Human Occupation of the North American Colorado Plateau ∼37,000 Years Ago

Abstract

Calibrating human population dispersals across Earth’s surface is fundamental to assessing rates and timing of anthropogenic impacts and distinguishing ecological phenomena influenced by humans from those that were not. Here, we describe the Hartley mammoth locality, which dates to 38,900–36,250 cal BP by AMS ¹⁴C analysis of hydroxyproline from bone collagen. We accept the standard view that elaborate stone technology of the Eurasian Upper Paleolithic was introduced into the Americas by arrival of the Native American clade ∼16,000 cal BP. It follows that if older cultural sites exist in the Americas, they might only be diagnosed using nuanced taphonomic approaches. We employed computed tomography (CT and μCT) and other state-of-the-art methods that had not previously been applied to investigating ancient American sites. This revealed multiple lines of taphonomic evidence suggesting that two mammoths were butchered using expedient lithic and bone technology, along with evidence diagnostic of controlled (domestic) fire. That this may be an ancient cultural site is corroborated by independent genetic evidence of two founding populations for humans in the Americas, which has already raised the possibility of a dispersal into the Americas by people of East Asian ancestry that preceded the Native American clade by millennia. The Hartley mammoth locality thus provides a new deep point of chronologic reference for occupation of the Americas and the attainment by humans of a near-global distribution.

Full text

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HYPOTHESIS AND THEORY
published: 07 July 2022
doi: 10.3389/fevo.2022.903795
Edited by:
Anderson Santos,
Rio de Janeiro State University, Brazil
Reviewed by:
Vladimir V. Pitulko,
Institute for the History of Material
Culture (RAS), Russia
Mario Dantas,
Universidade Federal da Bahia, Brazil
*Correspondence:
Timothy B. Rowe
rowe@utexas.edu
†Deceased
Specialty section:
This article was submitted to
Paleontology,
a section of the journal
Frontiers in Ecology and Evolution
Received: 24 March 2022
Accepted: 17 May 2022
Published: 07 July 2022
Citation:
Rowe TB, Stafford TW Jr,
Fisher DC, Enghild JJ, Quigg JM,
Ketcham RA, Sagebiel JC, Hanna R
and Colbert MW (2022) Human
Occupation of the North American
Colorado Plateau ∼37,000 Years
Ago. Front. Ecol. Evol. 10:903795.
doi: 10.3389/fevo.2022.903795
Human Occupation of the North
American Colorado Plateau ∼37,000
Years Ago
Timothy B. Rowe1,2*, Thomas W. Stafford Jr.3, Daniel C. Fisher4, Jan J. Enghild5,
J. Michael Quigg6†, Richard A. Ketcham1, J. Chris Sagebiel2, Romy Hanna1and
Matthew W. Colbert1
1High-Resolution X-Ray Computed Tomography Facility, Jackson School of Geosciences, University of Texas at Austin,
Austin, TX, United States, 2Vertebrate Paleontology Laboratory, Jackson School of Geosciences, University of Texas
at Austin, Austin, TX, United States, 3Stafford Research, LLC, Albuquerque, NM, United States, 4Museum of Paleontology,
Department of Earth and Environmental Sciences and Department of Ecology and Evolutionary Biology, University
of Michigan, Ann Arbor, MI, United States, 5Laboratory for Proteome Analysis and Protein Characterization, Department
of Molecular Biology and Genetics, Aarhus University, Aarhus, Denmark, 6The Gault School of Archaeological Research,
University of Texas at Austin, Austin, TX, United States
Calibrating human population dispersals across Earth’s surface is fundamental to
assessing rates and timing of anthropogenic impacts and distinguishing ecological
phenomena influenced by humans from those that were not. Here, we describe the
Hartley mammoth locality, which dates to 38,900–36,250 cal BP by AMS 14C analysis
of hydroxyproline from bone collagen. We accept the standard view that elaborate stone
technology of the Eurasian Upper Paleolithic was introduced into the Americas by arrival
of the Native American clade ∼16,000 cal BP. It follows that if older cultural sites exist
in the Americas, they might only be diagnosed using nuanced taphonomic approaches.
We employed computed tomography (CT and µCT) and other state-of-the-art methods
that had not previously been applied to investigating ancient American sites. This
revealed multiple lines of taphonomic evidence suggesting that two mammoths were
butchered using expedient lithic and bone technology, along with evidence diagnostic
of controlled (domestic) fire. That this may be an ancient cultural site is corroborated by
independent genetic evidence of two founding populations for humans in the Americas,
which has already raised the possibility of a dispersal into the Americas by people
of East Asian ancestry that preceded the Native American clade by millennia. The
Hartley mammoth locality thus provides a new deep point of chronologic reference for
occupation of the Americas and the attainment by humans of a near-global distribution.
Keywords: mammoth, butchery, taphonomy, pyrogenic residues, tomography, human dispersal
INTRODUCTION
Recognizing early human occupation sites in the Americas traditionally rests on discovery of in situ,
elaborately worked stone tools (Meltzer, 2009, 2015). The origins of this technology trace back to the
‘Upper Paleolithic revolution’ in Western Europe ∼45,000 years ago (Bar-Yosef, 2002). Elaborate,
stylized stone tools then spread from Western Europe into central Asia and Siberia and were later
introduced into the Americas by ancestors of the Native American clade ∼16,000 cal BP (Meltzer,
2009;Waters, 2019). The oldest evidence of Upper Paleolithic stone technology in the Americas is
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Rowe et al. North American Human Occupation 37 kya
generally equated with arrival of the first humans. However,
genomic evidence for two founding populations in the Americas
raises the possibility of two separate human dispersals, the first
preceding arrival of Native Americans by millennia (Raghavan
et al., 2014;Skoglund et al., 2015;Reich, 2018). If true, the
first arrival of humans was an event entirely separate from the
introduction of Upper Paleolithic technology into the Americas.
If humans who lacked elaborate stone tools occupied the
Americas long before arrival of the Native American clade, how
might we diagnose these older occupations in the archeological
record?
The Hartley locality is an open-air site on the Colorado Plateau
in northern New Mexico, perched 1970 meters (m) above mean
sea level on a slump block high on the wall of Rio Puerco canyon
(Figures 1,2). It was discovered by Mr. Gary Hartley, who was
hiking a game trail connecting the high grassy plain of La Joya
del Pedrigal to riparian habitat along Rio Puerco, ∼90 m below
(Supplementary Figure 1). A tusk was visible eroding through
the surface of a wedge of colluvial sediments filling a narrow
catchment trough along the top of the slump block. The tusk
pointed toward three ribs and a broken dentary also eroding
through the colluvial surface. A heavily reworked obsidian Clovis
projectile point, and scattered lithic debitage of Pedernal chert
and obsidian were exposed on the surface within a few meters
of the tusk. Preliminary excavation indicated more bones were
buried in the sediment wedge.
In situ associations between diagnostic Clovis projectile
points and mammoths are well-known (Frison and Todd, 1986;
Waters and Stafford, 2007;Hannus, 2018), and the Hartley
locality initially presented itself as a possible Clovis butchery
site (Huckell et al., 2016). However, surface accumulations
can be mixed and time-averaged (Kowalewski et al., 1998),
so the lithic accumulation might not be coeval with the
buried mammoth bones. To test the association and investigate
site formation processes, two seasons of hand excavation
(∼7 m3) focused on the region between and below the
exposed tusk and ribs. This revealed disarticulated, intermingled
partial skeletons of a young-adult female mammoth (Figure 3)
and a young calf (Supplementary Figure 2), mostly piled
in a meter-thick concentration of broken bones and fist-
sized cobbles, with the adult face and tusks stacked on top
(Supplementary Information).
The more obvious features of the bone assemblage suggested
this was a butchery site. However, six AMS 14C dates on
the adult mammoth bone collagen yielded ages ranging from
38,900 to 32,300 cal BP (Table 1 and Figures 4,5). These
dates are far too old to be cultural according to prevailing
archeological views (e.g., Meltzer, 2009;Waters, 2019). However,
they lie within the time range hypothesized by genomic evidence,
which projects a human presence in the Americas as early as
56,000 years ago (Wohns et al., 2022). The dates also suggest
the lithic debris on the colluvial surface was most likely a time-
averaged assemblage that accumulated long after the mammoth
bones were broken and buried, leaving open the question of
whether the mammoth bones reflected cultural activity. The
only lithic evidence recovered from excavating the Hartley site
consisted of six chert microflakes found in lower levels of
the excavation. Although they preserve evidence of percussion
flaking (below), taphonomic analysis of the bone assemblage
and surrounding sediments soon became the primary basis for
elucidating site formation.
We used established techniques (Shipman, 1981;
Behrensmeyer and Hill, 1988;Haynes et al., 2020), plus
state-of-the-art methods that had not previously been applied
to investigating ancient American sites. These include high-
resolution computed tomography (HRXCT and µCT) (Carlson
et al., 2003;Rowe and Frank, 2011;Frank et al., 2021), proteomic
analysis of the mammoth bone collagen, environmental scanning
electron microscopy (ESEM), and petrographic analysis of
microparticles from the matrix. These new methods significantly
expanded the range of criteria for site interpretation. Our
analyses revealed diagnostic evidence of diverse human activities
including systematic, highly patterned bone breakage and
secondary bone flake production, pyrogenic residues indicative
of a controlled (domestic) fire, and utilization of regional
vertebrate microfauna. The independent lines of evidence
described below support recognition of this as a cultural site.
METHODS
AMS 14C Measurements on Hartley
Mammoth Bone
Bones and teeth with unique scientific importance should
be dated not only multiple times, but should have direct
14C measured on different chemical fractions to assess how
chemical purification removes exogenous contaminants (Stafford
et al., 1991;Stafford, 2014;Waters et al., 2014). The specimen
selected was TMM 47004-1.40 (Stafford Research [SR] sample
no. 9067), a thick adult cortical bone fragment that we
identified as a butterfly fragment (below), from either an
ulna or tibia (Figure 6F). The bone was dateable because it
was moderately well preserved chemically (Tables 1,2and
Figures 4,5); it yielded 2.6wt% of collagen, compared to
23wt% for modern bone (Schwarcz and Nahal, 2021). The
sample also contained 0.4wt% nitrogen, compared to 4.5wt%
in modern bone (Schwarcz and Nahal, 2021). The molecular
weights of collagen fragments were in the >10 kDa range,
with minimum and maximum values being 20 and 50 kDa,
respectively. Dates were calibrated using OxCal 4.4, IntCal
20, Interface Build 133, Updated 24/11/2021, and calibrated
at 2-sigma, 95.4%, and 3-sigma 99.7% probabilities (Reimer,
2020).
Author TWS coordinated AMS 14C dating of the Hartley
mammoth bone to determine the age of the mammoth remains.
With exceptions as noted, TWS performed chemical purifications
by combusting pretreated samples to carbon dioxide (CO2) and
submitting 6 mm (outer diameter) fused quartz tubes of CO2for
graphitization and 14C measurement using three different AMS
14C facilities. One sample was sent to the University of Georgia
Accelerator Mass Spectrometer Radiocarbon Dating Laboratory
(UGAMS), and two samples were processed at the University
of California, Irvine, Accelerator Mass Spectrometer (UCIAMS).
TWS also sent subsamples of SR-9067 to the Oxford Radiocarbon
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FIGURE 1 | Hartley mammoth excavation. (A,B) Uppermost part of main bone accumulation. (C) Diagram of bone-bearing sediment wedge (not to scale).
(D) Schematic of Toreva block landslides. (E) Cross-section of Rio Puerco canyon. C, sandstone cobbles; R, rib; Tl, left tusk; Tr, right tusk; Fr, frontal; V, thoracic
vertebra; white star, 23.7 kg hammerstone/anvil.
Unit (ORAU; University of Oxford) for processing and dating by
Dr. Lorena Becerra-Valdivia (OxA, below).
Our initial exploratory 14C measurement was on 0.45 µm-
filtered gelatin from potassium hydroxide (KOH)-extracted
collagen, and it yielded an age of 28,745 ±86 RCYBP (UGAMS-
A24643), equivalent to 33,700 – 32,300 cal BP. Extensive 14C and
stable isotope analyses on the mammoth bone then followed.
A second 14C date was obtained using methods for chemical
purification of bone collagen described by Stafford (2014) and
Devièse et al. (2017, 2018a,b). Bone was crushed into ∼2–4 mm
fragments and decalcified in 0.6M hydrochloric acid (HCl) at
4◦C, washed with deionized (DI) water, extracted over 24 h with
0.01M KOH at 4◦C, washed to neutrality, acidified with 0.05M
HCl and the alkali-extracted collagen freeze-dried and weighed to
obtain a weight percent yield relative to 23wt% for modern bone.
Next, 20–25 mg of lyophilized collagen were heated in 0.05M
HCl at 90◦C until dissolution was complete, ∼20–30 min. The
solution (gelatinized collagen) was passed through a 0.45 µm
Millipore filter and freeze-dried. Finally, 15–20 mg of dry gelatin
were heated at 110◦C for 24 h in sealed Pyrex tubes to hydrolyze
the gelatin to free amino acids. This hydrolyzate was passed
through a column containing 1 cc of Restek XAD-2 resin, the
eluate filtered through a 0.45 µm Millipore membrane and
the XAD-purified hydrolyzate dried over a N2stream at 40◦C.
Subsamples collected from each step were used for amino acid,
δ13C, δ15 N, and 14C analysis. The sample was graphitized at the
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FIGURE 2 | Main bone accumulation, following removal of adult face. (Photogrammetry images). (A) Overhead view. (B) Oblique view. (C) Bi-directional rose
diagram showing random orientation of 40 elongate elements exposed; see text for explanation. Cr, calf ribs; Ct, calf tibia; Di, adult cranial diplöe; M2, adult upper
second molar; R1, adult first rib; V, adult thoracic vertebra; Sty, adult stylohyoid; white star, 23.7 kg hammerstone/anvil.
UC Irvine AMS facility, and the resulting date on the XAD-2
purified fraction was 30,860 ±220 RCYBP (UCIAMS-205817),
equivalent to 35,750 – 34,600 cal BP.
A third date on ultrafiltered (>30 kDa) gelatin was processed
at the UC-Irvine AMS facility using protocols described online
in the following document: https://sites.uci.edu/keckams/files/
2016/12/bone_protocol.pdf. It produced a measurement of
28,360 ±230 RCYBP (UCIAMS-191867), equivalent to 33,250 –
31,750 cal BP.
Finally, a series of three dates on subsamples of SR-
9067 were processed by Dr. Lorena Becerra-Valdivia at the
Oxford AMS facility and included ages on 60–90 µm-filtered
gelatin, ultrafiltered gelatin and the individual amino acid,
hydroxyproline, using protocols described in Hedges and van
Klinken (1992) and Devièse et al. (2017, 2018a,b). The first
sample used pretreatment code (P-code) ‘AG,’ which is a routine
procedure involving decalcification, base wash, reacidification,
gelatinization, and filtration of bone using previously cleaned
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FIGURE 3 | Recovered parts of adult mammoth. Numbers refer to cataloged bones with the prefix TMM 47004-1.X.
9 ml Ezee-filters (Elkay, United Kingdom), following Brock
et al. (2010). The result was 28,100 ±350 RCYBP (OxA-36306),
equivalent to 33,250 – 31,300 cal BP.
The second P-code, ‘AF,’ is another routine protocol that
includes ultrafiltration (using Vivaspin 15–30 kDa MWCO)
of the gelatin following Ezee filtration as described by
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Brock et al. (2010). The result obtained is 29,520 ±340 RCYBP
(OxA-36,358), equivalent to 34,650 – 33,200 cal BP.
The last protocol, P-code ‘NRC’ (non-routine chemistry),
involves separation of underivatized amino acids from
ultrafiltered, hydrolyzed collagen and isolation of
hydroxyproline using a preparative High Performance
Liquid Chromatography (prep-HPLC) approach. A full
description of this protocol, optimized at the ORAU, can be
found in Devièse et al. (2017, 2018a,b). Collected collagen and
hydroxyproline samples were dried, combusted, graphitized,
and AMS-dated as described by Brock et al. (2010), and with
HPLC carbon contribution corrections made per Devièse
et al. (2018b). The result is 32,750 ±430 RCYBP (OxA-X-
2737-56), equivalent to 38,900 – 36,250 cal BP. These results
show that the first five dates are in good agreement with
each other, while the hydroxyproline measurement is older.
Considering the NRC protocol to be more efficient than the other
protocols for removing exogenous carbon from bone (collagen)
samples (Rappsilber et al., 2007;He et al., 2015), the latter date
(32,750 ±430 RCYBP, equivalent to 38,900 – 36,250 cal BP), is
considered the most robust age estimate for sample SR-9067.
Amino Acid and Liquid
Chromatography-Tandem Mass
Spectrometry Proteomic Analyses
To assess quantitatively the preservation and composition
of the collagen isolated for AMS radiocarbon dating, JJE
performed quantitative amino acid and liquid chromatography-
mass spectrometry/mass spectrometry (LC–MS/MS) analyses on
the adult mammoth bone, SR-9067 (Table 2). Amino acids
were measured using the Biochrom 30+high-performance
liquid chromatography cation exchange system with ninhydrin
detection. While atomic C/N values are commonly used to
certify preservation quality of collagen (Schwarcz and Nahal,
2021), such analyses only indicate that the decalcified bone or
dentin being analyzed does not contain a preponderance of non-
proteinaceous material. Collagen is identified by its unique amino
acid composition, which is 9% percent (or 90 residues/1000)
hydroxyproline (HYP) and 33% (or 338 residues/1000) glycine
(GLY) (Table 2). Mammoth bone SR-9067 was decalcified with
two different reagents (0.6M HCl and 0.5M EDTA), and amino
acid compositions were measured for those residues (Table 2).
The results are values for slightly degraded collagen, where
HYP values have decreased from a modern value of 9% (or 90
residues/1000) to the 5% (or 49 residues/1000) present in the
Hartley mammoth collagen (Table 2). These diagenetic changes
in fossil collagen’s amino acid composition are well documented
(Stafford et al., 1991,Stafford, 2014) and involve proportional
decreases in HPY and relative increases in aspartic and glutamic
acids as overall collagen content decreases over time. All samples
were desalted by micropurification using Empore SPE Disks of
C18 octadecyl packed in 10 µl pipette tips. LC-MS/MS was
performed using either an EASY-nLC 1000 system (Thermo
Fisher Scientific, Waltham, MA, United States) connected to a
Q Exactive +Hybrid Quadrupole-Orbitrap Mass Spectrometer
(Thermo Fisher Scientific, Waltham, MA, United States) through
a Nanospray Flex Ion Source (Thermo Fisher Scientific,
Waltham, MA, United States) or an eksigent nanoLC 415 system
(Sciex, Torrence, CA, United States) connected to a TripleTOF
6600 mass spectrometer (Sciex, Torrence, CA, United States)
equipped with a NanoSpray III source (Sciex, Torrence, CA,
United States). Peptides were dissolved in 0.1% formic acid,
injected, trapped and desalted on a ReproSil-Pur C18-AQ trap
column (2 cm ×100 µm inner diameter packed in-house
with 3 µm resin; Dr. Marisch GmbH, Ammerbuch-Entringen,
Germany). The peptides were eluted from the trap column and
separated on a 15-cm analytical column (75 µm inner diameter)
packed in-house in a pulled emitter with ReproSil-Pur C18-
AQ 3 µm resin (Dr. Marisch GmbH, Ammerbuch-Entringen,
Germany). Peptides were eluted using a flow rate of 250 nl/min
and a 50 min gradient from 5 to 35% phase B (0.1% formic acid
and 90% acetonitrile or 0.1% formic acid, 90% acetonitrile and 5%
DMSO). The collected MS files were converted to Mascot generic
format (MGF) using the SCIEX MS Data Converter beta 1.1
(Sciex, Torrence, CA, United States) or RawConverter (Stafford
et al., 1991).
Computed Tomographic Scanning
All CT and µCT scanning was performed at the University of
Texas High-Resolution X-ray Computed Tomography Facility
(UTCT). Instrument specifications are detailed at: http://www.
ctlab.geo.utexas.edu. See Supplementary Information for scan
parameters for each specimen described in this report. CT
images were processed and measurements were taken using Aviso
2019.3, VG Studio Max 2.1, and NIH Image J.
Environmental Scanning Electron
Microscopy
Microparticles were mounted on SEM stubs and analyzed in
backscatter mode using a Philips XL30 ESEM TMP scanning
electron microscope, and a JEOL JSM6490 scanning electron
microscope, using EDAX Genesis energy-dispersive X-ray
analysis for elemental analysis.
SITE DESCRIPTION
Although extensively broken, the mammoth bones exhibit little
evidence of weathering or hydrologic transport abrasion,
and modest root etching. None of the bones exhibits
tooth marks or other evidence of carnivore scavenging
(Supplementary Information). The bones were buried by
low-energy slope-wash in a colluvial matrix (Supplementary
Information). CT and µCT revealed an anastomosing
postmortem burrow in one vertebra that we interpret
as termite-like insect activity. Occasional cicada burrows
penetrated sediments around and inside the broken bones.
Bone breakage from sediment loading is minor. We considered
scavenging, trampling, and other non-anthropogenic agents
in our assessment of site formation processes (Supplementary
Information), but the likelihood is exceedingly small that these
can account for the thorough, intensive, systematic and highly
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TABLE 1 | AMS 14C chronometric data for bone collagen from adult Hartley mammoth.
AMS lab
no.
Sample
no.
Taxon Skeletal
element
Sample ID Chemical
fraction
dated
Fraction
modern (Fm)
Fm ±1
SD
δ15Nh
(AIR)
δ13Ch
(VPDB)
C/N
(Atomic)
14C Age,
RC yr.
BP
±1
SD
CALBP 2 SD (95.4%)
UGAMS-
A24643
SR-9067 Mammuthus
spp.
Ulna or
Radius
diaphysis
FN B40,
N208.741
E98.122
0.45 µm-
filtered
Gelatin
from KOH-
Collagen
0.0279 0.0003 _ –19.6 _ 28,745 86133,700–32,300
OxA-
36306
SR-9067 Mammuthus
spp.
Ulna or
Radius
diaphysis
FN B40,
N208.741
E98.122
Ezee (60-
90 µm)-
Filtered
Gelatin
from
NaOH-
extracted
collagen
(Oxford P-
Code AG)
0.0303 0.0013 8.3 –19.4 3.4 28,100 350 33,250–31,300
UCIAMS-
191867
SR-9067 Mammuthus
spp.
Ulna or
Radius
diaphysis
FN B40,
N208.741
E98.122
Ultrafiltered
(>30 kDa)
Gelatin
0.0293 0.0008 8.1 -19.4 3.2 28,360 230 33,250–31,750
OxA-
36358
SR-9067 Mammuthus
spp.
Ulna or
Radius
diaphysis
FN B40,
N208.741
E98.122
Ultrafiltered
(>30 kDa)
Gelatin
(Oxford P-
Code AF)
0.0253 0.0011 8.5 –19.2 3.2 29,520 340 34,650–33,200
UCIAMS-
205817
SR-9067 Mammuthus
spp.
Ulna or
Radius
diaphysis
FN B40,
N208.741
E98.122
XAD-
Purified
Gelatin
hydrolyzate
from KOH-
extracted
collagen
0.0214 0.0006 8.1 –19.4 3.2 30,860 220 35,750–34,600
OxA-X-
2737-56
SR-9067 Mammuthus
spp.
Ulna or
Radius
diaphysis
FN B40,
N208.741
E98.122
Hydroxyproline
(Oxford
P-Code
NRC)
0.0175 0.0009 5.0 –25.5 5.8 32,750 430 38,900–36,250
Backgrounds and Known-Age Bones
UCIAMS-
205822
SR-8136 Eschrichtius
robustus
(Gray
Whale)
Rib Beaufort
Whale,
Alaska
XAD-
Gelatin
0.0016 0.0001 14.0 –14.3 3.2 51,680 260 Out of Range
UCIAMS-
205819
SR-9071 Alces
alces
Eurasian
Elk
Rib Miesenheim
IV,
Germany
XAD-
Gelatin
0.2541 0.0007 2.5 –20.2 3.2 11,005 25 13,100–12,800
See section “Methods” for explanation of P-Codes. Calibrations made using OxCal 4.4 (11/14/2021) IntCal 20. UGAMS, University of Georgia; UCIAMS, University of California-Irvine; OxA, Oxford University. Fn, Field
number (with excavation coordinates).
1We suspect this standard deviation is too narrow.
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FIGURE 4 | AMS 14C measurements on different Hartley mammoth bone collagen chemical fractions.
FIGURE 5 | Calibration of 14C measurement on hydroxyproline from Hartley mammoth bone collagen: 32,750 ±430 RC yr BP (OxA-X-2737-65) OxCal v4.4.4;
atmospheric data from Reimer (2020).
patterned bone breakage, or the stacking of the bone assemblage
described below.
Main Bone Assemblage
The main bone assemblage was derived mostly (95%) from
the adult skeleton (Figures 1–3and Supplementary Videos 1–
10). It included 44 broken cranial fragments, an intact upper
right second molar (Supplementary Video 7) and 12 isolated
tooth plates, 25 ribs broken into 52 fragments, 3 vertebrae
and 15 vertebral fragments, 32 percussion-impact bone flakes, 9
‘butterfly fragments’ (below), 20 unidentifiable bone fragments,
and 267 bags (6 ×10 cm and 10 ×17 cm) of small ‘bone scraps’
(below). Only two refits were found (Supplementary Video 1).
The adult’s face (tusks, premaxillae, and partial maxillae) is the
single largest, heaviest element present and was positioned on
top of the bone pile. It was sheared from the cranium at the
nares, and its maxillary alveoli are broken and empty. The calf
is represented by a partial left maxilla and dentary with intact
dentitions (Supplementary Figure 2), three isolated tooth plates,
left tibia diaphysis, and 10 rib fragments. This bone concentration
occupied an area ∼1.5 m × ∼1.5 m; its excavated thickness,
from the exposed tusk tip, is 1.1 m. It included numerous locally
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FIGURE 6 | Photographs of modified bones of adult Hartley mammoth. (A) Anterior rib with depressed blunt force impact wound (TMM 47004-1.1). (B) Mid-thoracic
rib showing large-gauge puncture wound to capitulum (TMM 47004-1.36). (C) Mid-thoracic rib with parallel chop marks (TMM 47004-1.37). (D,E) Rib butterfly
fragments (TMM 47004-1.152 and 47004-1.201). (F) Butterfly fragment derived from ulna or tibia (TMM 47004-1.40). (G) Rib butterfly fragment (TMM 47004-1.8).
(H) Dentary bone flake with two secondary flake scars (TMM 47004-1.215). (I) Distal rib in two views showing puncture in-filled with sediment (TMM 47004-1.163)
(Supplementary Video 9).
derived rounded sandstone cobbles weighing up to 3 kg. In
the midst of the pile, lying on top of four broken ribs, was
a 23.7 kg boulder interpreted as a hammerstone and/or anvil
(Figures 1,2). The long axes of rib fragments and elongate bone
fragments (n= 40) were mapped from photogrammetry images
(Figures 2A,B) and subjected to Rao’s spacing test (p>0.1),
Kuiper’s test (p>0.15), Rayleigh’s test (p= 0.927), and Watson’s
test (p>0). In each test, they failed to falsify the hypothesis
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TABLE 2 | Quantitative amino acid analyses on Hartley adult mammoth bone collagen.
Residues per thousand (R/1000)
Amino acid HCl-decalcified collagen EDTA-decalcified collagen Sigma bovine collagen1
Hydroxyproline 49 49 90
Aspartic acid 49 48 47
Threonine 20 20 17
Serine 37 36 31
Glutamic acid 85 83 73
Proline 136 136 114
Glycine 324 330 338
Alanine 132 130 113
Valine 26 26 26
Methionine 3 3 7
Isoleucine 10 10 12
Leucine 24 24 26
Tyrosine 0 0 4
Phenylalanine 13 12 20
Hydroxylysine 5 7 16
Lysine 34 34 21
Histidine 1 1 4
Arginine 51 51 39
Total 1000 1000 998
Specimen SR-9067 (TMM-47004-1.40, FN B40, N208.741 E98.122) compared to Sigma Modern Bovine Collagen. 1Amino acid values are, by convention, reported in
Residues per thousand (R/1000), as in this table; they are discussed in text as percentages.
of uniformity in orientation relative to the catchment channel
axis (Figure 2C).
Most (88%) of the larger (>5 cm) bones and bone fragments
in the main accumulation were buried in the lower two-
thirds of the excavation, more than 35 cm below the modern
colluvial surface. Every bone displays perimortem damage, save
the adult right stylohyoid (Supplementary Video 10), which
lay near the excavation floor. Two adult vertebral epiphyses
remained in articulation with each other but were separated
from their parent centra. Presumably, they were held together
by an intact joint capsule at time of burial, one of many
indications that burial was rapid. Buried beneath the adult face
were a partial right frontal with an attached fragment of the
nasal, the right parietal dome (Figure 7 and Supplementary
Video 6), and smaller fragments of pneumatic cranial wall
(diplöe) that collectively represent less than one-third of the
complete adult cranium.
Separation of the adult face from the cranium was caused
by the most profound skull fracture, followed by fragmentation
of the cranial vault. Dorsal surfaces of the frontal and
parietal preserve multiple depressed, blunt-force impact fractures
(Figure 7). One fragment of frontal plus nasal is depressed 5 cm
into the anterior endocranial cavity. En echelon fractures mark its
dorsal surface. Its persistent connection to the rest of the frontal
suggests that skin and/or periosteal membrane were intact at time
of burial. Other blunt impact fractures penetrated into diplöe of
the frontal and parietal; CT scans reveal corresponding breakage
of internal trabecular struts. The broken right parietal also bears
two en echelon blunt-force impact fields ∼10 cm in diameter,
implying perimortem skull breakage from repeated impacts to its
dorsal surface. Proboscidean skulls are commonly fragmented in
archeological sites to gain access to the brain and other soft tissues
(Agam and Barkai, 2016, 2018;Fisher, 2021).
The adult frontal, 23 postcranial elements, and a calf tibia
preserve circular cross-section punctures ∼2 mm to ∼1 cm in
diameter (Figures 6,8,9and Supplementary Video 9). Large-
diameter punctures occur in a cluster on the dorsal surface of
the frontal; CT scans reveal detached surficial bone fragments
displaced up to 15 mm into the diplöe. Small-diameter punctures
also perforate the dorsal surface of the frontal, and two penetrate
the orbital wall, entering obliquely from above. The locations and
3D geometries of these punctures are entirely inconsistent with
carnivore canine marks (Supplementary Information).
Eleven right and 14 left ribs were identified; proximal and
distal ends all exhibited perimortem damage. Costal epiphyses
were detached from all but one rib head; six isolated costal
epiphyses were recovered. On every proximal rib fragment
(n= 23), the capitulum is damaged, exposing medullary cavities
to sediment infilling. Six proximal rib heads (26%) display
circular punctures or gouges measuring ∼1 cm in diameter.
Ten ribs (40%) have small-diameter isolated punctures near
their heads and on their shafts. Fifteen ribs (60%) preserve
blunt-force impact fractures, and seventeen rib shafts (68%) are
spirally fractured, indicating they were fresh when fractured.
Short, broad, parallel chopmarks on one rib are consistent
with expedient tools (Blasco et al., 2013;Rosell et al., 2014;
Hannus, 2018). This damage suggests systematic rib detachment
from the vertebral column, costal cartilages, sternum, and from
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FIGURE 7 | 3D CT volumetric models and cross-sections of broken adult right frontal and right parietal. (A) Key to skull elements. (B–D) Partial right frontal in (B),
caudal (C), dorsal and lateral views (D), showing major fracture planes (red arrows F1, F2, F3, F4) where it was sheared from cranium (TMM 47004-1.31;
Supplementary Video 6). Blunt-force impact wounds (Bfw 1, Bfw 2, Bfw 3), puncture wound concentrations (P1, P2), and two small-gauge punctures into orbital
wall from obliquely above (P3, P4). (E) CT slice through frontal showing cemented grains in puncture channel P3 (red arrows), and offset bone from blunt force
impact (Bfw 1). (F) CT slice through parietal showing impact-depressed bone (white arrows) and broken underlying trabeculae. (G) Broken right parietal dome
showing right and left impact fields on dorsal surface (TMM 47004-1.213). Fr, frontal; Mx, maxilla; Oc, occipital surface of parietal; Or, orbit; Pa, parietal; Pmx,
premaxilla; Popr, postorbital process; Prpr, preorbital process; Spf, sphenorbital fissure; Tf, temporal fossa; Tu, tusks.
each other using cylindrical rods and expedient cutting and
chopping tools. Similar highly patterned carcass processing
sequences have been described that may vary among taxa, but
are largely standardized within a given taxon, and trace into the
Old World Lower Paleolithic (Blasco et al., 2013;Rosell et al.,
2014).
The three adult vertebral centra are missing their epiphyses;
five separated centrum epiphyses were recovered. CT scans
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FIGURE 8 | 3D CT volumetric models and cross-sections of cylindrical
punctures and cryptic punctures in vertebra and rib. (A) Thoracic vertebra with
deep puncture in left transverse process; CT slices (below) showing puncture
geometry in cross-section; and puncture cluster infilled by dense (bright)
sediment (TMM 47004-1.22). (B) Puncture in rib head (TMM 47004-1.36). (C)
Rib head (TMM 47004-1.2), with large-diameter (∼10 mm) cylindrical gouge,
and CT cross sections. Numbers refer to slice planes in CT datasets.
revealed ‘cryptic’ punctures in the centra not readily visible to
the naked eye owing to postmortem sediment infillings whose
color matches the bone. These include clusters of overlapping
large-diameter punctures in all three centra, and penetrations
extending up to 45 mm into the transverse processes (Figure 8
and Supplementary Video 8). Following disarticulation of the
vertebral column, the centra were separated from their epiphyses
and repeatedly punctured, probably to facilitate grease extraction
(Outram, 2001;Blasco et al., 2013;Fladerer et al., 2014;Rosell
et al., 2014;Hannus, 2018;Fisher, 2021).
The adult appendicular skeleton is represented mostly by
broken fragments, bone flakes, and ‘butterfly fragments’ (below).
The left scapula is represented by a detached metacromion
process, which is itself viewed as diagnostic of butchering
as it provides a handle to carry the large muscle (m.
infraspinatus) that attaches to it (Fisher, 2021). One complete
carpal, two associated sesamoids, and two isolated phalanges
were identified. The phalanges exhibit numerous small-diameter
perimortem punctures that subsequently filled with sediment.
Whereas carnivore canine penetrations are widest externally,
tapering internally to a point, CT scans show these punctures
are narrowest externally and broaden into wider chambers,
suggesting insertion and rotation of a pointed tool that disrupted
trabeculae, facilitating extraction of grease from cancellous bone
interiors. The punctures probably occurred in conjunction with
efforts to disarticulate grease-rich podial elements (Fisher, 1995)
and retrieve pedal fat pads.
Bone Flakes
A significant feature of the Hartley site is the high number
(n= 32) of bone flakes (Figure 9,Supplementary Figure 3,
and Supplementary Videos 1–5); we restrict the term ‘flake’
to bone fragments preserving technical attributes associated
with percussive forces (Fisher, 1995). Critics argue that, because
scavenging and weathering can also reduce bones, bone flakes
are not always culturally diagnostic (Meltzer, 2009;Waters et al.,
2018;Waters, 2019). However, those from the Hartley locality
preserve extraordinary patterning that geological processes or
scavenging cannot explain. Most of the bone flakes (n= 22;
69%) were derived from thick cortical bone of the limbs,
but cannot be assigned to a specific parent element with
certainty; seven (22%) are from ribs; and three (9%) are
from a dentary. Fifteen of the flakes (47%) have sharp edges
suitable for cutting.
A remarkable pattern in these flakes is their consistent
orientation with respect to the grain of their parent bone. The
grain is the general trend of the cortical Haversian canal system
and parent bone long-axis, and it controls fracture behavior
in percussion events (Hannus, 1989, 2018;Bradfield, 2013;
Shipman, 2018). Eleven (34%) flakes were struck parallel to the
grain, 16 (50%) were struck perpendicular to the grain, and only
five (16%) were of uncertain strike direction. In all, 78% (n = 25)
of the bone flakes were struck either perpendicular or parallel
to the grain of their parent bone. Such thorough, systematic
attention to grain during bone reduction is unknown in non-
cultural bone assemblages, including scavenged assemblages (e.g.,
Behrensmeyer et al., 1989).
Another remarkable pattern is that many bone flakes bear
one or more secondary impact scars exhibiting preferential
orientations to the grain of their parent flake. From a total of 50
secondary flake scars, 35 (70%) were struck parallel to the grain,
7 (14%) perpendicular, and eight (16%) were indeterminate as to
strike direction. In all, 84% of the secondary scars were struck
either perpendicular or parallel to the grain of their parent flakes.
A chi-square test of bone flake strike and secondary flake scar
directions returned a p-value of 0.001212 (the result is significant
at p<0.05). This indicates statistically significant patterned,
systematic bone selection and reduction organized around grain
and percussion-fracture behavior.
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FIGURE 9 | Arrested fractures in bone flake and microflake. (A) Bone flake derived from limb diaphysis in external view (top), percussion surface view (middle), and
internal view (bottom). (B) CT slices (numbered) showing arrested fractures (F1, F2, F3) propagating from percussion platform, and postmortem sediment loading
fracture (fpm) (TMM 47004-1.70; Supplementary Video 4). (C) Opposite surfaces of microflake (TMM 47004-3.5.3a: Supplementary Video 5). (D) µCT slices
showing arrested fractures (F1, F2). Pb, bulb of percussion; Pp, percussion platform; Pvc, periosteal vascular canals.
Following lithic terminology (Collins, 1999), we recognize
bone ‘microflakes’ (Figure 9) that are less than 3 cm maximum
length and exhibit some combination of the diagnostic features
of larger flakes. The Hartley site actually preserved a continuum
in flakes sizes but the term ‘microflake’ draws attention to the
potential taphonomic importance of small bone fragments that
are commonly overlooked (Outram, 2001).
Butterfly Fragments
Another important type of bone fragment, discussed in
medical literature, is the “butterfly fragment” (Winquist
and Hansen, 1980;Reber and Simmons, 2015;Cohen et al.,
2016). Like bone flakes, butterfly fragments result from blunt
force impact. Unlike bone flakes, which preserve technical
attributes associated with percussive forces and compressive
failure, butterfly fragments are products of tensile failure.
They result from medium to high velocity impact to a limb
diaphysis or rib and, if bending loads are sufficient, induce
tensile failure that can spall butterfly fragments from the
impact side of the diaphysis, the side opposite the impact,
or both (Supplementary Figure 4). We recovered five
butterfly fragments derived from limb elements and four
from ribs. Butterfly fragments are always produced by blows
perpendicular to the grain of the parent bone. Three of the
butterfly fragments derived from limb diaphyses bear secondary
flake scars struck parallel to the grain of the parent fragment.
With rare exceptions (Supplementary Information), only
humans are known to establish conditions in which limbs
and ribs can break under tensile failure, producing these
highly characteristic fragments. Sixteen of the bone flakes
plus butterfly fragments (n= 41, 39%) preserve use-wear
indicative of utilization as expedient tools (Supplementary
Information). Similar bone flakes from other proboscidean
butcher sites are described with compelling justification for
interpreting them as percussion-flaked artifacts (Hannus, 1989,
2018;Fisher et al., 1994;Johnson, 2005;Pobiner et al., 2008;
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Collins et al., 2013;Holen and Holen, 2013;Agam and Barkai,
2016;Holen et al., 2017;Fisher, 2021).
Arrested Fractures
To further enhance our understanding of bone flake formation
and diagnosis, we CT scanned a manually knapped flake and
microflake from fresh bovine bone, to document resulting
fracture patterns (Supplementary Figure 5). As background,
medical studies document healthy living bones containing
“microcracks” that span distances of a few tens of micrometers.
Typically, they are single features arising from in vivo mechanical
stress not produced by point-loading, and may be stimuli for
normal bone remodeling (Buckwalter et al., 1995;Poundarik
and Vashishth, 2015). µCT shows fresh bone placed under
static loads (trampling, scavenging, hide penetration) fails in
relatively simple fractures that can extend for millimeters
or centimeters, depending on load magnitude and bone size
(Hannus, 1989, 2018;Shipman, 2018). However, under dynamic
percussion-impact point-loading, where both force and loading
rates are sufficiently high, CT revealed that a “fracture network”
can propagate instantaneously into bone from the impact point,
but only one or a few of the fractures rupture and separate flakes
from parent bone. If all fractures in a network continued to
the point of complete separation, the result would be a larger
number of loose fragments, and CT scans of the entire assemblage
would show no evidence of the process that generated them. CT
reveals that this is not what happens. Simultaneous propagation
of fractures in a network resulting from a single impact depends
on the entire stress field generated by the impact. As soon as
one of these fractures “breaks through,” separating the original
mass into two components, the stress field dissipates, preventing
further propagation of remaining fractures in the network. Their
further development is thus arrested, and precisely for this
reason, they are retained in conjunction with the through-going
fracture separating parent bone and flake. Using CT and µCT,
we observed arrested fractures radiating from the impact point
in a manually knapped modern bovine flake and microflake
(Supplementary Figure 5), in flakes and a microflake from the
Hartley assemblage (Figure 9 and Supplementary Videos 1–
5), and in three flakes from the Riley mammoth (University
of Michigan UM 116967), which represents a very different
taphonomic setting (Fisher, 2021). We recognize that the impact
energy required to produce arrested fractures in bovine bone
would be lower than that required for large mammoth diaphyses,
but the comparability of arrested fractures in Hartley and Riley
specimens, and arrested fractures in bovine bone, suggest that
arrested fractures are indicative of much higher impact energies
than simple fractures.
“Bone Scraps”
Dry-screening (3.175 mm mesh) all excavated matrix yielded a
multitude of small (<3 cm) “bone scraps” that were derived from
cranial diplöe and postcranial medullary bone. Similar “bone
scrap” concentrations are reported in European sites (Outram,
2001;Fladerer et al., 2014). Hydrologic processes operating at
the site fail to account for this extreme degree of fragmentation.
However, crushing and boiling medullary bone is a common
practice in grease procurement that produces such “bone scraps”
as primary debris (Quigg, 1997;Costamagno et al., 2010;Fladerer
et al., 2014;Hannus, 2018). While crushing may not increase
the grease volume harvested, it increases efficiency because the
smaller fragments require less water and fire (Marquer et al., 2010;
Supplementary Material).
PYROGENIC PARTICLE ANALYSIS
Wet-screened (0.5 mm mesh) matrix samples from different
vertical levels within the main bone accumulation yielded
a diverse assemblage of micro-particles that are not eroded
constituents of the escarpment supplying clastic particles to the
colluvium. Microscopic examination of particles excavated from
Old World hearths, in concert with combustion experiments,
identified diagnostic residues of controlled fires fueled by
burning wood, plant material, and bone (Bellomo, 1993;Schiegl
et al., 1996;Alperson-Afil, 2008;Miller et al., 2010;Weiner,
2010;Aldeias et al., 2012;Aldeias, 2017). These particles
include siliceous aggregates, subspherical complex aggregates of
recrystallized ash, pulverized bone fragments, angular shattered
tooth and bone fragments, vitrified plant fragments, and charcoal
fragments.
Siliceous and Complex Aggregates
Siliceous aggregates are composites primarily of opal and quartz
grains cemented by microcrystalline calcite into water insoluble,
sub-spherical particles ranging in diameter from ∼1 to ∼8 mm
(Figure 10). Such particles are well-known from excavations
of Old World hearths, and their resistance to disaggregation
aids in diagnosing the presence of fire (Schiegl et al., 1996).
Siliceous aggregates form following the complete combustion
of wood into calcium carbonate ash and opal phytoliths, as
the pyrolyzed residue undergoes a series of diagenetic changes
(Schiegl et al., 1996). Experiments indicate that formation of
siliceous aggregates begins as wood is burned at combustion
temperatures between 500 and 600◦C, where calcium oxalate
(CaC204) present in wood and bark is reduced to calcium
carbonate (CaC03) as carbon monoxide (CO) burns off (Munro
et al., 2007;Weiner, 2010). At still higher temperatures, between
700 and 850◦C, the calcium carbonate is reduced to calcium oxide
(CaO). Calcium oxide is generally unstable, and upon cooling, it
reacts with atmospheric carbon dioxide (CO2) to form secondary
microcrystalline calcite, which becomes the primary binding
agent of the siliceous aggregates. Alternatively, if the heated
calcium oxide encounters water, it crystallizes to form calcium
hydroxide or lime mortar [Ca4(OH)2] (Weiner, 2010). Siliceous
aggregates were recovered in abundance from wet-sieved matrix
collected from within the main bone assemblage (Figures 10A–
D,I–K). ESEM analysis detected trace amounts, in descending
order of weight percentage, of aluminum, iron, magnesium,
sodium, potassium, and sulfur. Some of the siliceous aggregates
recovered from the Hartley site also contain bone fragment
inclusions (Figures 10E–K and Supplementary Videos 11, 12).
Another class of abundant water-insoluble particles recovered
from the sieved concentrate consists of “complex aggregates”
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FIGURE 10 | Photomicrographs and µCT imagery of pyrogenic microparticles. Siliceous aggregate in (A) photomicrograph, (B) volumetric 3D reconstruction from
µCT; (C,D) selected µCT slices in planes X and Y. Calcite (darker material) identified using ESEM in (B), and as lighter material in (C,D) (TMM 47004-3.11.8). (E–H)
Complex aggregate enclosing calcined bone fragment. (E) Photomicrograph, (F) µCT volumetric 3D reconstruction (G,H) µCT slices across aggregate containing
pulverized bone fragment. Note in (G,H) penetration of ash into fractured bone, indicating degassing, shrinkage, and fracture of the bone were
penecontemporaneous with calcium hydroxide crystallization (TMM 47004-3.12.1). (I–K) Ornamented fish cranial element cemented into siliceous aggregate in (I)
photomicrograph, (J) volumetric 3D reconstruction from µCT, and (K) CT slice showing cementation of fish bone into the aggregate (Supplementary Videos 9, 10).
nc, nerve canal in fish bone (TMM 47004-3.5.31). (L–N) Hollow fish radiale incorporated into complex aggregate of quartz grains cemented by microcrystalline
calcium hydroxide matrix (identified using ESEM). (L) Photomicrograph, (M) volumetric 3D reconstruction from µCT; (N) µCT slice showing laminated histology of
imbedded bone (TMM 47004-3.5.25).
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FIGURE 11 | Pyrogenic modifications to rodent incisor (volumetric reconstructions from µCT). (A) Occlusal view; (B) medial or lateral view; and (C) medial or lateral
view; (D) CT slice revealing separation fractures and infilling of pulp cavity by ash. Cp, combustion pits; M, matrix in pulp cavity; Sep, separation by differential
shrinkage at enamel-dentin junction; Sf, dentin shrinkage fractures in cross-section (TMM 47004-3.6.1b).
(Courty et al., 2012;Courty, 2017) that also range in size from
∼1 mm to ∼8 mm. ESEM analyses indicated their main
component is microcrystalline calcite and lime mortar, which,
together with iron oxide, cement together clastic grains of quartz,
calcite, muscovite, feldspar, and other minerals derived from the
Poleo Sandstone and Salitral Shale. Small fragments of bone are
incorporated into a number of these aggregates (Figures 10L–
N). ESEM analysis shows some bone inclusions to be calcined,
indicating they were burnt. µCT scans reveal ash intruded into
fractures in some of the bone fragments. ESEM and µCT analyses
of ash-encrusted bone fragments (Figures 10E–H) suggest that
degassing of water from green bone fragments induced hot
calcium oxide to crystalize into insoluble lime mortar.
Pulverized Bone Fragments
Pulverized bone fragments as small as 1 mm are also abundant
and display a spectrum of colors from tan to brown, red,
light purple, to the pure white of calcined bone. Combustion
experiments indicate that burning green bone produces the
smallest fragments in the largest quantities, as physiological
fluids contained in osteocyte lacunae, canaliculi, and vascular
canals vaporize when heated, causing fresh bone and teeth to
explode (Costamagno et al., 2010;Marquer et al., 2010). Bone is
a common fuel in archeological fires, and pulverized bone and
tooth fragments are virtually ubiquitous in pyrogenic residues
excavated from Old World archeological hearths (Costamagno
et al., 2010;Marquer et al., 2010;Fladerer et al., 2014). Calcination
commences at ∼550–600◦C (Hannus, 2018), and the presence
of lime mortar in some aggregates indicates they were heated to
temperatures exceeding 700◦C (Weiner, 2010).
Vertebrate Microfauna
Vertebrate microfauna recovered from the wet-screened
concentrate comprises an assemblage of angular fragments
of teeth, fish scales, and rare intact disarticulated bones
(Figures 11,12 and Supplementary Figure 7). The fish scales are
remarkable, considering the site is ∼70 m above the nearest river.
µCT scans show traits diagnostic of burning, including circular
combustion pitting and internal fractures from differential
shrinkage of bone, dentin, and enamel (Hanson and Cain, 2007;
Costamagno et al., 2010;Hannus, 2018). Fish scales and calcined
bone were also found cemented into both siliceous aggregates
and microaggregates (Figure 10 and Supplementary Videos
11, 12), with an adhering insoluble rind of microcrystalline
calcite and lime mortar not previously reported in association
with microvertebrate fossils. This indicates these tiny bones are
not contaminants from packrat middens or raptor roosts. Most
microfaunal specimens are unidentifiable to the level of species
or genus, but they hint at a greater cryptic biodiversity in the
economy of this locality.
Similar assemblages of pyrolyzed particles are common in
archaeological fires (Schiegl et al., 1996;Hanson and Cain, 2007;
Costamagno et al., 2010;Marquer et al., 2010;Courty et al., 2012;
Fladerer et al., 2014;Courty, 2017). They are not reported in
wildfires, where faunal concentration mechanisms are unknown,
and where wind, run-off, and pedogenic processes prevent
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FIGURE 12 | Burnt fish teeth, fish bones, and fish scales. Volumetric 3D reconstructions from µCT (above) and histological cross section (below). Red arrows
indicate internal heat fractures. (A) Gar scale and µCT cross section (TMM 47004-3.5.22c). (B) Cranial bone with enameloid covering, and µCT cross section (TMM
47004-3.5.17a). (C) Fin spine (Ictaluridae?) with enamel covering and µCT cross section (TMM 47004-3.11.10a). (D) Teleost scale and µCT cross section (TMM
47004-3.12.5a). (E) Broken tooth in two views, plus µCT slices (below) in sagittal (left) and horizontal (right) planes (TMM 47004-3.12.5e). (F) Broken teleost scale
and µCT cross section (TMM 47004-3.11.10g). (G) Teleost scale and µCT cross section (TMM 47004-3.11.10f). (H) Imbricating burnt teleost scales and µCT cross
section (TMM 47004-3.11.10c). B, bone; D, dentin; Dc, dental canal; E, enamel or enameloid; Nc, nerve canal; Sc1, scale 1; Sc2, scale 2; Sc3, scale 3.
the protected accumulation of ash and formation of siliceous
and complex aggregates. At the Hartley site, the concentrated
biodiversity and different burning states of pulverized bone,
fish scales and teeth, and mammal teeth also argue against
natural wildfire (Costamagno et al., 2010;Rosell et al., 2014;
Agam and Barkai, 2016). This assemblage is not characteristic
of flash heating by lightning, where much higher temperatures
(∼1800◦C) profoundly alter local mineralogy and offer no
mechanism for faunal concentration (Essene and Fisher, 1986;
Courty, 2017). In summary, this assemblage most likely formed
in a controlled fire associated with the mammoth butchering.
LITHIC EVIDENCE
Lithic evidence consisted of six microflakes of Pedernal chert
collected from levels between 40 and 65 cm deep into the
matrix surrounding the main bone assemblage. µCT revealed
diagnostic evidence of percussion flaking including prepared,
ground percussion surfaces, bulbs of percussion, eraillure
scarring, secondary flake scars with conchoidal separation
surfaces, and hinge fractures (Figure 13 and Supplementary
Video 13). µCT scans also revealed arrested fractures in
four of the lithic microflakes. Since surfaces can accumulate
objects over time, and burrowing can mix sediment, it is
conceivable that excavated chert microflakes are more recent
surface contaminants. However, they were not found associated
with burrows, and the fact that six were present favors their valid
association with the mammoth bones.
DATING THE HARTLEY MAMMOTH
LOCALITY
From a single adult long bone butterfly fragment (Figure 6F), six
radiocarbon measurements were made on different bone collagen
chemical fractions, including hydroxyproline (Table 1 and
Figures 4,5). Based on the hydroxyproline 14C measurement,
the adult mammoth dates to 32,750 ±430 RCYBP (OxA-X-
2737-65), with a calibrated date range of 38,900 – 36,250 cal
BP. The five other AMS dates are in close agreement with
each other and range from 30,860 +220 to 28,100 +350
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FIGURE 13 | Chert microflake in three views. (A) Internal aspect; (B) Striking
platform; (C) External aspect. Es, eraillure scar on bulb of percussion; Fs,
secondary flake scar showing ripple marks; H, secondary flake scar hinge;
Pb, bulb of percussion; Gpp, ground percussion platform. (TMM 47004-5.1;
Supplementary Video 13).
RCYBP (35,750 – 31,300 cal BP); however, the hydroxyproline
measurement is oldest. Because the hydroxyproline “non-routine
chemistry” protocol (see section “Methods”) is considered the
most efficient procedure for removing exogenous carbon from
bone collagen samples, 32,750 +430 RCYBP (38,900 – 36,250 cal
BP) is the most robust age estimate for the adult mammoth.
DISCUSSION
The methods employed here are important for archeology
generally as they can detect nuanced diagnostic evidence of
human presence in unexpected geographic and temporal settings,
and in the absence of stylized worked lithics. Pitulko et al.
(2016a,b, 2017) used CT and bone breakage patterns to diagnose
butchering of Mammuthus primigenius 45,000 years ago in
the central Siberian Arctic locality of Sopochnaya Karga (see
also Kenady et al., 2011). We augment and extend their
findings by highlighting the diagnostic importance of butterfly
fragments, and by demonstrating the application of CT and
µCT in detecting arrested fracture networks in bone flakes and
microflakes, particulate residues diagnostic of managed fires, and
in documenting microfauna.
Our interpretation of the Hartley locality as a cultural
site is consistent with other recent archeological discoveries
placing humans in the Americas during or before the Last
Glacial Maximum (LGM). These include multiple in situ
human footprints from New Mexico that date from ∼22,860
to ∼21,130 cal BP (Bennett et al., 2021), and footprints from
Argentina that date to ∼30,000 cal BP (Azcuy et al., 2021).
Simple stone tools discovered in Chiquihuite Cave, Mexico,
date from ∼26,500 to 19,000 cal BP and represent a previously
unknown tradition (Ardelean et al., 2020;Becerra-Valdivia and
Higham, 2020). At Coxcatlan Cave, Mexico, re-dating butchered
small mammals associated with minimally worked stone tools
established a 33,448 to 28,279 cal BP date for the site’s lowest
cultural level (Somerville et al., 2021). Simple flaked stone
artifacts are known from numerous ancient South American
sites. These include Toca da Tira Peia, Brazil, which dates to
∼20,000 cal BP (Lahaye et al., 2013), and Vale da Pedra Furada,
Brazil, which dates to ∼24,000 cal BP (Boëda et al., 2021); older
artifacts dating to ∼32,000 cal BP are also reported from this
site (Guidon and Delibrias, 1986;Guidon et al., 1994). At Toca
do Serrote das Moendas, Brazil, faunal remains associated with
human bones were dated to between ∼29,000 and ∼24,000 cal
BP (Kinoshita et al., 2014). And at Arroyo del Vizcaíno, Uruguay,
a fossil-rich 30,000 years old megafaunal locality with cut-marked
bones (Fariña et al., 2014) adds to a growing record of probable
human occupation sites in the Americas that predate arrival of
the Native American clade by millennia.
One method to test the hypothesis of an early human
occupation in the Americas is to compare it against a broader
context or system of independent generalizations (Ghiselin,
1969), such as genomic research on human populations using
aDNA. These studies corroborate our findings and permit
inferences regarding distinct cultural elements of the different
migrations. This resolves nagging anomalies in archeological
records, presents a far deeper time frame for attainment
by humans of a global distribution (with the exception of
Antarctica and most oceanic islands), and a longer time span
for human roles in extinctions and ecological change across the
Western Hemisphere.
Recent genomic evidence for two Old World founding
populations in the Americas (Skoglund et al., 2015) offers
independent corroboration that the sites described above may be
cultural. A unique ancestry signal discovered in Suruí, Karitiana,
and Xavante populations living today around the Amazon
basin rim was found to be shared with living populations in
Australia, New Guinea, and Andaman Islands. The ancestral
population that contributed this signal was termed a “genetic
ghost population” and given the name “Population Y” (Skoglund
et al., 2015). Population Y was estimated to have occupied eastern
Asia ∼50,000 years ago (Reich, 2018). Additional aDNA support
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Rowe et al. North American Human Occupation 37 kya
came from a ∼40,000 years old human bone from Tianyuan cave
in northern China that shares the Population Y signal (Yang
et al., 2017). Doubts raised about the Population Y hypothesis
(Posth et al., 2018) were dispelled by more recent genomic studies
that reproduced the earlier findings and documented in South
America the Australasian signal in every major linguistic group,
making it geographically widespread (Castro et al., 2021).
How and when Population Y ancestry reached South America
has been explained by alternative hypotheses (Skoglund et al.,
2015;Reich, 2018). The first is that Population Y contributed
ancestry to the Native American clade before its dispersal south
from Beringia. Some of the Native Americans then carried this
ancestry as they dispersed down the west coast and into South
America. In other words, Population Y predated dispersal but
did not live in the Americas, and its descendants now solely exist
there and in Australasia (Wohns et al., 2022). This has been the
favored hypothesis because it is consistent with the conventional
view that Native Americans were the first people to enter the
Americas ∼16,000 years ago.
However, several recent aDNA analyses that included a
∼45,000 years old human genome from Ust’-Ishim, Siberia (Fu
et al., 2014), ∼32,000 years old human genomes from the Yana
RHS site (Sikora et al., 2019), and younger ancient Asian genomes
(Poznik et al., 2016), revealed complex patterns of dispersal,
admixture, and turnover among West Eurasian and East Asian
populations in the occupation of Siberia and western Beringia.
Marine Isotope Stage 3 (MIS 3, ca. 50,000 – 30,000 cal BP) was
evidently a time of rapid expansion of modern humans across
Eurasia (Atkinson et al., 2008;Poznik et al., 2016;Pavlova and
Pitulko, 2020), but no Population Y ancestry was detected by any
of these studies (Sikora et al., 2019).
The second hypothesis is that unmixed descendants of
Population Y dispersed directly to the Americas during pre-
LGM time, predating the Native American arrival by millennia
(Skoglund et al., 2015;Reich, 2018). This early population was
later displaced by the Native Americans except in South America,
where it mixed with Native Americans and left a discernable
signal in every major living linguistic group. This now seems the
more likely, because the first hypothesis alone fails to explain
archeological sites that predate Native American arrival.
At present, the oldest human aDNA in the Americas is
from the Anzick child burial site that includes diagnostic Clovis
artifacts and dates to 12,905 – 12,695 cal BP (Becerra-Valdivia
et al., 2018). aDNA places this individual on the stem of the
Native American clade (Rasmussen et al., 2014); it preserves no
Population Y ancestry. The absence of a Population Y signal in
North America may be an artifact of the lack of human bones
and aDNA older than ∼13,000 cal BP, and under-sampling of
living populations. Until much older human aDNA is recovered,
uncertainty will attend the association of Population Y with any
of the older archeological sites. It is technically possible that the
older American sites represent entirely separate, unrecognized
pre-LGM lineage(s) that became extinct without leaving a
discernable genetic trace in younger populations (Raff, 2022).
It is not surprising that the cultural interpretation of these
“old” sites received criticism (Chatters et al., 2021;Potter
et al., 2021). Implicit in the criticisms is an expectation that
diagnostic Upper Paleolithic stone tools are the standard by
which all American cultural sites are recognized. However, if
Native Americans were the first to introduce this technology
into the Americas ∼16,000 years ago, such evidence should not
be present in older sites. Raff (2022) observed that “probably
most” American archeologists maintain a need to uniformly
apply a rigorous standard for accepting any and all sites. The
fallacy of such a view is that evidentiary standards and rationales
may have temporal and geographic boundaries. For example, the
33,000 cal BP site of Monte Verde I, located near the southern
tip of Chile, preserved simple worked-stone implements and
proboscidean bones in association with clay-lined hearth basins
(Dillehay and Collins, 1988). Few archeologists have been willing
to recognize this seemingly anomalous site as cultural. However,
viewed in light of the older Hartley site and its location on the
North American Colorado Plateau, Monte Verde I no longer
presents an anomaly in time or space. Given their ages, the
absence of elaborate stone tools is to be expected at both localities.
Another example of spatiotemporal evidentiary limits is the
meticulous re-dating of the North America Clovis interval to a
narrow window of only ∼200 calendar years, between 13,125
and 12,925 cal BP (Waters and Stafford, 2007;Becerra-Valdivia
et al., 2018;Waters et al., 2020). It follows that any cultural
sites in the Americas predating arrival by Native Americans
will necessarily be diagnosed using criteria other than Upper
Paleolithic stone tools.
In summary, taphonomic and genomic evidence accord in
detecting at least two founding populations for the Americas,
and in viewing the story of Native Americans expanding into
virgin country as “profoundly misleading” (Reich, 2018). The
position of the Hartley site deep in the North American Western
Interior suggests that the first human arrival in North America,
whether overland or via a coastal route, occurred well before
∼37,000 years ago. The Hartley site shares much in common
with Old World proboscidean butchering sites; it appears
that while hunting technologies evolved steadily, butchering
practices preserved more stable procedural efficiencies. The
Hartley locality exemplifies new methods and nuanced criteria
for diagnosing early human occupation sites in the archeological
record. It raises provocative new questions about when, where,
and how the Native American clade, with its unprecedented
technology, intersected with earlier human occupants of the
Americas. It also provides a new deep point of chronologic
reference for occupation of the Americas, for attainment by
humans of a global distribution, and a temporal recalibration of
human ecological impacts across the Western Hemisphere.
DATA AVAILABILITY STATEMENT
The datasets presented in this study can be found in online
repositories. The names of the repository/repositories and
accession number(s) can be found below: all excavated specimens
and samples are curated into the collections of the University
of Texas Vertebrate Paleontology Laboratory. All CT datasets
are archived at UTCT (http://www.ctlab.geo.utexas.edu/) and are
available on reasonable request.
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Rowe et al. North American Human Occupation 37 kya
AUTHOR CONTRIBUTIONS
TR coordinated the field work, preparation and curation of
fossils, wrote the manuscript, and processed the images. TS
conducted the 14C dating, assisted the field interpretation,
and wrote the manuscript. DF wrote the manuscript and
produced the experimental bone flakes. JE performed the
proteomic analysis and edited the manuscript. RH conducted
the ESEM analyses and edited the manuscript. JQ studied
bone modifications, microflakes and wrote the manuscript. JS
identified and curated the microfauna. RK edited the manuscript,
conducted the statistical analyses, and analyzed the CT data. MC
edited the manuscript and conducted the CT scanning and data
archiving. All authors contributed to the article and approved the
submitted version.
FUNDING
Funding was provided by the Jackson School of Geosciences,
and by National Science Foundation grants BCS 1541294, EAR
1258878, EAR-1160721, EAR 1919700, EAR 1561622, and EAR
1762458, and the W. J. J. Gordon Foundation.
ACKNOWLEDGMENTS
We thank K. Bader, J. Biorkmann, B. A. S. Bhullar, E.
Catlos, R. Cifelli, M. Collins, M. Cloos, G. N. Hartley, T.
Higham, E. Hoffman, S. Holen, B. Huckell, H. Hutchison,
C. Jass, J. Kappelman, C. Kerans, J. Koser, T. Liebert, E.
Lundelius, L. MacFadden, C. Merriam, G. Meyer, D. Mohrig,
J. Muus, R. Probiner, J. Singh, J. Stevens, J. Sturm, W. Taylor,
L. Becerra-Valdivia, and M. W. Young for contributions to
this report.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fevo.2022.
903795/full#supplementary-material
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Conflict of Interest: TR declares that he owns the land on which the site was
discovered. TS was employed by Stafford Research, LLC, Albuquerque, NM,
United States.
The remaining authors declare that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential
conflict of interest.
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