300000 year-old early Homo sapiens sparks debate over evolution – Ars Technica

Two views of a composite reconstruction of the earliest known Homo sapiens fossils from Jebel Irhoud based on micro computed tomographic scans of multiple original fossils. Dated to 300 thousand years ago these early Homo sapiens already have a modern-looking face that falls within the variation of humans living today. However, the archaic-looking virtual imprint of the braincase (blue) indicates that brain shape, and possibly brain function, evolved within the Homo sapiens lineage.

Philipp Gunz

View looking south of the Jebel Irhoud site in Morocco. The remaining deposits and several people excavating them are visible in the center. At the time the site was occupied by early hominins, it would have been a cave, but the covering rock and much sediment were removed by work at the site in the 1960s.

Shannon McPherron

The mandible from the individual dubbed Irhoud 11 is the first, almost complete adult mandible discovered at the site of Jebel Irhoud. The bone morphology and the dentition display a mosaic of archaic and evolved features, which the researchers believe place it close to the root of our own lineage.

Jean-Jacques Hublin

Here are some of the fossils being uncovered at Jebel Irhoud. In the center of the image, in a slightly more yellow brown tone, is the crushed top of a human skull (Irhoud 10) and visible just above this is a partial femur (Irhoud 13) resting against the back wall.

Steffen Schatz

Some of the Middle Stone Age stone tools from Jebel Irhoud. Pointed forms such as a-i are common in this period. Also characteristic are the so-called Levellois prepared core flakes.

Mohammed Kamal

Scientists Shannon McPherron (left) and Abdelouahed Ben-Ncer discussing the new fossils finds from Jebel Irhoud. The crushed skull from Irhoud 10 is just barely visible above the blue dustpan.

MPI EVA Leipzig

Daniel Richter drilling into the site of Jebel Irhoud as part of his work dating the deposits containing the fossils and stone tools. Richter applied thermoluminescence dating to heated flints coming from the excavations, and demonstrated that the site is about 300 thousand years old. The holes are drilled for dosimeters which measure the background radiation of the sediments for an entire year. Knowing the background radiation and the charge trapped in the heated flints, the age can be determined.

Shannon McPherron

Until this week, the earliest known fossils of Homo sapiens were about 200,000 years old. But two recent papers in Naturehave 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 five early humansthree adults, a child, and an adolescentwere 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 beena 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.

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, withlayers 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."

Composite reconstruction of a fossil skull from Jebel Irhoud, based on micro computed tomographic scans of multiple original fossils. The brain case is roughly the size of a modern human's, but it slopes backward instead of creating a taller, bulbous shape.

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."

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 humanityonly 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 suspectsthe 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."

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, its 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.

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.

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

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300000 year-old early Homo sapiens sparks debate over evolution - Ars Technica