The Genomic History of the Bronze Age Southern Levant.

We report genome-wide DNA data for 73 individuals from five archaeological sites across the Bronze and Iron Ages Southern Levant. These individuals, who share the "Canaanite" material culture, can be modeled as descending from two sources: (1) earlier local Neolithic populations and (2) populations related to the Chalcolithic Zagros or the Bronze Age Caucasus. The non-local contribution increased over time, as evinced by three outliers who can be modeled as descendants of recent migrants. We show evidence that different "Canaanite" groups genetically resemble each other more than other populations. We find that Levant-related modern populations typically have substantial ancestry coming from populations related to the Chalcolithic Zagros and the Bronze Age Southern Levant. These groups also harbor ancestry from sources we cannot fully model with the available data, highlighting the critical role of post-Bronze-Age migrations into the region over the past 3,000 years.

A Paleogenomic Reconstruction of the Deep Population History of the Andes.

There are many unanswered questions about the population history of the Central and South Central Andes, particularly regarding the impact of large-scale societies, such as the Moche, Wari, Tiwanaku, and Inca. We assembled genome-wide data on 89 individuals dating from ∼9,000-500 years ago (BP), with a particular focus on the period of the rise and fall of state societies. Today's genetic structure began to develop by 5,800 BP, followed by bi-directional gene flow between the North and South Highlands, and between the Highlands and Coast. We detect minimal admixture among neighboring groups between ∼2,000-500 BP, although we do detect cosmopolitanism (people of diverse ancestries living side-by-side) in the heartlands of the Tiwanaku and Inca polities. We also highlight cases of long-range mobility connecting the Andes to Argentina and the Northwest Andes to the Amazon Basin. VIDEO ABSTRACT.

Reconstructing Prehistoric African Population Structure.

We assembled genome-wide data from 16 prehistoric Africans. We show that the anciently divergent lineage that comprises the primary ancestry of the southern African San had a wider distribution in the past, contributing approximately two-thirds of the ancestry of Malawi hunter-gatherers ∼8,100-2,500 years ago and approximately one-third of the ancestry of Tanzanian hunter-gatherers ∼1,400 years ago. We document how the spread of farmers from western Africa involved complete replacement of local hunter-gatherers in some regions, and we track the spread of herders by showing that the population of a ∼3,100-year-old pastoralist from Tanzania contributed ancestry to people from northeastern to southern Africa, including a ∼1,200-year-old southern African pastoralist. The deepest diversifications of African lineages were complex, involving either repeated gene flow among geographically disparate groups or a lineage more deeply diverging than that of the San contributing more to some western African populations than to others. We finally leverage ancient genomes to document episodes of natural selection in southern African populations. PAPERCLIP.

Ancient DNA and deep population structure in sub-Saharan African foragers.

Multiple lines of genetic and archaeological evidence suggest that there were major demographic changes in the terminal Late Pleistocene epoch and early Holocene epoch of sub-Saharan Africa1-4. Inferences about this period are challenging to make because demographic shifts in the past 5,000 years have obscured the structures of more ancient populations3,5. Here we present genome-wide ancient DNA data for six individuals from eastern and south-central Africa spanning the past approximately 18,000 years (doubling the time depth of sub-Saharan African ancient DNA), increase the data quality for 15 previously published ancient individuals and analyse these alongside data from 13 other published ancient individuals. The ancestry of the individuals in our study area can be modelled as a geographically structured mixture of three highly divergent source populations, probably reflecting Pleistocene interactions around 80-20 thousand years ago, including deeply diverged eastern and southern African lineages, plus a previously unappreciated ubiquitous distribution of ancestry that occurs in highest proportion today in central African rainforest hunter-gatherers. Once established, this structure remained highly stable, with limited long-range gene flow. These results provide a new line of genetic evidence in support of hypotheses that have emerged from archaeological analyses but remain contested, suggesting increasing regionalization at the end of the Pleistocene epoch.

Dairying enabled Early Bronze Age Yamnaya steppe expansions.

During the Early Bronze Age, populations of the western Eurasian steppe expanded across an immense area of northern Eurasia. Combined archaeological and genetic evidence supports widespread Early Bronze Age population movements out of the Pontic-Caspian steppe that resulted in gene flow across vast distances, linking populations of Yamnaya pastoralists in Scandinavia with pastoral populations (known as the Afanasievo) far to the east in the Altai Mountains1,2 and Mongolia3. Although some models hold that this expansion was the outcome of a newly mobile pastoral economy characterized by horse traction, bulk wagon transport4-6 and regular dietary dependence on meat and milk5, hard evidence for these economic features has not been found. Here we draw on proteomic analysis of dental calculus from individuals from the western Eurasian steppe to demonstrate a major transition in dairying at the start of the Bronze Age. The rapid onset of ubiquitous dairying at a point in time when steppe populations are known to have begun dispersing offers critical insight into a key catalyst of steppe mobility. The identification of horse milk proteins also indicates horse domestication by the Early Bronze Age, which provides support for its role in steppe dispersals. Our results point to a potential epicentre for horse domestication in the Pontic-Caspian steppe by the third millennium BC, and offer strong support for the notion that the novel exploitation of secondary animal products was a key driver of the expansions of Eurasian steppe pastoralists by the Early Bronze Age.

A genetic history of the pre-contact Caribbean.

Humans settled the Caribbean about 6,000 years ago, and ceramic use and intensified agriculture mark a shift from the Archaic to the Ceramic Age at around 2,500 years ago1-3. Here we report genome-wide data from 174 ancient individuals from The Bahamas, Haiti and the Dominican Republic (collectively, Hispaniola), Puerto Rico, Curaçao and Venezuela, which we co-analysed with 89 previously published ancient individuals. Stone-tool-using Caribbean people, who first entered the Caribbean during the Archaic Age, derive from a deeply divergent population that is closest to Central and northern South American individuals; contrary to previous work4, we find no support for ancestry contributed by a population related to North American individuals. Archaic-related lineages were >98% replaced by a genetically homogeneous ceramic-using population related to speakers of languages in the Arawak family from northeast South America; these people moved through the Lesser Antilles and into the Greater Antilles at least 1,700 years ago, introducing ancestry that is still present. Ancient Caribbean people avoided close kin unions despite limited mate pools that reflect small effective population sizes, which we estimate to be a minimum of 500-1,500 and a maximum of 1,530-8,150 individuals on the combined islands of Puerto Rico and Hispaniola in the dozens of generations before the individuals who we analysed lived. Census sizes are unlikely to be more than tenfold larger than effective population sizes, so previous pan-Caribbean estimates of hundreds of thousands of people are too large5,6. Confirming a small and interconnected Ceramic Age population7, we detect 19 pairs of cross-island cousins, close relatives buried around 75 km apart in Hispaniola and low genetic differentiation across islands. Genetic continuity across transitions in pottery styles reveals that cultural changes during the Ceramic Age were not driven by migration of genetically differentiated groups from the mainland, but instead reflected interactions within an interconnected Caribbean world1,8.

Ancient West African foragers in the context of African population history.

Our knowledge of ancient human population structure in sub-Saharan Africa, particularly prior to the advent of food production, remains limited. Here we report genome-wide DNA data from four children-two of whom were buried approximately 8,000 years ago and two 3,000 years ago-from Shum Laka (Cameroon), one of the earliest known archaeological sites within the probable homeland of the Bantu language group1-11. One individual carried the deeply divergent Y chromosome haplogroup A00, which today is found almost exclusively in the same region12,13. However, the genome-wide ancestry profiles of all four individuals are most similar to those of present-day hunter-gatherers from western Central Africa, which implies that populations in western Cameroon today-as well as speakers of Bantu languages from across the continent-are not descended substantially from the population represented by these four people. We infer an Africa-wide phylogeny that features widespread admixture and three prominent radiations, including one that gave rise to at least four major lineages deep in the history of modern humans.

Palaeo-Eskimo genetic ancestry and the peopling of Chukotka and North America.

Much of the American Arctic was first settled 5,000 years ago, by groups of people known as Palaeo-Eskimos. They were subsequently joined and largely displaced around 1,000 years ago by ancestors of the present-day Inuit and Yup'ik1-3. The genetic relationship between Palaeo-Eskimos and Native American, Inuit, Yup'ik and Aleut populations remains uncertain4-6. Here we present genomic data for 48 ancient individuals from Chukotka, East Siberia, the Aleutian Islands, Alaska, and the Canadian Arctic. We co-analyse these data with data from present-day Alaskan Iñupiat and West Siberian populations and published genomes. Using methods based on rare-allele and haplotype sharing, as well as established techniques4,7-9, we show that Palaeo-Eskimo-related ancestry is ubiquitous among people who speak Na-Dene and Eskimo-Aleut languages. We develop a comprehensive model for the Holocene peopling events of Chukotka and North America, and show that Na-Dene-speaking peoples, people of the Aleutian Islands, and Yup'ik and Inuit across the Arctic region all share ancestry from a single Palaeo-Eskimo-related Siberian source.

Climate change-induced population pressure drives high rates of lethal violence in the Prehispanic central Andes.

Understanding the influence of climate change and population pressure on human conflict remains a critically important topic in the social sciences. Long-term records that evaluate these dynamics across multiple centuries and outside the range of modern climatic variation are especially capable of elucidating the relative effect of­and the interaction between­climate and demography. This is crucial given that climate change may structure population growth and carrying capacity, while both climate and population influence per capita resource availability. This study couples paleoclimatic and demographic data with osteological evaluations of lethal trauma from 149 directly accelerator mass spectrometry 14C-dated individuals from the Nasca highland region of Peru. Multiple local and supraregional precipitation proxies are combined with a summed probability distribution of 149 14C dates to estimate population dynamics during a 700-y study window. Counter to previous findings, our analysis reveals a precipitous increase in violent deaths associated with a period of productive and stable climate, but volatile population dynamics. We conclude that favorable local climate conditions fostered population growth that put pressure on the marginal and highly circumscribed resource base, resulting in violent resource competition that manifested in over 450 y of internecine warfare. These findings help support a general theory of intergroup violence, indicating that relative resource scarcity­whether driven by reduced resource abundance or increased competition­can lead to violence in subsistence societies when the outcome is lower per capita resource availability.

Pre-Columbian transregional captive rearing of Amazonian parrots in the Atacama Desert.

The feathers of tropical birds were one of the most significant symbols of economic, social, and sacred status in the pre-Columbian Americas. In the Andes, finely produced clothing and textiles containing multicolored feathers of tropical parrots materialized power, prestige, and distinction and were particularly prized by political and religious elites. Here we report 27 complete or partial remains of macaws and amazon parrots from five archaeological sites in the Atacama Desert of northern Chile to improve our understanding of their taxonomic identity, chronology, cultural context, and mechanisms of acquisition. We conducted a multiproxy archaeometric study that included zooarchaeological analysis, isotopic dietary reconstruction, accelerated mass spectrometry radiocarbon dating, and paleogenomic analysis. The results reveal that during the Late Intermediate Period (1100 to 1450 CE), Atacama oasis communities acquired scarlet macaws (Ara macao) and at least five additional translocated parrot species through vast exchange networks that extended more than 500 km toward the eastern Amazonian tropics. Carbon and nitrogen stable isotopes indicate that Atacama aviculturalists sustained these birds on diets rich in marine bird guano-fertilized maize-based foods. The captive rearing of these colorful, exotic, and charismatic birds served to unambiguously signal relational wealth in a context of emergent intercommunity competition.

Genomic analyses reveal range-wide devastation of sea otter populations.

The genetic consequences of species-wide declines are rarely quantified because the timing and extent of the decline varies across the species' range. The sea otter (Enhydra lutris) is a unique model in this regard. Their dramatic decline from thousands to fewer than 100 individuals per population occurred range-wide and nearly simultaneously due to the 18th-19th century fur trade. Consequently, each sea otter population represents an independent natural experiment of recovery after extreme population decline. We designed sequence capture probes for 50 Mb of sea otter exonic and neutral genomic regions. We sequenced 107 sea otters from five populations that span the species range to high coverage (18-76×) and three historical Californian samples from ~1500 and ~200 years ago to low coverage (1.5-3.5×). We observe distinct population structure and find that sea otters in California are the last survivors of a divergent lineage isolated for thousands of years and therefore warrant special conservation concern. We detect signals of extreme population decline in every surviving sea otter population and use this demographic history to design forward-in-time simulations of coding sequence. Our simulations indicate that this decline could lower the fitness of recovering populations for generations. However, the simulations also demonstrate how historically low effective population sizes prior to the fur trade may have mitigated the effects of population decline on genetic health. Our comprehensive approach shows how demographic inference from genomic data, coupled with simulations, allows assessment of extinction risk and different models of recovery.

Archaeological Central American maize genomes suggest ancient gene flow from South America.

Maize (Zea mays ssp. mays) domestication began in southwestern Mexico ∼9,000 calendar years before present (cal. BP) and humans dispersed this important grain to South America by at least 7,000 cal. BP as a partial domesticate. South America served as a secondary improvement center where the domestication syndrome became fixed and new lineages emerged in parallel with similar processes in Mesoamerica. Later, Indigenous cultivators carried a second major wave of maize southward from Mesoamerica, but it has been unclear until now whether the deeply divergent maize lineages underwent any subsequent gene flow between these regions. Here we report ancient maize genomes (2,300-1,900 cal. BP) from El Gigante rock shelter, Honduras, that are closely related to ancient and modern maize from South America. Our findings suggest that the second wave of maize brought into South America hybridized with long-established landraces from the first wave, and that some of the resulting newly admixed lineages were then reintroduced to Central America. Direct radiocarbon dates and cob morphological data from the rock shelter suggest that more productive maize varieties developed between 4,300 and 2,500 cal. BP. We hypothesize that the influx of maize from South America into Central America may have been an important source of genetic diversity as maize was becoming a staple grain in Central and Mesoamerica.

Late Holocene spread of pastoralism coincides with endemic megafaunal extinction on Madagascar.

Recently expanded estimates for when humans arrived on Madagascar (up to approximately 10 000 years ago) highlight questions about the causes of the island's relatively late megafaunal extinctions (approximately 2000-500 years ago). Introduced domesticated animals could have contributed to extinctions, but the arrival times and past diets of exotic animals are poorly known. To conduct the first explicit test of the potential for competition between introduced livestock and extinct endemic megafauna in southern and western Madagascar, we generated new radiocarbon and stable carbon and nitrogen isotope data from the bone collagen of introduced ungulates (zebu cattle, ovicaprids and bushpigs, n = 66) and endemic megafauna (pygmy hippopotamuses, giant tortoises and elephant birds, n = 68), and combined these data with existing data from endemic megafauna (n = 282, including giant lemurs). Radiocarbon dates confirm that introduced and endemic herbivores briefly overlapped chronologically in this region between 1000 and 800 calibrated years before present (cal BP). Moreover, stable isotope data suggest that goats, tortoises and hippos had broadly similar diets or exploited similar habitats. These data support the potential for both direct and indirect forms of competition between introduced and endemic herbivores. We argue that competition with introduced herbivores, mediated by opportunistic hunting by humans and exacerbated by environmental change, contributed to the late extinction of endemic megafauna on Madagascar.

Using a multimethod life history approach to navigate the osteological paradox: A case study from Prehispanic Nasca, Peru.

OBJECTIVES: We leverage recent bioarchaeological approaches and life history theory to address the implications of the osteological paradox in a study population. The goal of this article is to evaluate morbidity and mortality patterns as well as variability in the risk of disease and death during the Late Intermediate period (LIP; 950-1450 C.E.) in the Nasca highlands of Peru. We demonstrate how the concurrent use of multiple analytical techniques and life history theory can engage the osteological paradox and provide salient insights into the study of stress, frailty, and resilience in past populations. MATERIALS AND METHODS: Crania from LIP burial contexts in the Nasca highlands were examined for cribra orbitalia (n = 325) and porotic hyperostosis (n = 270). All age groups and both sexes are represented in the sample. Survivor/nonsurvivor analysis assessed demographic differences in lesion frequency and severity. Hazard models were generated to assess differences in survivorship. The relationship between dietary diversity and heterogeneity in morbidity was assessed using stable δ15 N and δ13 C isotope values for bone collagen and carbonate. One hundred and twenty-four crania were directly AMS radiocarbon dated, allowing for a diachronic analysis of morbidity and mortality. RESULTS: The frequency and expression of both orbital and vault lesions increases significantly during the LIP. Survivor/nonsurvivor analysis indicates cranial lesions co-vary with frailty rather than robusticity or longevity. Hazard models show (1) decreasing survivorship with the transition into the LIP, (2) significantly lower adult life expectancy for females compared to males, and (3) individuals with cranial lesions have lower survivorship across the life course. Stable isotope results show very little dietary diversity. Mortality risk and frequency of pathological skeletal lesions were highest during Phase III (1300-1450 C.E.) of the LIP. CONCLUSION: Results provide compelling evidence of increasing physiological stress and mortality in the Nasca highlands during the LIP, but also reveal substantial heterogeneity in frailty and the risk of death. Certain members of society experienced a heavier disease burden and higher mortality compared to their contemporaries. Elevated levels of disease and lethal trauma among females account for some of the sex differences in survivorship but cannot explain the large degree of female-biased mortality. We hypothesize that parental investment in males or increased female fertility rates may explain these differences.

The character of conflict: A bioarchaeological study of violence in the Nasca highlands of Peru during the Late Intermediate Period (950-1450 C.E.).

OBJECTIVES: This study uses osteological and radiocarbon datasets combined with formal quantitative analyses to test hypotheses concerning the character of conflict in the Nasca highlands during the Late Intermediate Period (LIP, 950-1450 C.E.). We develop and test osteological expectations regarding what patterns should be observed if violence was characterized by intragroup violence, ritual conflict, intermittent raiding, or internecine warfare. MATERIALS AND METHODS: Crania (n = 267) were examined for antemortem and perimortem, overkill, and critical trauma. All age groups and both sexes are represented in the sample. One hundred twenty-four crania were AMS dated, allowing a detailed analysis of diachronic patterns in violence among various demographic groups. RESULTS: Thirty-eight percent (102/267) of crania exhibit some form of cranial trauma, a significant increase from the preceding Middle Horizon era. There are distinct trauma frequencies within the three subphases of the LIP, but Phase III (1300-1450 C.E.) exhibits the highest frequencies of all trauma types. Males exhibit significantly more antemortem trauma than females, but both exhibit similar perimortem trauma rates. DISCUSSION: There was chronic, internecine warfare throughout the Late Intermediate Period with important variations in violence throughout the three temporal phases. Evidence for heterogeneity in violent mortality shows a pattern consistent with social substitutability, whereby any and all members of the Nasca highland population were appropriate targets for lethal and sublethal violence. We argue that by testing hypotheses regarding the targets and types of conflict we are better able to explain the causes and consequences of human conflict.

Early metal use and crematory practices in the American Southeast.

Long-distance exchange of copper objects during the Archaic Period (ca. 8000-3000 cal B.P.) is a bellwether of emergent social complexity in the Eastern Woodlands. Originating from the Great Lakes, the Canadian Maritimes, and the Appalachian Mountains, Archaic-age copper is found in significant amounts as far south as Tennessee and in isolated pockets at major trade centers in Louisiana but is absent from most of the southeastern United States. Here we report the discovery of a copper band found with the cremated remains of at least seven individuals buried in the direct center of a Late Archaic shell ring located in coastal Georgia. Late Archaic shell rings are massive circular middens thought to be constructed, in part, during large-scale ritual gatherings and feasting events. The exotic copper and cremated remains are unique in coastal South Carolina and Georgia where Archaic-age cremations are conspicuously absent and no other Archaic copper objects have been reported. Elemental data produced through laser ablation inductively coupled plasma mass spectrometry shows the copper originated from the Great Lakes, effectively extending Archaic copper exchange almost 1,000 km beyond its traditional boundaries. Similarities in mortuary practices and the presence of copper originating from the Great Lakes reveal the presence of long-distance exchange relations spanning vast portions of the eastern United States and suggest an unexpected level of societal complexity at shell ring localities. These findings are consistent with the hypothesis that elite actors solidified their positions through ritual gatherings and the long-distance exchange of exotic objects during the Archaic.

Archaeogenomic evidence from the southwestern US points to a pre-Hispanic scarlet macaw breeding colony.

Hundreds of scarlet macaw (Ara macao cyanoptera) skeletons have been recovered from archaeological contexts in the southwestern United States and northwestern Mexico (SW/NW). The location of these skeletons, >1,000 km outside their Neotropical endemic range, has suggested a far-reaching pre-Hispanic acquisition network. Clear evidence for scarlet macaw breeding within this network is only known from the settlement of Paquimé in NW dating between 1250 and 1450 CE. Although some scholars have speculated on the probable existence of earlier breeding centers in the SW/NW region, there has been no supporting evidence. In this study, we performed an ancient DNA analysis of scarlet macaws recovered from archaeological sites in Chaco Canyon and the contemporaneous Mimbres area of New Mexico. All samples were directly radiocarbon dated between 900 and 1200 CE. We reconstructed complete or near-complete mitochondrial genome sequences of 14 scarlet macaws from five different sites. We observed remarkably low genetic diversity in this sample, consistent with breeding of a small founder population translocated outside their natural range. Phylogeographic comparisons of our ancient DNA mitogenomes with mitochondrial sequences from macaws collected during the last 200 years from their endemic Neotropical range identified genetic affinity between the ancient macaws and a single rare haplogroup (Haplo6) observed only among wild macaws in Mexico and northern Guatemala. Our results suggest that people at an undiscovered pre-Hispanic settlement dating between 900 and 1200 CE managed a macaw breeding colony outside their endemic range and distributed these symbolically important birds through the SW.

High-precision chronology for Central American maize diversification from El Gigante rockshelter, Honduras.

The first steps toward maize (Zea mays subspecies mays) domestication occurred in the Balsas region of Mexico by ∼9,000 calendar years B.P. (cal B.P.), but it remains unclear when maize was productive enough to be a staple grain in the Americas. Molecular and microbotanical data provide a partial picture of the timing and nature of morphological change, with genetic data indicating that alleles for some domestication traits were not yet fixed by 5,300 cal B.P. in the highlands of Mexico. Here, we report 88 radiocarbon dates on the botanical remains from El Gigante rockshelter (Honduras) to establish a Bayesian chronology over the past ∼11,000 y spanning the transition to maize-based food production. Botanical remains are remarkably well preserved and include over 10,000 maize macrofossils. We directly dated 37 maize cobs to establish the appearance and local change of maize at the site. Cobs are common in deposits dating between 4,340 and 4,020 cal B.P., and again between 2,350 and 980 cal B.P. The earliest cobs appear robustly domesticated, having 10-14 rows, suggesting strong selection for increased yield. The later cobs are comparable to these earliest ones, but show clear emergence of diverse traits, including increased cob width, rachis segment length, and cupule width. Our results indicate that domesticated landraces of maize productive enough to be a staple grain existed in Central America by 4,300 cal B.P.

Early procurement of scarlet macaws and the emergence of social complexity in Chaco Canyon, NM.

High-precision accelerator mass spectrometer (AMS) (14)C dates of scarlet macaw (Ara macao) skeletal remains provide the first direct evidence from Chaco Canyon in northwestern New Mexico that these Neotropical birds were procured from Mesoamerica by Pueblo people as early as ∼ A.D. 900-975. Chaco was a prominent prehistoric Pueblo center with a dense concentration of multistoried great houses constructed from the 9th through early 12th centuries. At the best known great house of Pueblo Bonito, unusual burial crypts and significant quantities of exotic and symbolically important materials, including scarlet macaws, turquoise, marine shell, and cacao, suggest societal complexity unprecedented elsewhere in the Puebloan world. Scarlet macaws are known markers of social and political status among the Pueblos. New AMS (14)C-dated scarlet macaw remains from Pueblo Bonito demonstrate that these birds were acquired persistently from Mesoamerica between A.D. 900 and 1150. Most of the macaws date before the hypothesized apogeal Chacoan period (A.D. 1040-1110) to which they are commonly attributed. The 10th century acquisition of these birds is consistent with the hypothesis that more formalized status hierarchies developed with significant connections to Mesoamerica before the post-A.D. 1040 architectural florescence in Chaco Canyon.

Bayesian chronological analyses consistent with synchronous age of 12,835-12,735 Cal B.P. for Younger Dryas boundary on four continents.

The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemisphere deposited the Younger Dryas boundary (YDB) layer, containing peak abundances in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, platinum, and osmium. Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835-12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas. Two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of the YDB layer within the limits of dating uncertainty (∼ 100 y). The widespread distribution of the YDB layer suggests that it may serve as a datum layer.