The skull had the brow of a descendant but the face of an ancestor. When researchers finished reconstructing the DAN5/P1 cranium from Ethiopia’s Afar region, dated to between 1.6 and 1.5 million years ago, they found themselves staring at an evolutionary contradiction: a single individual who appeared to belong to two different chapters of human history at once.
Homo erectus, the species that first looked recognizably like us with its large brain, prominent brow ridge, and smaller teeth, has long been considered a turning point in human evolution. The species is often described as a package of changes arriving together, but this newly reconstructed skull from the Gona research area indicates the pieces did not always move in lockstep.
The brain inside DAN5/P1 was small, just 598 cubic centimeters, closer to the earlier Homo habilis than to the larger-brained Homo erectus fossils found in Kenya from the same time period. Yet its jutting brow ridge and certain features of the braincase matched Homo erectus. Its face, meanwhile, retained the flat cheekbones and narrow nasal opening of species that should have been fading from the scene.
An international team led by Karen L. Baab of Midwestern University published their findings in Nature Communications. After years of digital reconstruction using micro-CT scans, they pieced together a mostly complete Early Pleistocene Homo cranium from the Horn of Africa, only the sixth such specimen recovered from the continent. What emerged was a fossil that defies easy classification and complicates our understanding of how Homo erectus came to be.
“The presence of such a morphological mosaic contemporaneous with or postdating the emergence of the indisputable H. erectus craniodental complex around 1.6 Ma implies an intricate evolutionary transition from early Homo to H. erectus,” the researchers wrote.
How Researchers Reconstructed the Homo Erectus Skull
The DAN5/P1 fossil was discovered at the Dana Aoule North site in Gona, part of Ethiopia’s Afar region. Initially, only the braincase had been fully studied. The face remained a puzzle, broken into fragments during fossilization and cemented together by sediment over millennia.
To reconstruct it, the team took micro-CT scans of five facial bone fragments: portions of both sides of the upper jaw, a piece of the region below the eye sockets, part of the cheekbone, and several teeth including premolars and molars. From these scans, they generated 3D surface models that could be digitally manipulated and fitted together.
Reassembly required matching each piece according to anatomical landmarks. Sutures had to align. Break lines had to correspond. The frontal process of the upper jaw, for instance, had to be angled so it would properly meet the frontal bone while simultaneously matching curvatures implied by surrounding structures. After the face was assembled, the team attached it to the previously reconstructed braincase.
To account for uncertainty in how the face connected to the braincase, the researchers also created an alternative reconstruction with slightly different positioning. Both versions produced similar results in subsequent statistical analyses, lending confidence that the findings were not artifacts of reconstruction choices.
A Face from the Past on a Skull from the Future
Comparing DAN5/P1 to other early human fossils revealed its split identity. The braincase aligned with Homo erectus in several respects: a thick, projecting brow ridge with a continuous groove behind it, circular eye sockets, and the long, low profile characteristic of the species.
But the face pointed backward in time. The area below the eye sockets was flat from side to side rather than curving outward as in typical African Homo erectus. The nasal opening lacked the pronounced widening seen in roughly contemporaneous Kenyan fossils like KNM-ER 3733 and the adolescent KNM-WT 15000. The cheekbone’s attachment point sat more toward the front of the face than expected, a trait linked to earlier Homo species.
The roof of the mouth was large relative to brain size, placing DAN5/P1 closer to early Homo and to Homo erectus fossils from Dmanisi, Georgia, than to East African Homo erectus. The teeth were also substantial. The first and third molars fell within the size range of Homo habilis and at the upper end of variation for African Homo erectus.
Statistical analyses of skull shape placed DAN5/P1 within the Homo erectus range but nearest to two specific fossils: the adolescent KNM-WT 15000 from Kenya and the subadult D2700 from Georgia. When the researchers analyzed midface shape alone, DAN5/P1 clustered with Homo habilis specimens instead. Its single closest match in overall shape was KNM-ER 1813, a classic Homo habilis skull from Kenya.
“DAN5/P1 is, to our knowledge, the first African fossil to exhibit a mixture of H. erectus and early Homo features,” the team reported.
Two Populations, Two Evolutionary Speeds
The combination of ancestral and more advanced features in a single skull from this time period complicates the conventional timeline. By 1.6 million years ago, Homo erectus was supposed to have already arrived as a coherent package. Famous Kenyan fossils from roughly the same period, including KNM-ER 3733 and KNM-WT 15000, display the full suite of Homo erectus traits: larger brains, smaller teeth, wider nasal openings, and curved architecture below the eyes.
Yet DAN5/P1 lived at essentially the same time with a brain barely larger than Homo habilis, teeth to match, and a face that looked like it belonged to an earlier era. The brow ridge announced membership in Homo erectus. Almost everything else pointed to something more primitive.
The researchers interpret this as evidence that different populations of early humans were evolving at different rates across East Africa. The rift basin landscape, combined with low population densities, may have allowed groups in the Horn of Africa and the Lake Turkana region of Kenya to diverge over time. The Horn of Africa population, represented by DAN5/P1, retained more ancestral features. The Kenyan population developed more derived characteristics.
“The evolutionary contrasts in brain size and face morphology embodied by the broadly contemporaneous DAN5/P1 and Kenyan fossils, KNM-ER 3733 and KNM-WT 15000, imply complex population structure rather than simple coexistence of two different lineages,” the researchers wrote.
This interpretation casts Homo erectus as a species showing substantial regional variation rather than uniform characteristics everywhere it lived.
Advanced Tools Found at the Same Site as Small-Brained Fossil
Both simple and advanced stone tools turned up at the DAN5 site. Mode 1 tools, basic cores and flakes, appeared alongside Mode 2 Acheulean tools, the more sophisticated hand axes associated with Homo erectus.
Acheulean technology is typically linked to larger-brained populations. The longstanding assumption has been that expanding brains drove technological innovation. But at Gona, these advanced tools appear in the same context as a fossil with a brain volume comparable to Homo habilis, the earlier species credited only with simpler tools.
Evidence from the site also points to a broadened diet and access to animal resources. A similar pattern appears at Dmanisi in the Republic of Georgia, where small-brained fossils occur alongside evidence of meat consumption and varied tool use.
“The Gona evidence, along with that from Dmanisi, hints that key behavioral/technological innovations may precede major morphological transformations,” the team noted.
If accurate, the assumed connection between brain size and cognitive ability in early human evolution may have been less direct than researchers believed.
Surprising Similarities to a Much Younger Species
DAN5/P1 also shares certain features with Homo naledi, a much younger Middle Pleistocene species discovered in South African caves. Both possess small brains, flat nasal regions, forward-positioned cheekbones, and a distinct bump at the back of the skull.
The researchers caution against reading too much into these resemblances. Vast stretches of time and thousands of kilometers separate the two fossils, and shared primitive traits do not necessarily indicate a direct evolutionary link. But the parallels add to mounting evidence that small-brained Homo populations persisted in Africa long after larger-brained forms had emerged.
One Skull, Many Questions
Only six largely complete Early Pleistocene Homo skulls have been recovered from Africa. Each new discovery reshapes the narrative, and DAN5/P1 is no exception. Here was an individual whose brow announced membership in Homo erectus while whose face and teeth belonged to an earlier inheritance, found at a site that also yielded both simple and advanced stone tools.
The fossil confirms that the emergence of Homo erectus involved considerable variation across populations. Different groups existed simultaneously in East Africa, evolving at different rates and retaining different features. The transition from early Homo to Homo erectus appears to have been gradual and uneven, with different traits shifting at different speeds in different places.
DAN5/P1 does not settle the debate over where Homo erectus originated or how it spread across Africa and into Eurasia. Some researchers have proposed a Eurasian origin based on early Homo erectus-like features appearing in Georgia. The ancestral morphology preserved in DAN5/P1, however, remains compatible with local evolution of the species in East Africa.
Source : https://studyfinds.org/homo-erectus-skull-discovery-rewrites-human-evolution-timeline/
