Sally C. Reynolds

Expanded excavations at White Sands National Park have unearthed 20 stratigraphic horizons containing in situ human footprints, megafaunal trackways, and linear features created by travois. Advances in radiocarbon dating of conifer pollen, combined with ages of aquatic seeds, refines the chronology of the footprint-bearing sequence to between 23.5 and 21.7 thousand years ago (ka). Biomarker, pollen, and stratigraphic evidence indicate a complete overturning of the environment from a perennial lake to a playa and capture a large aridification episode coincident with Dansgaard-Oeschger event 2 at 23.2 ka, which shrank pluvial Lake Otero and allowed people to traverse the former shoreline area for nearly two millennia. This well-dated and highly resolved paleoclimate record provides compelling evidence that humans in North America experienced marked environmental changes during the Last Glacial Maximum.

Fossil human footprints provide direct evidence of past presence and behaviour, yet the processes governing their preservation remain poorly understood. In particular, it remains unclear whether repeated footprint-bearing surfaces reflect population abundance and occupation continuity, or instead arise through environmental and taphonomic filtering. This study addresses the problem using an agent-based model (ABM) of footprint formation and preservation within a dynamic lake-margin environment. The model simulates interactions between human activity, basin morphology, and time-varying hydrological conditions under contrasting climate forcing regimes and occupation structures. Preservation is treated as a probabilistic process occurring when wetting events coincide with sufficient footprint availability, allowing analysis within an environmental–behavioural state space. Results show that the footprint record is highly sensitive to the temporal organisation of climate variability, with different forcing regimes producing distinct preservation architectures even under comparable mean environmental conditions. Basin geometry exerts a fundamental control, with basins that are not too steep or shallow maximising shoreline mobility and preservation potential, while behavioural organisation shapes how footprints are distributed relative to preservation windows. In contrast, population size alone exerts comparatively limited influence. Comparison with artefact accumulation highlights a key contrast: artefacts integrate behaviour across time, whereas footprints are preserved episodically, capturing short-lived snapshots of activity. Footprint-bearing surfaces should therefore not be interpreted as direct proxies for population size or occupation continuity, but as discrete sampling events generated through the intersection of human activity and transient preservation opportunity. More broadly, the model suggests that fossil footprint assemblages are emergent products of coupled behavioural, geomorphic, and environmental systems, providing a framework for more robust interpretation of the human footprint record.

Quantitative analysis of footprint shape is central to ichnology, yet the application of geometric morphometrics is often limited by the difficulty of defining homologous landmarks on irregular or variably preserved impressions. Here we present a polygon-based approach to footprint morphometrics in which the footprint margin is digitised as a continuous outline and resampled into a standardised set of boundary points for analysis. Using a combination of simulated perturbation experiments and empirical datasets, we assess the sensitivity of this method to annotation uncertainty and compare it to natural variation within and between trackways. The results demonstrate a clear hierarchy of variance, in which annotation noise is lower than inter-observer variability, intra-trackway variation, and between-trackway differences. These findings suggest that polygon-based representations of footprint shape are robust to realistic levels of digitisation uncertainty. Application of the method to modern and fossil trackways, including the Laetoli footprints, shows that resampled polygon outlines retain meaningful morphological structure and can be used effectively in comparative analyses.

The figures in the original publication are adaptations of rock art motifs, and the sources of the original photographs were cited in the description of each figure

Humans are well known to have made paintings and engravings on rock surfaces, both geometric motifs with an unclear representation, and representative motifs that refer to their activities and aspects of their environment. This kind of art is widespread across time and space and has throughout history been subjected to various kinds of approaches. Typically, rock art research focuses on its role in the development of the hominin brain and the capability of abstract thinking, as well as on interpreting representative and non-representative motifs. Ethnography and cognitive research have often stressed that rock art is the result of ritual practises and the expression of a shamanic belief system. However, representative motifs may also shed light on a region’s ecological and human prehistory. Here, we give an overview of the general development of rock art study: we highlight the development of artistic behaviour in humans by discussing aesthetic preferences, and the creation of simple geometric motifs and eventually representative motifs, before describing the theories that developed from the earliest study of rock art. These have largely focused on classification and interpretation of the motifs, and often centred on Palaeolithic material from Europe. We then move on to discuss how ethnography among rock art creating communities often suggests important relationships between specific animals in both the realms of spiritual belief systems and within the local environment. Lastly, we highlight how rock art reflects the local penecontemporaneous environment when it comes to depictions of animals, plants, technologies, humans and their activities. We argue that animal depictions are a useful subject to study on a large scale, as it is the most widespread representative motif, and the most appropriate subject to study when the goal is to draw conclusions on environmental changes. Rock art can fill gaps in the local archaeological record and generate new questions of it, but also offer new insights into the history of local human–animal interaction: animal species depicted and/or referred to in rock art are likely to have been a selection of spiritually important animals and a comparison to known information on human interactions with local species may reveal patterns among which animals are selected for local rock art depictions and which are not. Interregional comparison can in turn shed light on whether humans in general tend to ascribe meaning to the same types of animals. We end the review with suggestions for future study, with a special role for computational methods, which are suitable for the analysis of large databases of visual imagery.

Intra-crystalline protein degradation (IcPD) analysis was undertaken on 80 fossil tooth enamel samples from four taxonomic groups (rhinocerotid, suid, equid, bovid) excavated from two archaeological cave sites in Zambia (Twin Rivers and Mumbwa Caves). Seventy-two (90%) of these fossils showed evidence of closed-system behaviour. The fossils’ relative extent of protein degradation between the sites was consistent with their known ages, with samples from Twin Rivers (Mid-Pleistocene) showing higher levels of degradation than Mumbwa Caves (late Mid-Pleistocene to late Holocene). At Twin Rivers, a potential trend between IcPD and excavation depth was observed, concordant with the working hypothesis of periodic deposition of sediments as slurry flows into a phreatic passage. However, greater depositional and taphonomic complexity was indicated by relatively wide ranges of IcPD values within individual excavation levels. These results are interpreted partly as the consequence of the excavation methods used, alongside reworking within the deposits, which had not previously been recognised. Whilst lack of stratigraphic control limited the investigation of taxonomic effect, one notable difference in the protein breakdown pattern of peptide chain hydrolysis was observed between rhinocerotid in comparison to the other studied taxa. We therefore recommend taxon-specific enamel amino acid geochronologies (AAGs) are developed in future. Whilst lack of comparator datasets meant it was not possible to create a calibrated, enamel AAG for the South-Central African region from these sites, Twin Rivers provides a case study illustrating the complexity of cave formation processes and the importance of direct dating for interpreting archaeological and palaeontological sequences.

Bennett et al. (2021, Science 373, 1528–1531) reported that ancient human footprints discovered in White Sands National Park, New Mexico date to between ∼23,000 and 21,000 years ago. Haynes (2022, PaleoAmerica, this issue) proposes two alternate hypotheses to explain the antiquity of the footprints. One is that they were made by humans crossing over older sediments sometime during the Holocene. This is incorrect as there are Pleistocene megafauna tracks interspersed with the human footprints, so they cannot be Holocene in age. The other hypothesis maintains seeds used to date the human footprints were exhumed from older sediments, transported across the Tularosa Basin, and deposited on moist ground that was traversed by Clovis people at ∼13,000 years ago. This scenario requires a series of events that are highly unlikely, if not impossible. We maintain the seeds were collected from their original depositional context and the ages of the footprints fall within the Last Glacial Maximum.

Madsen et al . question the reliability of calibrated radiocarbon ages associated with human footprints discovered recently in White Sands National Park, New Mexico, USA. On the basis of the geologic, hydrologic, stratigraphic, and chronologic evidence, we maintain that the ages are robust and conclude that the footprints date to between ~23,000 and 21,000 years ago. Madsen et al . ( 1 ) question the veracity of calibrated radiocarbon ages used to constrain the antiquity of human trackways discovered recently at White Sands National Park (WHSA) Locality 2, New Mexico, USA ( 2 ). The ages were derived from seeds of the aquatic plant Ruppia cirrhosa , which they suggest may suffer from hard-water (or reservoir) effects, making them too old, potentially by thousands of years. We were well aware of this possibility, investigated it, and presented several lines of evidence that argued against such a problem. Here we respond to each of their four primary points.