Homo antecessor (Latin for 'pioneer man') is an extinct species of archaic human recorded in the Spanish Sierra de Atapuerca, a productive archaeological site, from 1.2 to 0.8 million years ago during the Early Pleistocene. Populations of this species may have been present elsewhere in Western Europe, and were among the first to settle that region of the world, hence the name. The first fossils were found in the Gran Dolina cave in 1994, and the species was formally described in 1997 as the last common ancestor of modern humans and Neanderthals, supplanting the more conventional H. heidelbergensis in this position. H. antecessor has since been reinterpreted as an offshoot from the modern human line, although probably one branching off just before the modern human/Neanderthal split.

Despite being so ancient, the face is unexpectedly similar to that of modern humans rather than other archaic humans—namely in its overall flatness as well as the curving of the cheekbone as it merges into the upper jaw—although these elements are known only from a juvenile specimen. Brain volume could have been 1,000 cc (61 cu in) or more, but no intact braincase has been discovered. This is within the range of variation for modern humans. Stature estimates range from 162.3–186.8 cm (5 ft 4 in – 6 ft 2 in). H. antecessor may have been broad-chested and rather heavy, much like Neanderthals, although the limbs were proportionally long, a trait more frequent in tropical populations. The kneecaps are thin and have poorly developed tendon attachments. The feet indicate H. antecessor walked differently than modern humans.

H. antecessor was predominantly manufacturing simple pebble and flake stone tools out of quartz and chert, although they used a variety of materials. This industry has some similarities with the more complex Acheulean, an industry which is characteristic of contemporary African and later European sites. Groups may have been dispatching hunting parties, which mainly targeted deer in their savannah and mixed woodland environment. Many of the H. antecessor specimens were cannibalised, perhaps as a cultural practice. There is no evidence they were using fire, and they similarly only inhabited inland Iberia during warm periods, presumably retreating to the coast otherwise.

The Sierra de Atapuerca in northern Spain had long been known to be abundant in fossil remains. The Gran Dolina ("great sinkhole") was first explored for fossils by archaeologist Francisco Jordá Cerdá [es] in a short field trip to the region in 1966, where he recovered a few animal fossils and stone tools. He lacked the resources and manpower to continue any further. In 1976, Spanish palaeontologist Trinidad Torres investigated the Gran Dolina for bear fossils (he recovered Ursus remains), but was advised by the Edelweiss Speleological Club to continue at the nearby Sima de los Huesos ("bone pit"). Here, in addition to a wealth of bear fossils, he also recovered archaic human fossils, which prompted a massive exploration of the Sierra de Atapuerca, at first headed by Spanish palaeontologist Emiliano Aguirre but quickly taken over by José María Bermúdez de Castro, Eudald Carbonell, and Juan Luis Arsuaga. They restarted excavation of the Gran Dolina in 1992, and found archaic human remains two years later; in 1997, they formally described these as a new species, Homo antecessor. The holotype is specimen ATD6-5, a right mandibular fragment retaining the molars and recovered with some isolated teeth. In their original description Castro and colleagues posited that the species was the first human to colonise Europe, hence the name antecessor (Latin for "explorer", "pioneer", or "early settler").

The 25 m (82 ft) of Pleistocene sediments at the Gran Dolina are divided into eleven units, TD1 to TD11 ("trinchera dolina" or "sinkhole trench"). H. antecessor was recovered from TD6, which has consequently become the most well-researched unit of the site. In the first field seasons from 1994 to 1995, the dig team excavated a small test pit (to see if the unit warranted further investigation) in the southeast section measuring 6 m² (65 sq ft). Human fossils were discovered first by Aurora Martín Nájera; the 30 cm (12 in) layer they were found in is nicknamed the "Aurora Stratum" after her. A 13 m² (140 sq ft) triangular section was excavated in the central section starting in the early 2000s. Human fossils were also found in the northern section. In sum, about 170 H. antecessor specimens were recovered. The best preserved are ATD6-15 and ATD6-69 (possibly belonging to the same individual) that most clearly elucidate facial anatomy. Subsequent field seasons have yielded about sixty more specimens. The discovered parts of the H. antecessor skeleton are: elements of the face, clavicle, forearm, digits, knees, and a few vertebrae and ribs.

In 2007, a mandibular fragment with some teeth, ATE9-1, provisionally assigned to H. antecessor by Carbonell, was recovered from the nearby Sima del Elefante ("elephant pit") in unit TE9 ("trinchera elefante"), belonging to a 20– to 25-year-old individual. The site additionally yielded stone flakes and evidence of butchery. In 2011, after providing a much more in depth analysis of the Sima del Elefante material, Castro and colleagues were unsure of the species classification, opting to leave it at Homo sp. (making no opinion on species designation) pending further discoveries.

The stone tool assemblage at the Gran Dolina is broadly similar to several other contemporary ones across Western Europe, which may represent the work of the same species, although this is unconfirmable because many of these sites have not produced human fossils. In 2014 fifty footprints dating to between 800,000 and 1.2 million years ago were discovered in Happisburgh, England, which was provisionally attributed to an H. antecessor group given it was the only human species identified during that time in Western Europe. In 2025, another human face was discovered at the Sima del Elefante, dating to 1.1 to 1.4 million years ago. The discoverers opted to provisionally classify it as Homo aff. erectus (affinities to H. erectus) because it lacked the characteristic features of H. antecessor in the midface.

The face of H. antecessor is unexpectedly similar to that of modern humans compared to other archaic groups; so, in their original description, Castro and colleagues classified it as the last common ancestor of modern humans and Neanderthals, supplanting H. heidelbergensis in this capacity. The facial anatomy came under close scrutiny in subsequent years.