The material from the Grotte a Hominides at Thomas Quarry I includes a nearly complete adult mandible, an additional adult half mandible, a child mandible, vertebrae, a femur, and isolated teeth, many of them recovered from deposits interpreted as a former carnivore den. Gnaw marks on the femur indicate that carnivores accumulated and modified at least some of the hominin remains, helping to reconstruct the taphonomic history of the assemblage.
The fossils are dated to 773,000 years ago, plus or minus 4,000 years, using a high-resolution magnetostratigraphic record that documents in detail the Matuyama-Brunhes geomagnetic reversal. Earths magnetic field has switched polarity many times, and the Matuyama-Brunhes transition, at about 773,000 years, provides a globally synchronous marker that allows researchers to place the Moroccan hominins precisely within the late Early Pleistocene. In the Grotte a Hominides sequence, 180 magnetostratigraphic samples capture the shift from reverse to normal polarity and constrain the duration of the transition to roughly 8,000 to 11,000 years, with the hominin-bearing layers laid down during this interval. Associated animal fossils independently support the age estimate, reinforcing the magnetostratigraphic framework as the primary chronological control for the site.
The new work builds on more than three decades of Moroccan-French collaboration within the Prehistoire de Casablanca program, which has carried out excavations, stratigraphic logging, and geoarchaeological studies in the coastal formations southwest of Casablanca. These investigations have revealed a detailed succession of Plio-Pleistocene palaeoshorelines, dunes, and cave systems that record changing environments, faunal communities, and phases of hominin activity over more than a million years.
Thomas Quarry I is cut into the Oulad Hamida Formation and is known for Acheulean stone tool industries that represent the oldest such assemblages documented in northwestern Africa, with ages around 1.3 million years. The quarry lies close to other important localities such as Sidi Abderrahmane, a classic reference site for Middle Pleistocene prehistory in Northwest Africa, positioning the Grotte a Hominides within a broader network of stratified coastal deposits. Within this setting, the Grotte a Hominides is described as a cave system carved by a past marine highstand into earlier coastal formations and later infilled by sediments that preserved the hominin fossils in a stable, well-defined stratigraphic context.
The geological conditions along the Rabat-Casablanca littoral, including repeated sea-level changes, wind-blown sand accumulation, and rapid early cementation of coastal deposits, have promoted good preservation of bones and artifacts. As a result, the Casablanca region has become one of Africas richest archives for Pleistocene palaeontology and archaeology, documenting early Acheulean technology, environmental shifts, and several stages of human occupation.
Researchers used high-resolution micro-CT imaging, geometric morphometrics, and comparative anatomical analysis to study the Grotte a Hominides mandibles and teeth, assessing both external features and internal structures. Particular attention went to the enamel-dentine junction inside the teeth, which preserves informative shape features even when the outer enamel surface is worn and helps distinguish between hominin species.
Matthew Skinner notes: "Using microCT imaging we were able to study a hidden internal structure of the teeth, referred to as the enamel-dentine junction, which is known to be taxonomically informative and which is preserved in teeth where the enamel surface is worn away. Analysis of this structure consistently shows the Grotte a Hominides hominins to be distinct from both Homo erectus and Homo antecessor, identifying them as representative of populations that could be basal to Homo sapiens and archaic Eurasian lineages."
Dental comparisons show that several traits resemble those seen in hominins from Gran Dolina at Atapuerca in Spain, attributed to Homo antecessor and dated to a similar time, suggesting early population contacts between northwestern Africa and southern Europe. However, the team argues that by the Matuyama-Brunhes transition the African and European populations were already clearly separated, implying that any genetic or demographic exchanges must have occurred earlier in the Pleistocene.
Shara Bailey confirms that the overall morphology of the Grotte a Hominides teeth remains generalized and does not show Neandertal-specific features. She notes that "In their shapes and non-metric traits, the teeth from Grotte a Hominides retain many primitive features and lack the traits that are characteristic of Neandertals. In this sense, they differ from Homo antecessor, which - in some features - are beginning to resemble Neandertals. The dental morphological analyses indicate that regional differences in human populations may have been already present by the end of the Early Pleistocene".
Taken together, the morphological evidence leads the authors to interpret the Grotte a Hominides hominins as part of an African lineage closely related to Homo antecessor and situated near the divergence of African and Eurasian Middle Pleistocene lineages. This position would place the Moroccan fossils close to the ancestor that later gave rise to Homo sapiens in Africa and to Neandertals and Denisovans in Eurasia, combining archaic African features with traits that approach later forms.
The findings underscore the role of Northwest Africa in the early evolution of the genus Homo during a period when climate-driven environmental changes periodically opened corridors across what is now the Sahara. Palaeontological data from this interval indicate repeated links between Northwest Africa and savanna regions in eastern and southern Africa, challenging the idea that the Sahara always acted as a strict biogeographic barrier.
Chronologically, the Grotte a Hominides hominins are almost the same age as the Gran Dolina fossils, older than Middle Pleistocene hominins ancestral to Neandertals and Denisovans, and roughly 500,000 years earlier than the Homo sapiens remains from Jebel Irhoud in Morocco. Genetic studies place the last common ancestor of Homo sapiens, Neandertals, and Denisovans between about 765,000 and 550,000 years ago, and the Moroccan fossils appear to match most closely the older part of this time window.
The study concludes that the Grotte a Hominides fossils may be among the strongest current candidates for African populations close to the root of this shared ancestry, supporting a deep African origin for the lineage that led to modern humans.
Research Report:Early hominins from Morocco basal to the Homo sapiens lineage
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