Philipp Gunz

Shannon McPherron

Jean-Jacques Hublin

Steffen Schatz

Mohammed Kamal

MPI EVA Leipzig

Shannon McPherron

Until this week, the earliest known fossils of Homo sapiens were about 200,000 years old. But two recent papers in Nature have obliterated that date with a report of 300,000 year-old skull fragments from five individuals found in Morocco. The researchers who discovered the fossils call them "early Homo sapiens." But other scientists say this misrepresents the complex story of human evolution.

The Moroccan remains tell a complicated tale. While their faces are shaped almost exactly like those of modern humans, their skulls are sloped and elongated like much earlier species. While the media exploded with reports about how we've discovered the "earliest" Homo sapiens, the real story isn't that simple.

These papers are just part of a much larger debate about how and where humans evolved.

The discovery

The five early humans—three adults, a child, and an adolescent—were found in what would have been a roomy, pleasant cave about 300,000 years ago. Located on a Moroccan hillside between Marrakesh and the Atlantic coast, the site known as Jebel Irhoud was until recently been a mine and a quarry. Miners first discovered human remains there in the 1960s, but they were identified as 40,000 year-old Neanderthals. Max Planck Institute evolutionary biologist Jean-Jacques Hublin wasn't satisfied with this explanation.

Hublin said the shapes of the Irhoud skulls were all wrong for late Neanderthals. To him, the skulls looked like even more primitive versions of 150,000 year-old humans we'd already found in East Africa. Could the Irhoud people be even earlier examples of humanity in Africa? At the time his idea was anathema in the anthropology community, where conventional wisdom held that humans evolved in East Africa. If that were the case, it made no sense that a slightly earlier human was walking around in North Africa.

Unable to let go of his hunch, Hublin started periodically excavating at Jebel Irhoud in the 1980s. In 2004, he got lucky: Hublin's team uncovered an area of the site untouched by decades of quarrying. There, he told reporters at a press conference, they found a perfectly-preserved package of red clay about 3 meters deep, with layers containing the remains of five humans along with campsite debris such as stone tools, butchered animal bones (mostly gazelle), and charcoal from a fire. Some of the bones and tools were burned too, perhaps from cooking.

The charred remains were another stroke of luck. They meant that evolutionary biologist Daniel Richter, Hublin's colleague at Max Planck, could determine the age of these objects using a technique called thermoluminescence (TL) dating. Put simply, TL dating works by measuring how much radiation an object has absorbed since it was last heated. It works only on materials like rocks and sediments with crystalline structures.

By averaging the results of TL dating on several tools and sediment layers at Jebel Irhoud, Richter determined that the fossils were about 300,000 years old, from a period called the Middle Stone Age. This date was also found using another technique, electron spin resonance dating, used on the tooth enamel from some of the fossil finds.

The dates were solid, so Hublin and his colleagues analyzed the fossils to see where they fit in the human evolutionary tree. There were no traces of DNA in the fossils, so they had only cranial shapes to guide them. Perhaps the most striking thing about the Irhoud people was their faces. These ancient people could easily have wandered around in a modern city and passed as one of us—"as long as they wore a hat," Hublin joked. Their faces and tooth shapes were modern, but their elongated skulls looked more like much earlier hominins. At that point, Hublin and his colleagues dubbed them "early Homo sapiens." In an e-mail to Ars, Hublin clarified that they aren't modern humans, but instead "representative of populations directly ancestral to us."

Perhaps most important, these individuals were hunting in North Africa, far from Ethiopia and South Africa, where previous examples of ancient humans have been found. This undermines the hypothesis that humans evolved in sub-Saharan Africa and spread out from there into Eurasia. Hublin and colleagues call the Jebel Irhoud finds strong evidence for the Pan-African hypothesis, which holds that modern humans evolved all over the continent. Disputing the popular notion that there's an East African "Eden" or cradle of humanity, Hublin argued: "If there is an Eden, it's the size of Africa."

Trouble in paradise

No scientists I spoke with disputed the Irhoud fossil ages, but some were less than impressed with the magnitude of Hublin and his colleagues' discovery. University of Hawaii geneticist Rebecca Cann, known for dating humanity's last common female ancestor (so-called Mitochondrial Eve), called the Nature papers "incremental at best." An evolutionary biologist who preferred to remain anonymous added that calling any ancient human fossil in Africa "the earliest whatever" is "clickbait."

These scientists don't like the way Hublin and his colleagues suggest that the "earliest" Homo sapiens walked the Earth 300,000 years ago. Evolution is a constant process, with no perfect beginnings and endings, so there can never really be an "earliest" version of humanity—only transitional forms between one species and the next. Cann elaborated in a series of e-mails with Ars:

We figure the genetic lineage of our species is placed in Africa, with dates that vary depending on which set of loci/chromosomes/geographic group/SNP vs. [whole genome sequence] gets assayed. The rough estimate of the split between Neanderthal and Homo sapiens is placed at 500-600,000 years ago. So this site should have hominins on the Homo sapiens side, roughly half way down to modern. Most evolutionary biologists would say: "OK that's lots of variation over space/time, so expect transitional forms." What do I see? Transitions. [It's a] nothingburger.

Cann suspects the Jebel Irhoud people are just another transitional stage in hominin evolution, and hardly the "earliest" Homo sapiens. If anything, they're a middle stage, stuck halfway between our common ancestor with Neanderthals and modern humans. This is nice, but it's hardly news; as she put it, it's an evolutionary nothingburger. Other scientists felt that the results weren't a breakthrough, given that they just confirm evolution is a series of gradual changes. As Arizona State University evolutionary biologist Curtis Marean put it, the findings are "very important to know, but perhaps not unexpected."

Philipp Gunz, another Max Planck evolutionary biologist who worked with Hublin on the fossils at Jebel Irhoud, said the team isn't disputing any of this. Still, he thinks the "earliest" Homo sapiens label fits. "Our view is that Jebel Irhoud falls close to the root of the Homo sapiens lineage," he told Ars via e-mail. "I recognize that species do evolve over time, and I am convinced that the Homo sapiens fossils from Jebel Irhoud are a beautiful illustration of such changes within an evolving lineage."

Just one lineage?

For his part, Hublin thinks the problem ultimately boils down to semantics. "In the end if one does not call them 'sapiens,' what should they be called?" he asked via e-mail. "A new name of species like [scientists would have done] in the 19th century? Or a generic term mixing all sorts of unrelated fossils? All this seems a bit ridiculous when any geneticist would tell you that most likely all the hominins of the last 2 million years could interbreed."

You have very early skulls from Spain, some people call them Homo antecessor, that have some of the facial features of modern humans over 700,000 years ago. Maybe that early population is connected to the common ancestors of humans and Neanderthals. If that were the case, it’s not too surprising to see some similar facial features in a later African population. It might be closer to modern humans, but it might also represent a different offshoot of that early ancestral population.

Of course this interbreeding also calls the "earliest" claim into question. Given how much intermixing we see between ancient humans outside of Africa, why pin the Homo sapiens lineage on these individuals from Jebel Irhoud? Is it just the shape of their faces? University of Wisconsin, Madison, anthropologist John Hawks says that we can't even be sure the distinctly modern facial shape evolved within this lineage. He told Ars via e-mail:

Our flattened, delicate facial features may actually be from an ancestor who pre-dated both Neanderthals and the Jebel Irhoud line. If that's the case, we're likely to see a lot of early groups of hominins running around Africa and Eurasia with so-called human faces. That doesn't mean they all evolved into modern humans.

Cann and Hawks say it's also a mistake to assume that modern humans had a single lineage 300,000 years ago. The individuals at Jebel Irhoud might have contributed to our ancestry by interbreeding with other hominins, thus forming a hybrid population that later gave rise to modern humans. Our lineage is likely to be more of a braid than a single line. "There was population movement, expansion, introgression, contraction, and extinction followed by repopulation, [all] shifting the human gene pool," Cann explained. "That's evolution in action."

Added Hawks, "I don't think we should redefine 'modern human' to include things like Jebel Irhoud. That just avoids the interesting questions. How were these complex hominins interacting? How did they all coexist on this continent?"

Listing image by Philipp Gunz