How fossil find takes us one giant step closer to human origins
'Both dating of the jaw bone and the dating of the stone artefacts gave us an age of 300,000 years'
The earliest known existence of modern humans, or Homo sapiens, was previously dated to be around 200,000 years ago. It’s a view supported by genetic analysis and dated Homo sapiens fossils (Omo Kibish, estimated age 195,000 years, and Herto, estimated age 160,000 years), both found in modern-day Ethiopia, East Africa.
But new research, published today in two Nature papers, offers a fresh perspective. The latest studies suggest that Homo sapiens spread across the entire African continent more than 100,000 years earlier than previously thought.
This evidence pushes back the origins of our species to 300,000 years ago, and supports the idea that important changes in our biology and behaviour had already taken place across most of Africa by that time.
Our work focused on samples collected at the archaeological site Jebel Irhoud in Morocco. It’s a place that is well known for hominin fossils – that is, bones from early humans – first excavated in the 1960s.
However, the interpretation of the first fossils and identification of their age was compromised previously, due to uncertainty about the geological dating of the sediment layers in which the remains had been found.
More than 40 years later, in 2004, an international team of scientists reopened the excavation. They discovered 16 new Homo sapiens fossils and a large number of Middle Stone Age artefacts. Now in 2017 we’re able to report on these remains thanks to improved dating techniques.
The new analysis proposes a revised version of the evolutionary history of modern humans that involves the entire African continent, and long before the “out-of-Africa” spread of Homo sapiens to other continents (dated at around 100,000 years ago).
Jebel Irhoud is the richest African Middle Stone Age site associated with the earliest known representative of our species.
Bones with modern and ancient features
The fossil remains excavated included numerous skulls and jawbones, long bones such as femur (from the leg) and humerus (from the arm), and teeth of several individuals. They reveal a mixture of modern human and more archaic, or ancient, features.
Of particular note is the slim, “gracile” face seen in living humans, which is also present in Jebel Irhoud specimens. Compared with the more robust face and elongated skull of Neanderthals or older hominins, the faces of Jebel Irhoud specimens are slender, relatively short, and sit under a rounder braincase (the part of the skull in which the brain sits).
However, while tomographic scans (which create 3D images through digital sectioning) reveal that the facial shape in the fossil samples is practically indistinguishable from humans nowadays, there are differences in the skull structure. In particular, the structure of the cranium, or skull bone, is different.
Composite reconstruction of the earliest known Homo sapiens fossils from Jebel Irhoud (Morocco) based on micro computed tomographic scans of multiple original fossils. Dated to 300,000 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. Credit: Philipp Gunz, MPI EVA Leipzig.
Compared with modern humans, in the Jebel Irhoud specimens we see a more elongated shape of the braincase, plus elongated temporal bones (on the sides of the skull, forming the temple), flatter parietal bones (along the sides and top of the skull) and different occipital shape (at the rear of the skull). This results in a longer and lower braincase.
Anatomically, this analysis places the 300,000-year-old Jebel Irhoud Homo sapiens somewhere between Homo erectus and African archaic Middle Pleistocene hominins. In this way it challenges the hypothesis that Homo sapiens derived from a later intermediate species that lead to the emergence of both the modern human and Neanderthal lineages.
These differences have implications for our understanding of Homo sapiens evolution. They suggest that our facial shape was established early in our history, whereas certain cognitive functions may have appeared later with the evolution of the Homo sapiens lineage and modifications of the braincase.
How we dated the Jebel Irhoud samples
In addition to the fossils, stone artefacts found at the Jebel Irhoud site consisted of Middle Stone Age material of Levallois technology (a distinct method of creating stone tools), with a high proportion of retouched tools, especially pointed forms.
Materials from Jebel Irhoud were accurately dated using two distinct techniques.
Thermoluminescence (TL), which works by measuring the irradiation dose received from the surrounding sediment since the material was last exposed to heat or fire, was applied to the stone artefacts.
Techniques known as Coupled Uranium-Series and Electron Spin Resonance (together referred to as US-ESR) were applied to the fossil remains directly. By calculating the diffusion of natural elements (uranium and thorium) into the dental tissues as fossilisation occurred over time, combined with the amount of surrounding radiation from the sediment the enamel crystal has been exposed to, we are able to work out the burial time of the remains.
While TL established clearly a chronology for the sediment layers which contained the hominin fossil remains, the US-ESR dates gave a direct age of one of the Jebel Irhoud jawbones (Irhoud 3 mandible). This was possible through new high-resolution radioactivity measurements of the geological context and important methodological improvements which allowed us to understand the impact of irradiation onto the tooth enamel crystal structure.
Both dating of the jaw bone and the dating of the stone artefacts gave us an age of 300,000 years.
A complex human journey
The new find is consistent with a picture of complex, Africa-wide origin of Homo sapiens.
The observed skull shapes and calculated ages of the bones and tools have implications for interpreting other human-like fossils found in Africa. The enigmatic partial skull of Florisbad from South Africa is also a very important Middle Stone age sample, and presents a mix of modern and ancient features.
Whether Florisbad should be classified as Homo heidelbergensis or Homo helmei has been a subject of prolonged debate. But with the anatomical features observed in Jebel Irhoud specimen, Florisbad skull can be more securely described as an early Homo sapiens form.
This discovery is also interesting in light of the newly dated Homo naledi from South Africa. This species, which survived until 250,000 years ago, overlaps in time with the Jebel Irhoud specimens. As Homo naledi represents a different branch of the genus Homo (not a direct ancestor to Homo sapiens), this is evidence that more archaic forms of hominins coexisted with the early representative of our species.
Similarly, Homo erectus Salé – also discovered in Morocco, not far from Jebel Irhoud – dates back to 250,000 years ago and might have coexisted with the early form of Homo sapiens, although the identification and age of the Salé specimen remains highly debated.
More than ever, as humans our complex evolutionary journey to end up as the lone surviving hominid species appears to be anything but linear. With African archaic Middle Pleistocene hominins overlapping in time with the Jebel Irhoud specimens, our discovery advocates for a complex African-wide evolutionary history of Homo sapiens. This new evidence adds more detail to the debate around the birthplace not only for our species, but also for the entire genus homo.
The famous drawing of a linear and simplistic evolution from ape-like individual morphing to an upright modern human is anything but accurate.
This work was performed by teams of scientists from around the world, including those in Australia, Germany, USA, Morocco, France, UK and Italy.
We acknowledge resources from AusSMC in preparing this article.