Since Charles Darwin published The Origin of Species in 1959, we have come a long way towards learning about the evolution of many species on Earth. It was the first widely accepted theory of evolution since Darwin's research on finches in the Galapagos Islands provided groundbreaking concrete evidence of natural selection. Through the use of deductive reasoning and application of the scientific method, advances in dating technology and environmental reconstruction have contributed to a breadth of knowledge about the ancestors of many living species. This knowledge has helped us understand how evolution may have occurred for a variety of species throughout the history of our estimated 4.5 billion-year-old planet. Hominins, which is the designation used in taxonomy to refer to all species regarded as human-like in ancestry or similarity, are of particular interest. Understanding hominins allows those in today’s fields of science to reflect upon an understanding of our evolution to gain insight into the potential future of modern humans, Homo sapiens.
Thus, when 15 artifacts of a new species, Homo naledi, was discovered within the Dilanedi Chamber of the Rising Star Cave in South Africa in 2013 (Berger et al., 2015), much of the collective scientific community paid close attention. The stages involved in answering some of the essential questions may determine exactly where it fits into the hominin phylogenetic tree, the family tree-like diagram which displays evolutionary relationships on a geological time scale. In addition, these fossils were thus far the largest collection of any single species of hominin that had been discovered in Africa as of 2015 (Berger et al., 2015). Any new discovery means another insightful clue into our evolutionary chain. This would help us gain an understanding of the differences between us and our ancestors that may not only explain our current behavior but one day aid in the survival of our species. In the case of Homo naledi, these stages of discovery involved a variety of archaeological research methods, such as “optically stimulated luminescence (OSL) dating in combination with paleomagnetic analyses, [sent to multiple independent labs to prevent bias], to provide ages for the fossils and surrounding deposits in the Dinaledi Chamber, and build upon the geological context.” (Dirks et al., 2017) Basically, they used reliable scientific dating methods. This data would then be used to make further comparisons to what we know of other extinct hominins, living hominid primates, and humans, followed by evaluations and further research by other members of the scientific community.
Homo sapiens are the only known living hominin species in existence, with 99.9% genetic similarities with every other living human being today. We have also apparently taken over Earth, with an estimate of 7.9 billion of us wreaking havoc on our planet’s resources, interfering with natural processes that have maintained its order and disrupting ecosystems. So what makes us so unusual? We know humans differ from other mammals in our locomotion, obligate bipedalism, which means we walk habitually upright (without a tail), conserving our energy and freeing up our hands for tool making, hunting, and carrying objects or infants. As omnivores, we are also generalist eaters, able to eat a variety of food that ensures survival in varying environments. We have unusual manual dexterity and longer opposable thumbs, giving us an ability to grasp and manipulate objects with precision which helped us make finer tools and crafts as well as accomplish other tasks, contributing to an increased chance of survival in different environments against different predators and those we compete with for resources. Finally, we have our advanced brain, with its relatively large ratio to body mass, which some scientists speculate may have contributed to our cognitive functions, our emodiversity (a wide range of emotions), emotional processing, and our advanced ability to communicate in more complex manners. This sets us apart from our primate relatives and aided us in advanced social behaviors which includes transmission of knowledge and caring for members of our group. As a result, the locomotion, diet, hand structure, and cranial capacity would be some of the key traits of the Homo naledi to focus on in determining not only where it might fit into the hominin phylogenetic tree, but also, in discovering which of these traits may have contributed to the other. For instance, it may provide further insight on whether our advanced intellectual abilities contributed to the continued technological advancement of our tools and allowed us to adapt more easily to a wider range of environmental conditions.
When they first discovered Homo naledi, the earliest hypothesis was that it may have been 2 million years old. Through “measuring the concentration of radioactive elements, and the damage caused by these elements (which accumulates over time), in three fossilized teeth, plus surrounding rock and sediments from the cave chamber,” scientists now report that the fossils are most likely between 236,000 and 335,000 years old (Dirks et al., 2017). The earliest anatomically modern human found to date was found in Jebel Irhoud, Morocco, around artefacts and fossils that, through thermoluminescence dating, is estimated to be between 254,000 and 349,000 years old. (Richter et al., 2017) This is notable, because it means that Homo naledi and Homo sapiens may have coexisted on our planet. Of course, the distance between the Rising Star Cave in South Africa and Jebel Irhoud, Morocco, is around 11,000 kilometers or 6,800 miles, so we aren’t quite sure as of yet if they ever actually interacted. Further evidence may be harder to find since fossilization doesn’t naturally occur unless specific environmental conditions are met.
Despite the Homo naledi existing around the late Middle Pleistocene, it seems to show signs of mosaic evolution - a concept that evolutionary changes took place in some parts without simultaneous changes in others. It shares traits typical of both the Australopithecus genus, known to have lived between 2-4 million years ago, and the Homo genus, known to have lived from 2.4 million years ago until now. This makes Homo naledi quite unusual compared to other hominins of its era. For instance, its hand has a wrist and long thumb that is considered adaptive for "intensified manual manipulation" such as that required in tool-making, typical of the Homo genus. However, it has longer, more curved finger bones, typical of the Australopithecus genus, which indicates a regular use of its hands throughout life for grasping, climbing, and suspension. (Kivell et al., 2015) To date, no actual tools have been discovered near the fossils, which, despite its high likelihood, makes it difficult to say for certain if they created or used any. Homo naledi has locomotor traits similar to others of the Homo genus, including the longer lower limb, strong gluteus maximus insertions, and a humanlike ankle and foot, which suggests “enhanced locomotor performance for a striding gait.” (Berger et al., 2015) This way that we walk is what sets hominins like modern Homo sapiens apart from primates. As far as the Homo naledi diet, based on the discovery of significant dental chipping, it likely consumed tougher food that was harder to break down, similar to the diet of other Australopithecus, but with more abrasive particles such as dust or grit, indicating a distinct ecological niche from other South African hominins. (Berthaume et al., 2018) It either lived in an environment with relatively limited food options or wasn't as creative with its methods of obtaining food. Its cranial capacity is, much like the Australopithecus, relatively small at 465-560 cc (Garvin et al., 2017) compared to the Homo sapiens’ average of 1350 cc. To compare, Homo erectus, known to have lived as long as 2 million years ago, had a cranial capacity of around 1000 cc. The Homo naledi is so far the only species discovered dated to that timeframe of 254,000 and 349,000 years ago with as many ancient features of the Australopithecus. Ultimately, despite its similarities with the ancient Australopithecus, overall, it seems to share more in common with the Homo genus, hence its species designation, Homo naledi.
Another human trait that seems most distinguishable from other living mammals is our ability to communicate, cooperate, and collaborate with each other in the advanced way that we do. Other mammals certainly travel in groups and communicate with each other, such as primates or dolphins, however, we tend to develop advanced ritualistic behaviors and practices that are learned through what some refer to as “cultural transmissions.” Cultural transmission is the process through with attitudes, beliefs, language, and social norms are taught to each other within groups. This leads to speculation of why as many as 15 Homo naledis were found in close proximity to one another. Initial observation indicated that there was no visible evidence of bite marks, cuts, scrapes, burning, or other damage to the artifacts by carnivores, rodents, hominins, or stone tools, other than “limited incidental recent breakage by cavers (Dirks et al., 2015).” The logical hypothesis would be that they were either trapped or buried there, implying what might potentially be a cultural mortuary practice. Deliberately disposing corpses in a specific location in a ritualistic manner might, then, imply the capacity for mourning, an emotional response, or attachment. It may imply a sense of awareness of one’s own mortality that is more commonly attributed to uniquely “human” traits - traits more commonly found in Homo sapiens. (Egeland et al., 2018)
Even in the absence of concrete evidence of cultural behavior, the key finding here is that, as published by Berger et al., “Humanlike manipulatory capabilities and terrestrial bipedality, with hands and feet like H. naledi, an australopith-like pelvis and the H. erectus-like aspects of cranial morphology that are found in H. naledi. Enlarged brain size was evidently not a necessary prerequisite for the generally human-like aspects of manipulatory, locomotor, and masticatory morphology of H. naledi” (Berger et al., 2015). Whether their brains were small or large, scientists have discovered that many species in the Homo genus seem to share a capacity for social behavior, forming foraging groups for more efficient hunting, gathering, and organization. (Holloway et al., 2018) This revelation is quite paradigm-shifting. This also suggests that the ancient Australopithecus could have also had at least some of the social or organizational skills attributed to more recent species of hominins. Once more, this leads to the question, how did we, Homo sapiens, not only survive, but also climb our way to the top of the food chain while other hominins went extinct? Is it possible that the extinction of Homo naledi is partly due to the fact that, unlike the Homo erectus that migrated out of Africa as early as 1.8 million years ago, it never migrated out of an increasingly challenging environment? Hopefully, as more discoveries are made and more advanced dating methods surface, we may get closer to finding the answer.
References
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