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Quaternary

The Quaternary Period is the most recent geological period of the Era, spanning from approximately 2.58 million years ago to the present day. Defined by its at the Stratotype Section and Point (GSSP) in Monte San Nicola, , it encompasses profound climatic oscillations, including over 50 cycles of glaciation and interglacials driven by —variations in Earth's , , and . These cycles resulted in the repeated advance and retreat of massive continental ice sheets, particularly in the , causing global sea-level fluctuations of up to 130 meters and reshaping landscapes through erosion, deposition, and tectonic influences like the formation of the around 2.7 million years ago. The Quaternary is formally subdivided into two epochs: the Pleistocene Epoch (2.58 million years ago to 11,700 years before A.D. 2000), marked by intensified cooling trends that began around 3–2.5 million years ago with the onset of glaciation, and the Holocene Epoch (11,700 years ago to present), the current interglacial period characterized by relative stability until recent anthropogenic warming. Key climatic events include the around 26,500–19,000 years ago, when ice sheets covered much of and , and abrupt transitions like Heinrich events (cold stadials) and Dansgaard-Oeschger interstadials, which highlight the period's high-amplitude variability. This dynamic environment profoundly influenced biota, with the Pleistocene hosting diverse such as woolly mammoths, saber-toothed cats, and giant ground sloths, many of which faced toward the epoch's end—up to 73% of large genera in alone—due to a combination of climate shifts, habitat loss, and human hunting pressures. A defining hallmark of the Quaternary is the evolution and global dispersal of humans, with early hominins like appearing near its onset and anatomically modern Homo sapiens emerging in around 300,000 years ago, adapting to fluctuating climates through innovations in tool use, fire control, and migration. These adaptations enabled H. sapiens to cross land bridges like during glacial lows, reaching the by at least 21,000 years ago, and to develop complex cultures, including the advancements around 40,000 years ago. The period's study, known as Quaternary science, integrates disciplines like , , and to reconstruct past environments via proxies such as ice cores, sediments, and fossils, providing critical insights into ongoing and human impacts, including the proposed as a new geological age beginning with the .

Definition and Extent

Geological Boundaries

The Quaternary Period represents the current geological period, extending from 2.58 million years ago () to the present and forming the uppermost division of the Cenozoic Era. Its lower boundary is formally defined at the base of the , corresponding to the Global Stratotype Section and Point (GSSP) located in the Monte San Nicola section near , , . This boundary, ratified by the (ICS) and the (IUGS) in June 2009, is assigned an astronomically tuned age of 2.58 . The GSSP is positioned just above the Gauss-Matuyama magnetic polarity reversal, a key paleomagnetic marker dated to approximately 2.581 Ma, which provides a correlation point for the base of the Quaternary. This reversal, transitioning from the reversed Matuyama Chron to the normal Gauss Chron, aligns with the onset of intensified glaciation and broader trends. Supporting evidence derives from marine isotope stratigraphy, where oxygen isotope ratios (δ¹⁸O) in benthic from deep-sea sediments indicate a shift toward cooler conditions, particularly within Marine Isotope Stage (MIS) 103—an stage marking the boundary's position. These isotopic records, reflecting increased volume and declines, underscore the Quaternary's initiation as a period of pronounced climatic variability. The upper boundary of the Quaternary remains undefined, as it encompasses the present day and continues as the ongoing geological period. Discussions persist on its potential limits, including whether it might conclude at the top of the Epoch or extend indefinitely pending future stratigraphic redefinitions based on emerging global markers.

Subdivisions

The Quaternary Period is primarily divided into two epochs: the Pleistocene Epoch, spanning from 2.58 million years ago (Ma) to 11.7 thousand years ago (ka), and the Holocene Epoch, from 11.7 ka to the present. This division was formalized by the (ICS) in 2009, with the base of the Quaternary defined at the Global Boundary Stratotype Section and Point (GSSP) in , marking the start of the Stage. The Pleistocene Epoch is further subdivided into four stages based on the 2009 ICS updates, which extended the epoch's base to include the Gelasian. The Early or Lower Pleistocene consists of the Gelasian Stage (2.58–1.80 Ma) and the Calabrian Stage (1.80–0.774 Ma). The Middle Pleistocene, officially named the Chibanian Stage in 2020 (previously the Middle Pleistocene Subseries), spans 0.774–0.129 Ma. The Late or Upper Pleistocene, known as the Tarantian Stage (though its GSSP remains pending ratification), covers 0.129 Ma to 11.7 ka. These subdivisions reflect global stratigraphic standards ratified by the ICS Subcommission on Quaternary Stratigraphy. The Epoch is informally divided into three stages, ratified by the in 2018, each defined by significant climatic events recorded in proxy data. The Greenlandian Stage (11.7–8.2 ka) marks the initial post-glacial warming following the Pleistocene-Holocene boundary. The Stage (8.2–4.2 ka) begins at the 8.2 ka cooling event, a abrupt temperature drop evident in ice cores and linked to freshwater influx from melting ice sheets. The Stage (4.2 ka to present) starts with the 4.2 ka event, a global episode documented in speleothems, lake sediments, and cultural records, such as the decline of Mesopotamian civilizations. Boundaries within the Quaternary are established using multiple stratigraphic criteria to ensure global correlatability. plays a key role, such as the first evolutionary appearance of the foraminifer Neogloboquadrina acostaensis and an influx of cool-water mollusks (e.g., ) at the base of the , indicating cooling trends. defines others, including the top of the Olduvai normal polarity subchron at the Calabrian base (Vrica GSSP, ) and the Matuyama-Brunhes at the base (Chiba GSSP, ). Chemostratigraphy, particularly oxygen isotope ratios in marine sediments, supports correlations, such as the shift at the 4.2 ka boundary reflecting widespread aridification. Regional variations exist in stratigraphic nomenclature, particularly in , where glacial stages are used alongside global divisions. The Wisconsinan Stage, for example, represents the last major glaciation (approximately 110–11 ka), corresponding to the Tarantian and encompassing multiple ice advances across the , as defined in type sections in and correlated via and till . These local schemes aid in mapping ice-margin fluctuations but are tied to epochs for international consistency.

Historical Development

Early Observations

In the late 17th century, Danish anatomist and Nicolaus Steno laid foundational observations for understanding sedimentary layers, proposing in his 1669 work De solido intra solidum naturaliter contento dissertationis prodromus that rock strata form sequentially through deposition in horizontal layers, with older layers underlying younger ones, though he did not yet conceptualize a distinct Quaternary period. These principles of superposition and original horizontality provided the groundwork for later stratigraphic interpretations of superficial deposits, but Steno attributed such formations to divine processes rather than a specific recent geological . By the mid-18th century, Italian geologist Giovanni Arduino advanced the classification of superficial strata in the Italian Alps, distinguishing in his 1760 letters a fourfold division of rock formations: Primary (crystalline rocks), Secondary (fossiliferous limestones), Tertiary (softer, fossil-rich sands and clays), and Quaternary (recent alluvial and volcanic deposits near the surface), though he did not formally name the uppermost division as such. Arduino's observations in the Veneto region highlighted the Quaternary-like uppermost layers as the most recent and least consolidated, containing modern-like fossils, marking an early recognition of post-Tertiary materials without linking them to a unified period. Entering the early 19th century, French naturalist emphasized extinctions and catastrophic events in his studies of Parisian basin fossils, interpreting unconsolidated "diluvial" deposits—boulder-strewn gravels and silts—as evidence of sudden floods that wiped out , aligning with a catastrophist view that multiple revolutions punctuated Earth's history. Cuvier's 1812 Recherches sur les ossemens fossiles linked these deposits to episodic disasters, including possible ties to biblical narratives, fostering initial misconceptions that such layers resulted from a single global rather than gradual or glacial processes. Swiss naturalist Louis Agassiz revolutionized interpretations of these deposits in 1837, presenting at the Swiss Natural History Society meeting evidence from Alpine erratics—massive boulders transported far from their origins—as indicators of vast ice sheets that once covered Europe, proposing the first comprehensive glacial theory and an "ice age" to explain erratic distributions and striated bedrock. Agassiz's Discours de Neuchâtel argued that these phenomena were not diluvial but products of cold climate episodes, challenging flood-based explanations and introducing the idea of cyclical refrigeration in recent geological time. This period also saw tensions between catastrophist views and emerging uniformitarian principles, as articulated by British geologist in his 1830–1833 , which rejected biblical flood interpretations of superficial deposits in favor of slow, ongoing processes like and shaping the over vast time, though Lyell initially downplayed glacial influences on Quaternary-like features. The debate highlighted misconceptions, with diluvialists invoking Noah's Flood for erratic and alluvial evidence, while uniformitarians like Lyell advocated present-day analogies, setting the stage for later 19th-century formalizations of the Quaternary.

Formal Establishment

The term "Quaternary" was first proposed in 1829 by French geologist Jules Desnoyers to describe post- marine and continental deposits in the Seine Basin of , distinguishing them as the most recent layer in the stratigraphic sequence following the Primary, Secondary, and divisions established earlier. Desnoyers' emphasized the youth of these sediments, which included alluvial and coastal formations lacking the assemblages typical of older rocks, marking an initial step toward formalizing the uppermost interval. In the mid-19th century, debates intensified over subdividing this Quaternary interval, particularly with British geologist Charles Lyell's introduction of the term "Pleistocene" in 1839 to denote an epoch characterized by significant faunal turnover, where approximately 70% of molluscan species resembled modern forms, based on Sicilian strata. Lyell contrasted the Pleistocene with his earlier "Newer " concept and reserved the "Recent" for the immediate post-glacial modern period, sparking discussions on the timing and extent of ice ages and the placement of the -Pleistocene boundary amid varying interpretations of glacial evidence across . These debates highlighted tensions between uniformitarian views and emerging evidence of climatic oscillations, ultimately solidifying the Quaternary as encompassing both glacial and phases. By the early , the International Geological Congress (IGC) facilitated the integration of the Quaternary into the broader timescale, with key advancements at the 13th IGC in in , where the International Union for Quaternary Research (INQUA) was founded to standardize interdisciplinary studies of this period. INQUA's establishment promoted coordinated research on Quaternary , , and , influencing subsequent IGC resolutions that affirmed the Quaternary's status as the youngest system within the Era. Post-World War II developments revolutionized Quaternary dating, notably with Willard Libby's radiocarbon method in the late 1940s, which enabled precise calibration of sediments up to about 50,000 years ago by measuring the decay of in organic materials. Complementing this, uranium-series dating emerged in the , applying alpha-counting techniques to deposits like corals and speleothems to date older Quaternary intervals up to 500,000 years, providing critical timelines for Pleistocene climate cycles. These methods resolved longstanding chronological uncertainties and supported INQUA-led efforts in global correlation. Boundary definitions evolved further from the through the , shifting the Quaternary base from 1.8 million years ago (Ma) at the Vrica section in to 2.58 Ma at the Monte San Nicola Global Stratotype Section and Point (GSSP) in , ratified in 2009 by the . This adjustment incorporated the Stage—previously the uppermost —into the Quaternary, aligning it with the onset of significant glaciation and maintaining hierarchical consistency in the timescale. The change, endorsed through INQUA and IGC collaborations, enhanced the period's scope to better reflect paleoenvironmental transitions.

Stratigraphic Framework

Type Localities

The Global Stratotype Section and Point (GSSP) for the base of the Quaternary System, which coincides with the base of the Stage and Pleistocene Series, is located at Monte San Nicola in , . This site was ratified by the in 2009, with the boundary defined at the base of the marly layer overlying the MPRS 250 in marine sediments. The Matuyama/Gauss magnetic polarity chronozone boundary (C2r/C2An) lies between 61.2 m and 64 m below the GSSP, providing a key marker horizon at approximately 61.2 m depth, tied to Marine Isotope Stage 103 and an age of 2.58 Ma. The GSSP for the Pleistocene-Holocene boundary, defining the base of the Series/, is in the North Ice Core Project (NGRIP) from central . Ratified in , it is placed at a depth of 1492.45 m, corresponding to an age of 11,700 years before 2000 CE (b2k), marked by an abrupt shift in excess values signaling the end of the stadial and the onset of warming. This offers a high-resolution record of climatic transition, with the boundary uncertainty estimated at ±99 years (2σ). Key regional stratotypes further delineate Quaternary subdivisions. The Calabrian Stage, the earliest stage of the Pleistocene, has its GSSP at the Vrica section near Crotone in Calabria, Italy, ratified in 2011. Defined at the base of the marine claystone conformably overlying sapropel layer 'e' (Mediterranean Precession-Related Sapropel 176), this boundary occurs approximately 8 m below the top of the Olduvai magnetochron subzone and aligns with the Marine Isotope Stage 65/64 transition at 1.80 Ma. Biostratigraphic markers include the highest occurrence of Discoaster brouweri below the boundary and the lowest common occurrence of left-coiling Neogloboquadrina pachyderma above it. The Stage, encompassing the Middle Pleistocene Subseries, is defined by its GSSP at the Chiba composite section in , ratified in 2020. The boundary is at the base of the Ontake-Byakubi-E tephra bed, 1.1 m below the Matuyama-Brunhes midpoint, corresponding to Marine Isotope Stage 19c at 774 ka. In , regional correlation often relies on sites like Bignell Hill in , where in sequences aids in linking terrestrial records to global , though it serves more as a reference for late Quaternary eolian deposits rather than a formal stage type. Selection of these type localities follows strict criteria established by the , emphasizing stratigraphic completeness with continuous sedimentation across the boundary and sufficient thickness of strata above and below (typically tens of meters). Sites must feature primary markers for global correlation, such as biostratigraphic datums (e.g., species turnovers), magnetostratigraphic reversals, or chemostratigraphic signals, supplemented by multiple records like assemblages or isotopic profiles to ensure robust identification. Accessibility, , and potential for numerical dating (e.g., via layers or orbital tuning) are also evaluated, with proposals requiring voting and in peer-reviewed journals. Challenges in establishing Quaternary type localities arise from depositional discontinuities, particularly in terrestrial sections where erosional gaps and hiatuses disrupt continuity, complicating boundary placement compared to the more complete records in marine sediments or ice cores. For instance, continental archives often exhibit fragmented successions due to glacial erosion or fluvial incision, necessitating reliance on offshore or polar sites for global standards, while regional correlations must bridge these gaps using auxiliary proxies like magnetostratigraphy. This contrast highlights the preference for sites with uninterrupted proxy sequences, such as the foraminifera-rich marine clays at Vrica or the annually resolved layers in NGRIP, to minimize interpretive ambiguity.

Correlation Methods

Correlation methods in Quaternary stratigraphy enable the synchronization of sedimentary records across diverse geographic regions and timescales, integrating relative and absolute dating techniques to establish a coherent global framework. These approaches are essential for aligning terrestrial, marine, and sequences, accounting for the period's dynamic depositional environments influenced by glacial-interglacial cycles. By combining biostratigraphic, magnetostratigraphic, chemostratigraphic, and numerical methods with orbital tuning and statistical modeling, researchers achieve chronologies with resolutions ranging from millennia to centuries, facilitating paleoclimatic reconstructions and evolutionary studies. Biostratigraphy utilizes fossil assemblages, particularly index fossils with narrow temporal ranges, to correlate Quaternary strata. In marine sediments, the planktonic foraminifer Globorotalia truncatulinoides, which first appeared in the around 2 Ma, serves as a key biostratigraphic proxy for correlations throughout the Quaternary, with its coiling direction and abundance variations reflecting oceanographic shifts during glacial-interglacial transitions. On land, and mammalian fossils provide regional biostratigraphic markers, though marine microfossils like offer broader applicability due to their global distribution in ocean sediments. Magnetostratigraphy employs reversals in Earth's geomagnetic to anchor long-scale correlations, providing a global reference independent of . The Brunhes-Matuyama reversal, dated to 0.78 million years ago (Ma), serves as a critical separating the normal Brunhes Chron from the reversed Matuyama Chron, identifiable in both and deposits. This event, recorded in deep-sea cores and sequences, enables precise alignment of Quaternary records spanning the mid-Pleistocene, with subchrons like the Jaramillo (0.99–1.07 Ma) offering additional tie points for earlier subdivisions. Chemostratigraphy leverages stable isotope variations in sedimentary components to identify synchronous events, particularly useful for high-resolution marine correlations. Oxygen isotope ratios (δ¹⁸O) from benthic foraminifera shells track global ice volume changes, defining marine isotope stages (MIS) such as MIS 1 () to MIS 22 (). Carbon isotopes (δ¹³C) complement this by revealing ocean circulation and productivity shifts, with patterns in deep-sea records allowing inter-basin synchronization. These isotopic signals, preserved in carbonate tests, provide a continuous chemostratigraphic framework for the past 2.58 Ma, often integrated with other methods for enhanced precision. Numerical dating provides absolute ages through radiometric techniques, calibrated to international standards for Quaternary timescales. Radiocarbon (¹⁴C) dating is effective for organic materials younger than 50 ka, commonly applied to late Pleistocene and Holocene sediments like peat and shells. For older volcanics exceeding 100 ka, potassium-argon (K-Ar) and argon-argon (⁴⁰Ar/³⁹Ar) methods date tephra layers interbedded with sediments, yielding ages for early Quaternary events. Optically stimulated luminescence (OSL) dates sediment burial by measuring trapped electrons in quartz grains, ideal for aeolian and fluvial deposits up to 200 ka, bridging gaps where organic material is scarce. Orbital tuning aligns sedimentary cycles to Milankovitch forcing parameters— (~100 and 400 cycles), (~41 ), and (~19–23 )—to refine chronologies beyond radiometric limits. In deep-sea cores, δ¹⁸O variations are tuned to insolation maxima at 65°N, as in the LR04 benthic stack, achieving resolutions of 1–2 for the past 5 . This method assumes quasi-periodic climate responses to orbital changes, validated by of proxy records, and extends correlations to non-marine sequences via ash or matching. Integration of these methods occurs through Bayesian age-depth modeling, which combines stratigraphic constraints, radiometric dates, and orbital targets to generate probabilistic chronologies for composite records. Tools like or OxCal incorporate prior information on rates and hiatuses, producing median ages with uncertainty envelopes for multi-proxy sequences. For instance, in lake or cores spanning the Quaternary, this modeling harmonizes ¹⁴C, OSL, and tuned MIS data, minimizing errors in age estimates and enabling robust global correlations.

Paleoenvironmental Conditions

Climate Variations

The Quaternary Period is characterized by pronounced climatic oscillations, primarily manifesting as alternating glacial and stages. Since the Mid-Pleistocene Transition (MPT) around 1.2 to 0.8 million years ago, these cycles have followed a dominant ~100,000-year () periodicity, with larger ice volume amplitudes compared to earlier phases. Prior to the MPT, during the , glacial-interglacial cycles were paced by a ~41 obliquity-dominated , exhibiting lower amplitudes. These long-term patterns are superimposed by shorter-term variations, including millennial-scale events, shaping global temperature, precipitation, and dynamics throughout the period. The primary driver of these fluctuations is Milankovitch forcing, involving variations in Earth's orbital parameters—, obliquity, and —that modulate seasonal insolation, particularly at high northern latitudes. These insolation changes initiate growth or decay, amplified by feedbacks such as ice-albedo effects, where expanding increases surface reflectivity and cools the planet further, and variations in atmospheric greenhouse gases like CO₂ and CH₄, which rise during deglaciations to enhance warming. Disruptions to ocean circulation, notably slowdowns of the Atlantic Meridional Overturning Circulation (AMOC), contribute to abrupt shifts by redistributing heat and altering patterns. Paleoclimate proxies provide robust evidence for these variations. Ice cores from , such as and EPICA Dome C, reveal δ¹⁸O records that track and changes over the past 800,000 years, showing systematic depletions during glacials and enrichments in interglacials. Speleothems and lake levels further document regional shifts, with expanded lake systems and growth indicating wetter conditions during interglacials. Notable events punctuate this record. The (), spanning approximately 26.5 to 19 ka, marked peak ice volume and global cooling of about 4–7°C relative to today, driven by low summer insolation. The stadial (12.9–11.7 ka) represented a rapid return to near-glacial conditions in the , interrupting with temperature drops up to 10°C in , likely triggered by AMOC weakening from meltwater influx. In the , the (roughly 900–1300 CE) featured regionally elevated temperatures in the North Atlantic, though not globally synchronous, amid overall warmth. Regionally, Quaternary climate exhibits a bipolar seesaw pattern, where Northern Hemisphere coolings coincide with Antarctic warmings due to heat redistribution via ocean currents like the AMOC. Interglacials often saw intensified in low latitudes, with enhanced East Asian summer precipitation linked to stronger summer insolation and reduced Northern Hemisphere ice cover. The Holocene began with the Climatic Optimum (approximately 9–5 ka), a phase of peak warmth exceeding modern levels in many mid-to-high latitude regions, driven by high orbital insolation. This was followed by Neoglacial cooling starting around 5 ka, with gradual temperature declines culminating in the (late Holocene), reflecting decreasing insolation and feedback amplification. These shifts influenced broader environmental responses, such as ice sheet retreat.

Landscape Evolution

During the Quaternary Period, extensive ice sheets profoundly shaped continental landscapes through glacial erosion and deposition. The Laurentide Ice Sheet in North America and the Fennoscandian Ice Sheet in Europe carved characteristic U-shaped valleys by abrading pre-existing V-shaped fluvial valleys, while depositing terminal and recessional moraines that mark former ice margins. Fjords, such as those in Norway and along the Canadian coast, formed where glacial erosion deepened and widened coastal valleys below sea level, later flooded upon deglaciation. These landforms, prominent from the Pleistocene glaciations, illustrate the scale of ice sheet dynamics, with the Laurentide covering up to 13 million square kilometers at its maximum extent. In periglacial environments of high latitudes, freeze-thaw cycles produced distinctive features beyond direct ice cover. polygons, formed by thermal contraction cracking in fine-grained , created polygonal networks up to several meters across in and sub-Arctic regions like and . Solifluction lobes, resulting from saturated flow over , accumulated on slopes in areas such as the Canadian Shield and uplands, contributing to hillslope instability and redistribution. These processes dominated during cold phases, altering surface and across vast unglaciated terrains. Eolian processes during glacial periods led to widespread loess deposition, forming fertile plains in regions like the Chinese Loess Plateau and North American Midwest, which preserve paleoclimate signals through and proxies. Fluvial systems responded to Quaternary climate oscillations by incising river terraces during lowstands and aggrading valleys during highstands, as seen in the and river systems where multiple terrace flights record glacial-interglacial cycles. Coastal landscapes experienced eustatic sea-level fluctuations, with a drop of approximately 120 meters at the exposing continental shelves and forming raised beaches upon post-glacial transgression; examples include elevated shorelines in and along the U.S. Atlantic coast, now 10-20 meters above present levels due to relative sea-level changes. These shifts drove delta progradation and , reshaping margins worldwide. Tectonic activity interacted with Quaternary climates to modify landscapes in active zones. In the , faulting and produced rift valleys and escarpments, with explosive eruptions around 320-280 thousand years ago burying and preserving hominin sites in the central rift, such as those at Gademotta, while uplifting plateaus influenced drainage patterns. Volcanic fields in the Main Ethiopian Rift, such as the Silti-Debre Zeyit region, added basaltic flows and calderas, altering local topography and sediment supply during the Pleistocene. These processes amplified geomorphic responses to climatic variations in rift settings. Post-glacial isostatic rebound continues to reshape formerly glaciated regions. In , uplift rates reach up to 1 centimeter per year near the , elevating coastlines and reversing drainage directions as the crust relaxes from Pleistocene ice loading. Conversely, peripheral areas like in experience subsidence at rates of several millimeters per year, leading to relative sea-level rise and inundation of lowlands. This ongoing adjustment, modeled through glacial isostatic adjustment (), affects over 20% of the Northern Hemisphere's land surface. While natural processes dominated Quaternary landscape evolution, human activities since the introduced modifications like accelerated from early in the Mediterranean and , though these pale in comparison to glacial and periglacial legacies.

Biological Record

Plant Life

During the Quaternary Period, angiosperms continued to dominate terrestrial ecosystems, having established their prevalence since the through adaptive advantages in growth rates and nutrient utilization. This dominance persisted amid repeated glacial-interglacial cycles, shaping global vegetation patterns. Flowering plants, particularly grasses, adapted to varying climates, with C4 photosynthetic pathway grasses—originating between 7 and 5 million years ago in the —expanding significantly during glacial cooling phases due to increased aridity and cooler temperatures that favored their water-efficient metabolism. These expansions transformed landscapes, particularly in mid- to low-latitude regions, where C4-dominated grasslands replaced or interspersed with C3 vegetation, as evidenced by analyses from paleosols and remains. In periods, warmer and moister conditions facilitated the northward expansion of forests into mid-latitudes, where coniferous and trees supplanted and vegetation, as reconstructed from profiles across northern and . Similarly, tropical rainforests reached their peak extent during the , benefiting from elevated precipitation and temperatures that supported dense, multilayered canopies in equatorial regions like Amazonia. These shifts highlight the sensitivity of to orbital-driven variability, with interglacials promoting woody accumulation and in forest biomes. Key events marked profound changes in Quaternary flora. The Pleistocene megafaunal extinctions around 11,000 to 10,000 years ago, coinciding with the end of the , reduced grazing pressure on herbaceous plants, enabling rapid encroachment in former open habitats like the , as indicated by increased pollen percentages in cores. Additionally, the 4.2 ka aridification event triggered widespread , diminishing mesic such as forests and wetlands in mid-latitude and tropical zones, leading to contractions in woodland cover and expansions of xerophytic communities. These disruptions underscore how abrupt shifts and interactions drove turnover. Pollen records from lake sediments provide critical evidence of these floral dynamics, revealing transitions from graminoid-dominated to more heterogeneous tundra-steppe mosaics during deglaciations, with forbs and shrubs increasing as temperatures rose. Such archives, spanning the Pleistocene-Holocene boundary, document cyclic expansions and contractions of plant communities in response to insolation changes and retreat. Modern distributions of Quaternary reflect legacies of these cycles, with approximately 375,000 accepted of vascular worldwide (as of 2025), many tracing origins to glacial refugia that preserved . hotspots like the and served as Quaternary refugia, where stable microclimates allowed relictual to survive glacial maxima, fostering high and current . These areas highlight how periodic isolation and reconnection influenced and range limits. Human impacts emerged post-8 ka with the onset of , including in the and , which selectively promoted grasses like and millet while altering wild flora through habitat clearance and . This transition, tied to warming, began integrating human selection into the natural Quaternary record of plant evolution. Associated faunal changes, such as reduced herbivory, further influenced these vegetative shifts.

Animal Life

The Quaternary period is marked by significant faunal diversity, particularly during the Pleistocene epoch, when dominated many ecosystems. Iconic species such as woolly mammoths (Mammuthus primigenius), saber-toothed cats ( fatalis), and giant ( americanum) exemplified adaptations to cold glacial environments. Woolly mammoths, for instance, possessed thick fur coats, subcutaneous fat layers, and small ears to minimize heat loss, enabling survival in tundra-steppe habitats across and . Saber-toothed cats featured elongated canines for subduing large prey like and horses, while giant sloths had robust limbs and claws suited for foraging on tough vegetation in forested regions of . These adaptations highlight how evolved in response to fluctuating climates and landscapes throughout the period. A pivotal event in Quaternary animal history was the extinction, part of the broader Quaternary occurring roughly 50,000 to 10,000 years ago. This episode led to the disappearance of approximately 72% of large genera (>44 kg) in , 83% in , and up to 88% in , including mammoths, mastodons, and giant kangaroos. The causes remain debated but are widely attributed to a combination of rapid climate warming at the end of the and hunting pressures, with overhunting disrupting population dynamics of already stressed species. In the and , where arrival coincided with these losses, archaeological of hunting supports the "" hypothesis, though climatic shifts in vegetation and also played key roles. This event profoundly altered ecosystems, reducing large herbivore-mediated nutrient cycling and fire regimes. Faunal migrations were facilitated by land bridges exposed during glacial lowstands, notably the Bering Land Bridge (Beringia) connecting Asia and North America. During the Pleistocene, this corridor enabled bidirectional exchanges of mammals, with taxa such as horses (Equus spp.) and camels (Camelops spp.) migrating from North America to Asia, while others like woolly rhinoceroses moved eastward. Genetic evidence from ancient DNA confirms gene flow between Beringian and continental populations of horses and camels, indicating multiple waves of dispersal that enriched Holarctic faunas. These interchanges contributed to the dynamic assembly of Pleistocene mammal communities, influencing evolutionary trajectories across continents. In the , following the Pleistocene extinctions, certain animal populations demonstrated resilience and recovery. (Rangifer tarandus) and (Bison spp.) persisted or expanded in northern latitudes, adapting to post-glacial warming through migratory behaviors and dietary flexibility in expanding grasslands. Marine mammals, particularly baleen s such as bowhead (Balaena mysticetus) and right whales (Eubalaena spp.), underwent genetic diversification and population expansions after the (~21,000 years ago), as retreating ice sheets opened new feeding grounds in nutrient-rich zones. Stable analyses of whale bones reveal increased abundance linked to enhanced primary in Holocene oceans. These recoveries underscore the period's transition to more stable, ecosystems. Key fossil records illuminate Quaternary animal life and interactions. The La Brea Tar Pits in California, USA, preserve over 3.5 million specimens from the Pleistocene, capturing predator-prey dynamics through trapped carnivores like dire wolves (Canis dirus) and saber-toothed cats scavenging or hunting herbivores such as mammoths and camels. Bone accumulation patterns indicate repeated visits by predators to tar-trapped prey, providing insights into trophic webs. In Asia, the Zhoukoudian cave deposits near Beijing, China, yield rich Pleistocene faunal assemblages, including deer, hyenas, and elephants, accumulated primarily by carnivore activity in a karstic environment. These sites reveal regional biodiversity and human-animal overlaps without direct tool associations. Overall, Quaternary biodiversity trends show an increase in small mammals and birds, which were less impacted by megafaunal extinctions due to higher reproductive rates and niche flexibility. and lagomorph diversity rose in post-glacial forests and grasslands, filling ecological voids left by larger herbivores. assemblages similarly expanded, with migratory species adapting to new habitats. In contrast, faunas appear relatively stable across the period, though their record remains understudied compared to vertebrates, with and fossils indicating consistent roles in and despite climatic shifts. These patterns reflect a shift toward more diverse, smaller-bodied communities in the .

Human Origins

The Quaternary period marks a pivotal phase in hominin evolution, beginning with the emergence of early tool-using species shortly after its onset at approximately 2.58 million years ago (Ma). Although some earlier hominins like Australopithecus afarensis, dated to about 3.9–2.9 Ma in East Africa, straddle the Plio-Pleistocene boundary and are debated as pre-Quaternary, the genus Homo arose within the period itself. Homo habilis, often considered the earliest member of the genus, appeared around 2.3 Ma in East Africa, characterized by larger brain sizes and rudimentary stone tool use that facilitated scavenging and processing of food resources alongside contemporaneous fauna. Subsequent developments saw the rise of Homo erectus around 1.9 Ma, which persisted until about 110 thousand years ago (ka) and represented a major adaptive leap with increased body size, endurance running capabilities, and the first clear evidence of controlled fire use. This species initiated the first major out-of-Africa migrations, with fossils indicating dispersal to by at least 1.85 Ma, reaching as far as by around 1.8 Ma and enabling exploitation of diverse habitats from savannas to woodlands. In , later archaic hominins evolved, including Neanderthals from approximately 400–40 ka, who thrived across and western with robust builds suited to cold climates, and Denisovans, known primarily from genetic evidence in and dated to a similar timeframe of roughly 400–50 ka. Anatomically modern humans, Homo sapiens, originated in around 300 ka, with early fossils from sites like in showing a mosaic of modern and archaic traits. Major dispersals occurred between 70–50 ka, as populations expanded into , interbreeding with local archaic groups and reaching the by approximately 21–23 ka via Beringian land bridges during lowered sea levels, with recent evidence supporting coastal migration routes. These migrations coincided with the development of sophisticated cultural technologies, including simple flake tools from 2.6 Ma, bifacial handaxes from 1.7 Ma that enhanced hunting efficiency, and symbolic art around 40 ka, such as cave paintings in that reflect cognitive complexity. Genetic studies reinforce this timeline, tracing the of —often termed ""—to 150–200 ka in , supporting an African origin for modern humans. Evidence of interbreeding is evident in the 2–4% Neanderthal-derived DNA found in non-African populations, resulting from encounters during Eurasian dispersals, with similar but lower Denisovan contributions in some Asian and Oceanian groups. These genetic exchanges highlight hybridization as a key factor in human adaptability. Hominin success in the Quaternary was closely tied to environmental fluctuations, particularly glacial-interglacial cycles driven by Milankovitch forcing, which prompted behavioral innovations like expanded use during colder stages to maintain warmth, cook food, and deter predators—evidence of habitual fire control dates back to the Middle Pleistocene around 400 ka. Such adaptations allowed and later species to occupy high-latitude environments and exploit seasonal resources, paralleling shifts in associated distributions.

Contemporary Significance

Holocene Dynamics

The Holocene epoch commenced around 11.7 ka with an abrupt warming that ended the cold interval, initiating a period of relative climatic stability compared to the preceding Pleistocene fluctuations. This transition was accompanied by accelerated melting of residual ice sheets, resulting in a global sea-level rise of approximately 120 meters over the epoch, with much of it occurring in the early stages through meltwater pulses. The early , encompassing the phase from 11.7 to 8.2 ka, featured sustained warmth that promoted further and the establishment of forests across northern latitudes. However, this warmth was interrupted by the 8.2 ka cooling event, a century-scale downturn of about 1–2°C in the , triggered by a massive freshwater outburst from Agassiz into the North Atlantic, which disrupted ocean circulation. Subsequently, the Holocene Hypsithermal (approximately 9–5 ka) marked the epoch's thermal peak, with global temperatures 1–2°C warmer than present in many regions, driven by enhanced summer insolation. This interval saw the "greening" of the , where strengthened supported vegetation and lake expansions across . Mid-Holocene transitions included events, such as the 5.9 ka and especially the 4.2 ka droughts, which reduced intensity and contributed to societal stresses, including the decline of the Indus Valley Civilization through habitat shifts and resource scarcity. Holocene climate variations were primarily influenced by residual Milankovitch orbital forcings, which modulated seasonal insolation contrasts, and solar variability. For example, the (1300–1850 CE) involved cooling of up to 1°C linked to prolonged low sunspot activity, such as during the . Proxy records from tree rings and corals reveal centennial- to millennial-scale temperature fluctuations of 1–2°C, underscoring the epoch's natural variability amid overall stability.

Anthropocene Debate

The term Anthropocene was popularized by atmospheric chemist and biologist Eugene F. Stoermer in 2000, proposing it as a new geological epoch to reflect humanity's dominant influence on Earth's systems, beginning around the late 18th century with the marked by rising atmospheric carbon from coal combustion. A more recent proposal by the advocates for a start date of 1950 CE, coinciding with the "Great Acceleration" in human activity, identifiable through global stratigraphic markers like the spike in radiocarbon (¹⁴C) from nuclear bomb tests and plutonium-239 fallout from atmospheric detonations. This mid-20th-century boundary emphasizes the rapid intensification of human impacts, including unprecedented rates of , resource extraction, and technological proliferation. Alternative boundaries have been suggested to capture earlier human-driven changes. One proposal places the onset at 1610 , linked to the "Orbis Spike"—a brief dip in atmospheric CO₂ caused by massive following the deaths of up to 50 million people in the due to European and , which temporarily altered global carbon cycles through the . Another aligns it with the around 1800 , based on the proliferation of fly ash and carbon signatures from coal burning in sediments worldwide. These options highlight debates over the precise "" that stratigraphically defines the , balancing social, ecological, and geochemical evidence. Stratigraphic records provide robust evidence for human-induced changes. Widespread deposition of plastic microplastics in and terrestrial sediments since the serves as a durable marker, persisting in layers far beyond natural organic remains. Shifts in isotopes (δ¹⁵N) reflect the global spread of synthetic fertilizers from the mid-20th century, altering and chemistry on a planetary scale. In urban and archaeological strata, homogenization of bone assemblages—dominated by livestock remains like chicken bones—illustrates faunal restructuring through industrialized and waste patterns. As of March 2024, the Subcommission on Quaternary Stratigraphy voted 12-4 against formalizing the Anthropocene as a new epoch, rejecting the 1950 CE proposal despite years of review by the Anthropocene Working Group; this decision was upheld by the International Commission on Stratigraphy, leaving it unratified in the Geologic Time Scale as of late 2025. Ongoing debates center on whether these changes warrant a full epochal boundary or represent a transient event within the Holocene, given that human influences like agriculture began millennia ago. Critics argue that the scale of current alterations, including irreversible biodiversity loss through habitat destruction and species invasions—termed the "sixth mass extinction"—justifies formal recognition, as extinction rates now exceed background levels by 100 to 1,000 times. The also encompasses the "Homogenocene," a parallel describing the homogenization driven by and , which mixes faunal and floral communities across continents. For instance, the intentional and accidental spread of via shipping and has reduced , creating more uniform ecosystems worldwide, with non-native now comprising up to 20% of regional biotas in heavily traded areas. This faunal mixing, accelerated since the , underscores the Anthropocene's signature of anthropogenic connectivity overriding natural biogeographic barriers.