At one point, any new human fossil from hundreds of thousands of years ago might have drawn intrigue. If the new bones looked different from others that had been found before, they may have even been hailed as a new archaic human species, and given a taxonomic name in the genus Homo.
But some scientists say evidence is mounting that paleoanthropologists in the past may have been too quick to categorize hominin fossils as distinct species.
So when a chunk of a 400,000-year-old skull was unearthed at the Gruta da Aroeira archaeological site in Portugal, the scientists who reveal its discovery in a paper published Monday in the journal Proceedings of the National Academy of Sciences didn't try to assign a taxonomic name to the specimen as a reflection of that new thinking.
"There has been a tendency to think of minute differences in the morphology of fossils as indicative of speciation, but the more we find, the more we see that these features are not diagnostic in that sense because they do not allow us to divide these fossils into discrete clusters," study author João Zilhão, an archaeologist at the Catalan Institution for Research and Advanced Studies (ICREA) in Portugal, tells The Christian Science Monitor in a phone interview.
Instead, he says, "They're part of a pool."
Or, as Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis and one of Dr. Zilhão's co-authors calls it, "a unified humanity across the Old World."
This new fossil is not an entire skull. It includes part of the skullcap, a bit of the jaw and nasal floor, and two fragmentary teeth. But comparing that specimen to others reveals that the individual had traits similar to those thought to be uniquely Neanderthal as well as some more like those associated with Homo erectus.
This is not the first specimen from the Middle Pleistocene to be found with a mixture of traits. Other fossils unearthed in recent years from Spain, France, Italy, and Germany have also shown such confounding variation in features.
Some specimens might have been ancestors of Neanderthals, showing which features found on the later hominin evolved first. Living mostly in modern-day Europe, Neanderthals were the last archaic human group to go extinct, some 40,000 years ago.
But Dr. Trinkaus says it might be simpler than that. "What this fossil does for me is it reinforces what I've maintained for some time that this is all just normal variation," he tells the Monitor in a phone interview. In that view, some groups of archaic humans would have been particularly uniform in their traits, while others may have interbred or developed a variety of traits.
Trinkaus points to the gray wolf as an example. Although gray wolves are found living across continents today, and there are regional variations in their coat colors and other features, it's considered all one species, Canis lupus, he says. "Nobody questions it." (Indeed, taxonomists classify the domestic dog as a subspecies of C. lupis.)
And to Trinkaus, archaic members of the genus Homo were actually one species, too.
Fred Smith, a paleoanthropologist at Illinois State University who was not involved in this study, agrees that the lines between groups of archaic humans are fuzzy. "One of the things that all the genetic evidence that's coming out now is showing us is that these things that we thought fit in these little boxes may sort of fit in a box, but they're exchanging genes with other boxes," he says in a phone interview with the Monitor. "And I think what we're seeing in this time period in Europe is a reflection of the fact that there's a good bit of genetic exchange going on that means that morphology in a particular area is not going to be homogeneous."
But what does genetics and interbreeding have to do with species? Well, the most common way for biologists to differentiate between species is to determine if the two groups of organisms can reproduce and have viable offspring. So if this variation in traits is due to such intermingling, then that would support Trinkaus's claim that these might not be such distinct species after all.
"My opinion would be that this fossil stresses the need to overcome the species question in order to understand the humans living in Eurasia about half a million years ago," Zilhão says.
Not all anthropologists entirely agree with this idea though.
"Given that some of the authors are known to be unenthusiastic about assigning distinct species names to archaic human fossils, it’s not surprising that the paper sits on the fence taxonomically, but to be fair the specimen does show a new mix of traits for this period of time, and does differ from the folks down the road at the Sima de los Huesos," an archaeological site in the Atapuerca Mountains in northern Spain, Chris Stringer, a paleoanthropologist at the Natural History Museum, London who was not involved in this study, writes in an email to the Monitor about the new specimen.
Dr. Stringer also points out that the fragment is missing some of the key parts of the skull that paleoanthropologists use to classify early humans. And if these features were there, Stringer would use them to assign a species name to the specimen whereas the paper's authors likely would not, he says.
Another thing that makes this specimen and others particularly hard to classify is the issue of timing, Shara Bailey, a paleoanthropologist at New York University who was not involved in the research, explains in a phone interview with the Monitor.
Although this new specimen is one of the most precisely dated among European Middle Pleistocene hominin fossils, the authors of the paper still suggest a possible range of 390,000 to 436,000 years ago. And, Dr. Bailey points out, some of the other specimens with mixed attributes have even larger possible age ranges.
"I think that it would be very difficult to say that any of these populations that are represented by the fossils we have were necessarily coming in contact with one another," Bailey says, as tens of thousands of years is a long time compared to human lifetimes. "We don't even know if they lived at the same time."
And even if they did, the region was undergoing a glacial period through much of that time, she adds. As such, there were physical barriers that may have been separating populations.
An exchange of technology?
This new find also could be a piece of the puzzle to understand how technology spread across archaic human populations in Europe. The cranial fossil was found alongside some Acheulean bifaces, or hand axes, and animal bones that bear burn marks, hinting at the technological capabilities of the group this individual belonged to.
"Because the two major cultural technological innovations of the time (the making of bifaces and the habitual use of fire) are found at the same time associated with both groups, the Aroeira find confirms that culture spread across population boundaries, they were not 'brought' by one specific population only," Paola Villa, a curator at the University of Colorado Museum who was not part of this study, writes in an email to the Monitor.
The distinction is a debated one, even among the authors on the new paper, Trinkaus says. And it hinges on the question of whether technological exchange and a mixture of biological traits are linked or indicative of one another.
But, he says, "the bottom-line is that what this shows is this is a unified humanity."