A publication of the Archaeological Institute of America
Fossils from the Gran Dolina railway cut in northern Spain's Sierra Atapuerca are from a hitherto unknown species of early human, according to the site's excavator José Bermúdez de Castro of the National Museum of Natural Sciences, Madrid, and his colleagues. They named the newly identified species Homo antecessor (from the Latin for pioneer or explorer), and claim that it is directly ancestral to both modern humans and Neandertals.
The Gran Dolina fossils--nearly 80 postcranial, cranial, facial, and mandibular bones as well as teeth of at least six individuals--were excavated between 1994 and 1996 (see "The Peopling of Eurasia," ARCHAEOLOGY, January/February 1996). A key specimen is a partial facial skeleton of a juvenile, estimated to be ten to 11 years old, recovered in 1995. The fossils exhibit both seemingly modern features, such as sunken cheekbones with a horizontal rather than vertical ridge where upper teeth attach and a projecting nose and midface, and more primitive ones, including prominent brow ridges and premolars with multiple roots. The level in which the fossils were found, TD6, is dated by a reversal in the earth's magnetic field to more than 780,000 years ago. So far this level has been exposed only in a test pit of six square meters, but the excavators are confident that many more human fossils will be found when the larger excavations reach the level some years in the future.
Traditionally, Middle Pleistocene Europeans were considered to be an early or archaic Homo sapiens form that was transitional between Homo erectus and modern humans. More recently it has been suggested that European hominids (represented by the fossils from Mauer, Vèrtêsszôllos, Bilzingsleben, Arago, and Petralona together with African ones (including fossils from Bodo, Broken Hill 1, and Dali) can be clasified as a distinct species, Homo heidelbergensis, that is the stem for both Neandertals and modern humans. (The name heidelbergensis comes from the Mauer mandible, which was found near Heidelberg in 1907.)
The preceding phase of human evolution is debated. Some scholars have labeled the 1.6-million-year-old skeleton of a boy found at Nariokotome, Kenya, and other remains as Homo ergaster, a species separate from Homo erectus. These scholars see Homo ergaster as the ancestor of Homo heidelbergensis, which developed into modern humans in Africa and into Neandertals in Europe and Southwest Asia. For them, Homo erectus is to the side of the modern human pedigree. Others believe the African fossils designated Homo ergaster are really Homo erectus, and that African fossils (such as the skull from Bodo, Ethiopia) with ties to modern humans are Homo heidelbergensis, which is descended from Homo erectus and is ancestral to both modern humans and Neandertals.
If the Gran Dolina fossils do represent a new species, the human family tree must be revised. According to Bermúdez de Castro and his colleagues, a few dental and cranial features suggest Homo antecessor is close to Homo ergaster. The Gran Dolina excavators further argue that while Homo antecessor has similarities to later Homo heidelbergensis, the ancestor of the Neandertals, it has more traits in common with modern humans than does Homo heidelbergensis. In their view, Homo ergaster gave rise the Homo antecessor in Africa; about one million years ago Homo antecessor spread via the Middle East to Europe, including Gran Dolina. In Europe Homo antecessor evolved into Homo heidelbergensis and then to Neandertals. In Africa Homo antecessor evolved into Homo sapiens via such fossils as the Bodo skull and a cranium from Kabwe, Zambia. In this scenario both Homo erectus and Homo heidelbergensis are off the line leading to modern humans.
"We realized right away that the face was modern-looking," says Juan Luis Arsuaga of the Universidad Complutense, Madrid. "We tried to put the fossils in Homo heidelbergensis, but they were so different that we could not." But many paleoanthropologists--Philip Rightmire of the State University of New York, Christopher Stringer of the Natural History Museum in London, Jean-Jacques Hublin of the National Center for Scientific Research in Paris, and the University of Michigan's Milford Wolpoff--have expressed reservations about the designation of a new species and the revision of the evolutionary tree. They suggest that "modern" features are in fact juvenile traits that might not be present in adults and they say that more evidence, especially from adults, is needed before the interpretation can be accepted.
Antonio Rosas of the National Museum of Natural Sciences in Madrid counters that some adult fragments from Gran Dolina do show modern traits, including the hollowed cheekbones, which do not appear in either the earlier Nariokotome boy (often identified as Homo ergaster) or later fossils from the Sima de los Huesos at Atapuerca, which date from about 300,000 years ago and are considered to be Homo heidelbergensis. This may indicate that modern human midfacial and subnasal morphology is retention of a juvenile pattern that developed in Homo antecessor, which was not yet present in Homo ergaster and never developed in Homo heidelbergensis or Neandertals. If so, this bolsters the identification of Homo antecessor as a distinct species and as the direct forebear of both modern humans and Homo heidelbergensis.
"This controversy is welcome," says Arsuaga, "because it will help us to understand human evolution better." There are, he says, "two main groups of paleoanthropologists today. Those who consider that human evolution is like a ladder with only one species at a time--Homo habilis, Homo erectus, Homo sapiens--will never accept more species. The other group sees human evolution as a tree with many branches. Some authors think that Homo erectus represents a separate branch and that Neandertals and modern humans are two separate branches with a common ancestor. This common ancestor used to be called 'archaic' Homo sapiens, but now many people believe that Neandertals are a different species (Homo neanderthalensis) from ourselves. So the common ancestor can no longer be called Homo sapiens. We agree in all of this, but the common ancestor is now called Homo heidelbergensis, after the Mauer mandible. Our opinion is that Homo heidelbergensis is a Middle Pleistocene European species, which is ancestral only to Neandertals and not to modern humans. The last common ancestor to Neandertals and modern humans is older, and it is the newly named species Homo antecessor (of Lower Pleistocene age). We have only placed the common ancestor slightly back in time. What is important are the Dolina fossils and not the species we have named, and among the newly published fossils there is a wonderful partial face that has much to tell about human evolution."--MARK ROSE