One of the Biggest Mysteries in Human Evolution

A marked prominence of the superorbital region is manifested in the most remarkable manner in the Neanderthal cranium, which must have given the human visage an unusually savage aspect, so wrote Professor Hermann Scharfhausen in his 1861 paper, titled, On the Crania of the Most Ancient Races of Man. Indeed, the superorbital region, or brow ridge, is a very distinctive morphological trait in most of our hominin ancestors. There are many questions regarding this unique feature. Why did ancient humans have such large brow ridges? What purpose does this feature serve? Is the lack of brow ridges related to self-domestication in modern humans? It's very hard to come to firm conclusions about ancient human history. When talking about hominid history, it's hard to be definitive. There are few samples to study in the fossil records and huge gaps in our understanding of how human anatomy changed, and when. For example, Homo erectus is a key species in the hominin fossil record for the study of human evolution, being one of the first species discovered and perhaps the most documented, but also because of its long range and having dispersed out of Africa earlier than any other human species. Aldervi hominid number 9, known as OH9, is a fossilized skull cap of an early hominin, found in Aldervi Gorge by Louis Leakey in 1960. It is believed to be about 1.4 million years old. Its cranial capacity is estimated at more than 1,067 cubic centimeters, the largest value among all known African Homo erectus specimens. OH9 is significant because of the features it carried, and its correlation to the species classification it resides in. But most anthropologists have regarded it as Homo ergusta, or as Homo erectus ergusta. To the extent that proponents of the use of Homo ergusta define ergusta as a separate species, instead of an African Homo erectus. Cranial bone thickness has been widely qualified for Homo erectus, but rarely quantified. It's quite often that throughout craniums, the thickness varies between those different hominids. Yet in OH9, compared to other Homo erectus, it had the biggest cranial capacity and one of the largest superorbital thicknesses, of 18.5 millimeters. Indeed, OH9 has a robust brow ridge that allows it to stand out among other Homo erectus specimens. What's more, the brow ridge made it difficult to determine whether this cranium should be classified as Homo erectus or a different species. Cranial bone thickness is key when determining whether a specimen found is Homo erectus. According to Spanish anthropologist Roberto Cieza, a few hypotheses around the purpose of the large brow ridge include, the capacity to dynamically express affiliative prosocial emotions, through highly mobile eyebrows, protection of the skull and the eyes against blows, and or a signaling effect, accentuating aggressive stares. Thus, its large size could have been sexually selected through generations. A team of researchers set out to learn more about why ancient humans had these distinctive brow ridges, and why they eventually lost them. Oversized brow ridges had a social function, and as they shrunk in size, humans were able to better communicate subtle emotions. And it turns out the erectus brow ridge is overbuilt. Simply put, the brow ridges of our ancestors did not seem to serve a mechanical function. They were possibly a social signal, and a sign of strength and dominance. For Homo erectus, prominent brows were just plain sexy, a desirable trait in a mate. I wanted to thank the sponsor of this video, Atlas VPN. Right now, Atlas VPN is running a huge discount. This means you can get a three-year subscription for just $1.83 a month plus three months for free with a 30-day money-back guarantee. What is a VPN? VPN stands for Virtual Private Network, a service that protects your internet connection and privacy online. I personally use Atlas VPN anytime I am using a public Wi-Fi network to protect from hackers and get around having websites blocked or having speeds throttled. No one likes to be watched or tracked, even if they have nothing to hide. That's why it's important you step up your privacy game. Hackers have many methods to steal your data on public hotspots, but with a VPN your online traffic is invisible to them. And remember, for a limited time Atlas VPN is running a huge discount, which means you can get a three-year subscription for just $1.83 a month plus three months for free with a 30-day money-back guarantee. Time is running out, so get your deal by clicking the link in the video description below. Whereas the strong brow ridge on Homo erectus signaled strength, our relatively massive foreheads allow for empathy. It's an instance where evolution of our physical anatomy reveals more about our minds than our environments. But as time passed, the need to assert dominance was less important and the need to empathize and communicate with others became more important. And that's why the heavy brow ridges of our ancestors may have morphed into the lofty foreheads of today. Over the years, scientists have put forth a number of theories about why ancient humans had a protruding brow ridge. Most of these theories focused on structural and mechanical explanations. A thick brow bone may have protected ancient humans from blows to the head, reinforced the jaw, shielded the eyes, or even prevented our ancestors' hair from obscuring their vision. To test these ideas, researchers created a 3D model of an ancient human skull from X-ray analysis of a fossilized skull known as Cabeway 1, which is currently housed in the Smithsonian. The skull came from an individual of the Homo heidelbergensis species, which lived between 700,000 and 200,000 years ago, and may be a common ancestor of Neanderthals and modern humans. The researchers then played around with the computer model, shrinking the size of the brow ridge to see if it would affect the mechanical stresses of chewing. But they found that a smaller ridge did not reduce stress on the skull. The team also discovered that the brow was larger than necessary to fill in the space between the forehead and eye sockets. Maybe, the pronounced brow ridge did not serve a structural or mechanical function. Maybe the purpose of the large brow was social. Indeed, Homo erectus's thick brow bones may have been caused by higher levels of testosterone. The species' entire skeletons were, in fact, thicker than those of modern humans. The thick brow ridge isn't necessarily designed for social signaling. It could just be a consequence of other differences in body chemistry. The pattern of bone thickness distribution, observed in Homo erectus and early Homo or African Homo ergaster erectus, appears to be a unique trait among early hominids. The significant features of the OH9 skull also consisted of the structure of the skull. The occipital bun thickness, which is the back of the head, was also fairly thick compared to most Homo erectus, with a thickness of 18.5 mm. OH9 having the largest early cranium capacity, implied that major differences in the development of cognitive capabilities existed between Homo erectus and anatomically modern humans. These traits, like the robust brow ridge and skull thickness, confirmed that Homo erectus kept evolving in different areas of anatomy. Homo erectus is now known to have been the first hominid of virtually modern human body size and proportions. Yet the time between the demise of Homo herbilli, about 1.6 million years ago, and the appearance of Homo heidelbergensis, now put at approximately 600,000 years ago, has been aptly termed the muddle in the middle. This 1 million year period of time was when hominids left Africa, and populated most of the old world. Homo erectus was the species responsible, and we are only now learning how this amazing event happened. In fact, some anthropologists even prefer to give Homo erectus a new name, Homo sapiens erectus. According to researchers, our communicative forehead started off as a side effect of our faces getting gradually smaller over the past 100,000 years. This process has become particularly rapid in the last 20,000 years. Modern humans are the last surviving hominin. While our sister species the Neanderthals were dying out, we were rapidly colonizing the globe and surviving in extreme environments. This had a lot to do with our ability to create large social networks. Eyebrow movements allow us to express complex emotions as well as perceive the emotions of others. A rapid eyebrow flash is a cross-cultural sign of recognition and openness to social interaction, and pulling our eyebrows up at the middle is an expression of sympathy. Tiny movements of the eyebrows are also a key component to identifying trustworthiness and deception. Indeed, eyebrows are the missing part of the puzzle of how modern humans manage to communicate with each other than extinct hominins.