  Fierce debates surround each of these questions.  Some scholars insist that people who produced the Clovis Culture, which began between 11,400 and 11,060 cal BP were the first arrivals, while others see a pre-Clovis arrival as early as 17,000 cal BP.  One camp argues that the first migrants must have entered through the continental interior between glacial ice masses, whereas a growing number insist on a Pacific Coast route of entry.  Extinction of more than 80 genera of large mammals is either attributable, at least in part, to human hunters, or generalized human foragers played little or no part in that great decline in the hemisphere’s biodiversity.

One reason these debates persist is that much of the evidence is circumstantial.  The Pacific coastline that existed during the terminal Pleistocene is now almost all under water; archaeological finds in the supposed corridor are few and of uncertain age.  Pre-Clovis finds are almost all of uncertain age or human association. Clovis artifacts are widespread—in fact almost ubiquitous from south-central Canada to northern Venezuela, but, except for a few major kills of mammoth, mastodon, and gomphothere, and isolated horse and American camel, they are rarely associated with faunal remains.  In the eastern US and Central America, bone preservation is almost nonexistent in such sites.  Human remains could contribute greatly to this discussion by providing direct evidence of people’s behavior, dietary preference, and position in the food chain, but until 2007, only two fragmentary sets of bones dating to Clovis times had been found.  These consisted of four fragmentary elements from an infant and two leg bones of an adult.  Human skeletal remains had never been found in association with bones of the animals they supposedly brought to extinction.

That picture changed in 2007, when a team of cave divers, Alejandro Álvarez, Alberto Nava, and Franco Attolini, discovered Hoyo Negro, an immense collapse chamber in the Sac Actun cave system, one of the largest submerged tunnel systems in the world. This 60 meter diameter, 35 to 55-meter-deep chamber lies at the confluence of three long tunnels, the floors of which are at 10-12 meters below modern sea level.  During the last glaciation, this system was entirely above water and animals could enter it from the many sinkholes (called “cenotes”) that dot this region.  The nearest Pleistocene entry to the cave is 600 meters away.  Over the centuries, many animals fell into the bell-shaped chamber.  Scattered over the bottom of this natural trap the divers found bones of large animals and the skull of a human. Divers have been exploring the caves of the eastern Yucatan Peninsula for nearly 30 years, having now mapped nearly 1500 kilometers of submerged passages.  During their explorations, they have often seen and photographed scattered bones and even complete, articulated skeletons of large mammals, including gomphotheres, giant ground sloths, manatees, and tapirs.  Human skeletons are common. Most are from the recent Maya people but a few are thought to be very ancient.  What was different about Hoyo Negro was that the bones of people and animals occurred together and were at a depth of more then 40 meters below modern sea level.  The bones of the human and one gomphothere were commingled.  In addition to being a paleontological treasure trove, the chamber’s floor is dotted with stalagmites, charcoal, tree branches, and cones of bat guano and calcite sediment.  These provide the potential for a detailed record of local paleoecology and a chronology of postglacial sea-level rise.

Scientific research on Hoyo Negro began in 2011 with the formation of Proyecto Arqueológico Subacuático Hoyo Negro, Tulum, Quintana Roo (Underwater Archaeology Project Hoyo Negro, Tulum, Quintana), under Mexico’s underwater archaeology area at the Instituto Nacional de Antropología e Historia (National Institute of Anthropology and History).  Proyecto Hoyo Negro is a consortium of scholars from universities and government agencies in Mexico, the United States, and Canada.  Included are paleontologists, archaeologists, physical anthropologists, geochemists, paleobotanists, physicians, and computer modeling specialists.  Our goal has been to collect the human skeleton and to wrest the maximum possible information about terminal Pleistocene environmental conditions humans first encountered.  To achieve this we seek to identify each of the animal species in the cave and determine its geologic age by radiometric dating and place in the local food web through stable isotope analysis.  In addition, we need to obtain data on changing sea levels, and climatic and hydrological conditions from speleothems and calcite sediment.  To do this requires operating in some of the most difficult conditions the archaeological world has to offer.

Hoyo Negro poses significant technical challenges, from both field and analytical perspectives.  The site is in the dark and, except for a few fossils scattered in the upper tunnels, it is more than 40 meters under water.  To work directly on sediments and fossils requires certification in both technical diving and cave diving, which are qualifications few scientists possess. This restrictive environment means that the scientific team must rely on its skilled divers to be its eyes and hands.  It also means that many of the fossils visible in the cave may never be safely collected; collection must be planned carefully and executed flawlessly.  In addition, the tropical karst setting and its flooding history pose major challenges for dating the finds.  Bones have lain in warm, first fresh and now salt water for at least 10,000 years.  Collagen preservation is almost nonexistent and the bioapatite fraction of bones is often contaminated by calcium carbonate in the cave waters.  Charcoal is common but has demonstrably been floated repeatedly and moved about, making it next to useless for determining minimum ages of underlying finds.

Hoyo Negro’s challenges were met in the 2015-2016 field seasons using a combination of high-resolution photography, 3-Dimensional computer modeling, and highly selective collecting.  Work began with photography and modeling. First, the dive team laid out a grid of lines spaced 2 meters apart across the chamber at a depth of 30 meters.  They then took thousands of overlapping high resolution digital photographs of the entire floor and portions of the walls and connecting tunnels.  These were stitched together by the 3-Dimensional modeling team at the Qualcomm Institute of the University of California, San Diego, using structure-from-motion software (Agisoft) to create a composite of the 60-meter diameter floor.  In addition, each cluster of animal bones on the cave floor is being photographed close up to produce higher resolution 3-Dimensional models.  From these photographs and models it has been possible for the archaeological team to taxonomically identify specimens, select specimens for collection, and design mechanisms for specimen recovery.

The human skeleton, that of a 15-17 year-old woman now known as Naia, was recovered in three dive seasons of 2016.  Using the 3-Dimensional model it was possible to take the dimensions of each element and cut forms in collection boxes to precisely fit.  Divers entered the water with dive cards including numbered photographs of the elements to be collected.  Each number on a card matched a slot in a box designed to receive that element.  Boxes were sealed at depth under water and kept sealed until opened in the laboratory in Campeche, more than 300 km away.  This precaution was necessary to avoid air spaces to allow water to slap back and forth in the boxes and damage the fragile bones.  In all, 78 bone pieces, including 98 skeletal elements and 28 teeth were recovered.  This includes nearly all of the major elements.  Only the feet, part of one leg, a few ribs and vertebrae, and some hand bones are missing.

Dating by a combination of radiocarbon dating of tooth enamel, uranium-thorium dating of overlying calcite formations, and chronology of sea-level change place this skeleton between 13,000 and 12,000 cal BP, with a probable age between 12,900 and 12,700 cal BP. This makes Naia the oldest securely dated human skeleton in the Americas and the only complete skeleton to date within the Clovis culture window of 11,400-10,600 cal BP.  As a young person, her bones and teeth documented the course of her growth and her movements across the landscape while she was still a child.

Naia is thus providing extensive information about the lives of women and children at this early time.  She was very gracile and often suffered protein deficiency, despite living only 7 kilometers from the coast at the time she died.  This brings the Pacific coast migration theory into question.  She suffered a spiral fracture in her left forearm of the sort typically associated with physical abuse today and had already been a mother at her young age.  We have additional insights to share about the diet and lifeways of her people but these are embargoed pending review of an article on the subject.  They provide insights into the questions about the diet, predation strategy and patterns of movement that will contribute to the debates about humans’ part in the extinction of American megafauna.

To date, 13 species of mammal, not including bats, have been identified on the Hoyo Negro floor and 5 in the connecting tunnels.  These include 7 extinct megafaunal species: the gomphothere Cuveronius tropicus, sabertooth Smilodon cf fatalis, the short-faced bear Arctotherium, the Shasta ground sloth Nothrotheriops shastensis, a mylodont ground sloth (probably Paramylodon), a large wolf-like South American canid, and a previously unknown species of mylodont sloth.  Pumas, tapirs, peccaries, a coati, opossums, and as yet unidentified small cats complete the menagerie. In addition to the bones, divers have found tracks in the upper tunnels belonging to the short-faced bears.  Bones of these animals had never before been seen north of Venezuela and no information about body structure or locomotor patterns had been known.  The tracks are thus a magnificent addition to the paleontological record.

In late 2015 and 2016, with boxes designed with measurements taken from the 3-Dimensional models and following procedures described above for the human skeleton, the team recovered parts of three short-faced bears and the mylodont sloth.  Skulls are targeted primarily because they are the most useful for identification and for the analyses best accomplished with tooth dentin and enamel.  Radiocarbon dating of one bear places it in the same geologic time range as Naia, indicating that at least one of the megafauna was probably her contemporary.  The sloth is a new genus, which we have named Nohochichak xibalbahkah (the second name pronounced shi bal bah kah).  Unfortunately, preliminary dating of bioapatite provides a maximum possible age of around 36,000 cal BP, so this animal was not Naia’s contemporary.

Work is continuing as this Shanghai conference takes place. During the November-December 2017 season, parts of 10 more animals and several stalagmites and samples of calcite overgrowth are scheduled for collection.  This time, the most important fossils are the sabertooth, the mylodont and Shasta sloths, and limb bones of the bears and Nohochichak.  Publications on the human, and the sloth and carnivore faunas from the site are expected in the coming year.  Stalagmites and calcite samples will add to the Holocene record already taken from the site to provide data on sea level and climate for what may be as much as 30,000 years of the terminal Pleistocene.