Humanity's strange face: Stone Age skull stokes debate over what it takes to be human.

In June 2003, three cave researchers prepared for what they hoped would be a return to the Stone Age. The explorers strapped on scuba gear and plunged into a lake in Romania's Carpathian Mountains. These intrepid souls, led by Stefan Milota of Pro Acva Grup in Timisoara, Romania, swam one at a time into a rocky passage that snakes up into an adjacent limestone hillside. At the top of the 80-foot-long channel, they emerged into the musky air of a pitch-dark cave that most of the group had first visited the previous year. Helmet-mounted lights cast a glow over a panorama of bones scattered across the ground.

The group had chanced upon the cave for the first time while exploring the hillside's many fluid-filled conduits. Millennia ago, the deep space would have been accessible on foot, but since then, a massive rockslide has plugged the above-ground entrance.

Upon reaching the cavern in 2002, the investigators saw cave bear bones lying everywhere. The huge creatures may have hibernated there and died after awakening, perhaps lacking the energy to lumber to the surface.

Curiously, a bear skull and leg bone lay atop separate boulders, suggesting that someone had placed them there. Proceeding into another of what turned out to be a series of chambers, the explorers collected the big prize of that first modern visit: a lower jaw that, though massive and thick, looked to be human.

Radiocarbon measurements yielded an age of about 35,000 years for the fossil, making it Europe's oldest securely dated representative of what anthropologists call modern Homo sapiens (SN: 5/10/03, p. 302). However, closer analysis revealed a mix of primitive and modern traits, according to Erik Trinkaus, an anthropologist at Washington University in St. Louis.

Milota and his coworkers returned last year, hoping to find new fossils and map the cave they dubbed Pestera cu Oase, or cave with bones. An even bigger prize awaited them. Amid a pile of bear bones in one chamber sat a partial skull that mimicked the previous find in its mix of seemingly modern and primitive human characteristics. The face was intact, with many teeth in place, and surrounded by several pieces of the individual's brain case. The specimen even retains internal structures of the nose and ears. A piece of another person's cranium lay nearby.

Scientists have yet to establish radiocarbon ages for these fossils. But the new H. sapiens specimens are probably just as old as the Oase (pronounced wa-zee) jaw, says Trinkans, who now leads efforts to analyze the Romanian discoveries. Together, the cave finds challenge the popular theory that people evolved into our modern form by making a neat evolutionary jump from a more primitive version of H. sapiens sometime around 50,000 years ago, Trinkaus asserted on March 31 at the Paleoanthropology Society's annual meeting in Montreal.

The Oase skull's strange combination of modern and archaic characteristics underscores scientific confusion about how to define anatomically modern humans, Trinkaus adds.

"Paleontologists have created an artificial [anatomical] Rubicon that the Oase fossils violate," he says. "The blend of traits on these specimens contradicts the existence of a straightforward evolutionary process [during the Stone Age] in which modern humans came out of Africa and replaced everyone else."

FUNNY FACE The Oase skull--which probably came from a 15- or 16-year-old male--resists easy interpretation. "The anatomy of this individual was not particularly modern, while still qualifying overall as that of a modern human," Trinkans says.

The fossil displays a relatively narrow, high-set nose and distinct chin typical of people today. The shape and [positioning of the specimens internal ear structures also mark it as a modern H. sapiens. Another telling sign appears on the Oase skull's forehead, which lacks a brow ridge typical of Neandertals. Most researchers regard Neandertals as a separate species, Homo neanderthalensis.

Yet the Romanian fossil also flaunts some strikingly primitive traits. Its molar teeth are considerably larger than those of European H. sapiens that lived 30,000 to 20,000 years ago. In fact, Trinkaus says, the chewing surfaces of the Oase skull's molars are wider than those of the generally bulkier Neandertals.

Moreover, at the back of the Oase individual's mouth, a welter of bumps tops its third molars. No fewer than 11 protrusions of various sizes jut up from each chewing surface. Corresponding H. sapiens molars are far smoother.

A third-molar tooth from another human ancestor, which was recently unearthed in the central Asian nation of Uzbekistan, also displays nearly a dozen bumps. Preliminary work suggests that this tooth belonged to a Neandertal that lived at least 40,000 years ago. Michelle Glantz of Colorado State University in Ft. Collins described that find at the Paleoanthropology Society meeting.

Bump-covered third molars appear in some current species of mammals and may evolve as a by-product of genetic mechanisms that promote larger teeth, Trinkaus speculates.

Aside from the Oase specimen's teeth, the heft of its jaw and the shape of parts of its braincase also hark back to H. sapiens that lived 100,000 years ago or more, he says.

The mix of old and new features on the Romanian fossils adds to suspicion that, on the evolutionary path toward today's people, interbreeding occurred among H. sapiens, Neandertals, and other ancient Homo species to varying extents in different regions, according to Trinkaus and a colleague in the Oase project, Joao Zilhao of Cidade University in Lisbon, Portugal.

That theory previously received a boost with the discovery in Portugal of a 24,500-year-old skeleton that Trinkaus and Zilhao view as a prime example of interbreeding between modern H. sapiens and Neandertals (SN: 5/8/99,p. 295). The child's bones display a potpourri of traits from both species, in the researchers' view.

The Oase fossils contain a different mosaic of characteristics, with more pieces from archaic H. sapiens than from Neandertals. If human evolution hinged on groups of various Stone Age species moving from place to place and interbreeding to some extent along the way, then unpredictable mixes of anatomical features would have been generated in any locality that attracted prehistoric crowds, Trinkaus contends.

He thus takes a skeptical view of traditional efforts to reconstruct neatly branching evolutionary trees of human ancestors by determining whether fossils contain predominantly primitive or advanced traits. Analyses of modern and ancient DNA are also incapable of unraveling the extent to which Neandertals and other Homo species interbred with H. sapiens, in Trinkaus' opinion.

That leaves a big evolutionary question mark punctuating the Romanian discoveries. "We don't fully understand what's going on with the combination of features on these bones," the St. Louis researcher says.

CHANNELING EVOLUTION One way to make sense of fossils such as the Portuguese child and the Oase skull is to stop assuming that each ancient Homo species existed on a separate branch of an evolutionary tree, says Trenton W. Holliday of Tulane University in New Orleans. Instead, he proposes, think of human evolution as a braid of several large, winding tributaries, some of which are connected by small streams. Each tributary represents a Homo species; the streams signify gene flow among them.

Most of the larger channels eventually dried out, leaving only one main tributary on which H. sapiens drifted into the present.

Holliday's scenario relies on evidence that many plant and animal species remain cohesive over time, despite some interbreeding with related species that creates fertile offspring with hybrid genetics. In some instances, these offspring display a combination of anatomical features from both parent species; in others, they look like one parent species but possess a blend of DNA sequences from both species.

Interbreeding flourishes where two or more related species share common ground. Scientists refer to these areas as hybrid zones. Novel DNA arrangements can arise in hybrid zones and then spread into the larger populations of each species. In a process that's thought to be especially common among plants, a hybrid population may ultimately evolve into a new species.

Even low levels of genetic give-and-take among closely related Homo species, as suggested by a recent DNA analysis (SN: 3/9/02, p. 149), would have yielded confusing blends of skeletal traits that vary from one region to another, Holliday asserted in the December 2003 Current Anthropology.

Other investigators doubt that interbreeding among ancient Homo species would have yielded individuals with the patchwork of traits observed on the Portuguese and Romanian fossils. Genes directly regulate the shape of only selected skeletal features, notes Daniel Lieberman of Harvard University. Bone responds to myriad influences as a person grows (SN: 11/25/00, p. 346). Parentage from different species is thus hard to detect in fossils, Lieberman says.

The Oase skull may carry hybrid traits, according to some researchers who have received detailed information about it. For all its anatomical eccentricities, the Oase material "fits well within the envelope of modern H. sapiens," says Inn Tattersall of the American Museum of Natural History in New York City.

Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, agrees. Skulls from at least 60,000 years ago in northern Africa display primitive facial features, such as large jaws and teeth, he notes, yet they're attributed to modem H. sapiens.

Another perspective, exemplified by Milford Wolpoffof the University of Michigan in Ann Arbor, folds Neandertals and all other Homo fossils from the past 2 million years into one species, H. sapiens. If that's true, then interbreeding had nothing to do with human evolution.

CULTURED RELATIVES The debate over the nature of modern H. sapiens doesn't end with fossils. A related issue concerns whether archaeological discoveries support the assumption that a cultural and behavioral revolution between 50,000 and 30,000 years ago set people apart from Neandertals and other species in our evolutionary family.

Western European sites from that period show many signs of cultural advances. These include art and ornaments of various kinds, graves, bone tools, and stone blades.

Christopher S. Henshilwood of the African Heritage Research Institute in Cape Town, South Africa, and Curtis Marean of Arizona State University in Tempe reviewed Stone Age finds from Europe and elsewhere in the December 2003 Current Anthropology. They concluded that modern human behavior emerged as H. sapiens evolved over the past 200,000 years, not in a late revolution.

People living in tropical parts of Africa prior to 50,000 years ago were the intellectual equals of those who later trekked into Europe, Henshilwood and Marean assert. There, a frigid climate, competition for resources in larger populations, and other environmental pressures encountered by ancient Europeans instigated innovations in tool making and hunting, these researchers propose.

They point to a growing body of evidence that many behaviors previously attributed only to late Stone Age people stretch back at least 100,000 years in Africa and occurred in European and Middle Easter Neandertals from around 130,000 to 28,000 years ago.

In 75,000-year-old sediments at South Africa's Blombos Cave, for instance, Henshilwood has unearthed etched pieces of ocher and--as described in the April 16 Science--numerous perforated ostrich eggshell shards. Late Stone Age Europeans had no monopoly on body decorations and other symbolic products, he argues.

Although intriguing, such finds offer at best hazy glimpses of ancient societies, Trinkaus remarks. However, he suspects that cultural evolution has deep roots in the Stone Age.

Meanwhile, the fossil hunt at Oase resumes this summer. Researchers seek radiocarbon ages for the bones of bears and H. sapiens and an explanation of how people's remains ended up in the cave. "We're beyond the glory stage at Oase," Trinkaus says. "We're entering the hard-work stage."