The term “human” is not a scientific term, although it is used quite often to describe hominin characteristics. In this paper I define being human as having a specific set of biological characteristics. These biological characteristics emerged with Homo erectus and remained in almost all the subsequent hominin populations with the notable exception of Homo floresiensis, due to the insular dwarfism that occurred on the island of Flores that species has been found on. These characteristics are: an average height of over 5 feet, smaller dentition, a robust bone and muscular structure, a nuchal crest, an average cranial capacity of over 800 cm3, and greater encephalization. In this paper I will discuss how these characteristics made a shift in human evolution and why I consider these hominins to be human, compared to the earlier Australopithecines and their different characteristics.
Sahelanthropus tchadensis existed around 7.0 mya and is thought to be the earliest known bipedal hominin. There is only one fossil representative of the species: a cranium found in modern day Chad, Africa at a site called Toros-Menalla (Lewis et al. 208). This is an important discovery as this cranium has features of a chimpanzee cranium, such as a small cranial capacity estimated around 350 cm3, yet it also has features similar to later hominin species, such as non-ape like teeth and a more vertical face. The most important feature, however, is the intermediate location of the foramen magnum, suggesting bipedalism, although it is not definitive evidence. Either way, this species was something different than apes, possibly beginning the hominin lineage. The next group of hominins to emerge are the Australopiths, ranging from 4.2 mya to around 1 mya. This is the longest surviving group of hominins to have lived, spanning over 3.2 million years (Lewis et al. 213). None of these Australopiths have any of these biological characteristics that define being human, and as such aren’t considered to be human species. The first of these to discuss is Australopithecus afarensis. The most significant site for A. afarensis is the Laetoli site in Tanzania; here researchers found one of the most complete fossilized skeletons of any hominin ever discovered (Lewis et al. 214). From this skeleton, and a minimum number of 60 other individuals, paleoanthropologists have determined that A. afarensis shared more characteristics with late Miocene apes, than later hominins, who display more derived traits (214-215). In comparison with my definition of “human”, A. afarensis has large dentition with sharp canines, a smaller estimated cranial capacity of 420 cm3 and a height of 3-4 feet for females and 4- 4.5 feet for males (216). As stated above, these characteristics are all reminiscent of earlier primates and aren’t very similar to “humans” or more human-like hominins. These hominins, while playing an important role in human evolution, would be considered pre-human. The next group of hominins include Australopithecus africanus discovered in South Africa, and a very derived group of hominins given the name Paranthropus. A. africanus is similar to the earlier A. afarensis in that it has a small brain (~440 cm3), large dentition, and is not much more like the Homo genus than A. afarensis is (220). The derived Paranthropus species ranging from 2.5-1.2 mya had very large dentition, suggesting a diet of very hard foods. They also had massive mandibles and big skulls in general; their brain size, however, was at its maximum at around 510-530 cm3. These characteristics, while different from the earlier australopiths, are actually less human-like and show an odd branch of our evolutionary history. There is no evidence for Paranthropus after around 1.0 mya so they are thought to have gone extinct with no direct descendants, and no connection to the Homo genus.
The last of the australopiths to be discovered was Australopithecus sediba, discovered in 2008 in South Africa; the fossils there were radiometrically dated to just less than 2.0 mya. This is an important discovery because of the physical characteristics of A. sediba’s skeletal structure. The cranial capacity remains small estimated at 420 cm3 and it retains some more primitive features, such as curved fingers and primitive traits in the feet, yet it does also have some new features unseen in other Australopiths. These new characteristics are short fingers and possible brain reorganization, these traits more closely resemble derived traits of the Homo genus (222). This evidence is very new however and there are still more fossils to discover from these sites. While these traits are similar to those of the Homo genus, they still aren’t very close to the biological characteristics defining humanity set at the beginning.
The next genus to be discussed is the one containing our own species, Homo sapiens sapiens, the Homo genus. The first of the genus is Homo habilis, emerging as early as 2.3 mya. This was an important discovery because this was the earliest evidence of stone tool use yet, and its name translates from Latin into “handyman”. The Oldowan stone tool group is the earliest stone tool tradition and there is evidence for it dating back to 3.3 mya. The archaeologists who discovered H. habilis believed these hominins to be our direct ancestors as well as the inventors of stone tools, but the new evidence suggests an earlier species, possibly an Australopithecine (223). While it was placed in the new genus, many characteristics of Homo habilis more closely resemble the Australopiths than the later Homo species. Most notably, the cranial capacity did increase by about 20% to an average of 631 cm3, but still nowhere near the 1000+ cm3 of the later species; this species still had many ancestral traits, and most likely looked like the australopiths as well (Lewis 222, 223). Somewhere around 2 mya, a new species emerged, Homo erectus, this species is the first hominin to leave Africa and start spreading itself around the Middle East, Europe, and Asia. Also, starting around 1.7-1.6 mya, a new type of tool manufacturing developed; it is called the Acheulian type of tools. Homo erectus begins this massive shift in biological characteristics and cultural developments; the average H. erectus male weighed well over 100 pounds, averaged over 5.5 feet, and had a very robust body structure, being very large compared to all previous hominins. Homo erectus’ cranial capacity averaged 900 cm3, but specimens recorded are as high as 1250 cm3 which is very similar to the size of Homo sapiens sapiens 1400 cm3 (Lewis 236). Despite this large brain size, it should be noted that the brains of H. erectus are less encephalized than later members of the Homo genus. Furthermore, there is evidence of less fur on the body of Homo erectus indicating an adaptation to a warmer environment, as well as for endurance and more efficient body regulation. Also, the appearance of a nuchal crest on this species indicates a life of running that is new to the hominins. This nuchal crest allows for the stabilization of bodily muscles while moving quickly, allowing for faster movement, which prevailed as a favorable advantage. For me these massive shifts in biological characteristics marks the onset of humanity in its sense. These allowed us to emerge from Africa, as well as manipulate our environment in a new way that completely set us apart from our Australopith ancestors. The new type of stone tool development, the Acheulian industry, also was a significant upgrade for the hominins and clearly spread from Africa with Homo erectus around Asia and Europe. These new tools showed a more advanced method of making tools as well as specific intent on which type of tool was going to be made and what how it was to be used, which is a significant development. Homo erectus also was a more efficient scavenger, able to make wide use of more resources, most likely leading to the wide range of success in traversing the Old World (Lewis 249). The next hominin, and what is considered to be our direct ancestor is Homo heidelbergensis ranging from 850 kya to 200 kya and located in Europe and Africa. The cranium of this species is similar to that of Homo erectus, yet also has some differences that make it unique, such as the occipital area on the skull has less of an angle, the cranial vault bones are thinner, and the base of the skull has generally more modern development than that of Homo erectus. A cultural development to note with H. heidelbergensis is that of the site of Sima de los Huesos, in northern Spain. There is the earliest evidence there of deliberate body disposal of the dead anywhere yet, that dates to around 600-400 kya (Lewis 257). This shows a new thinking pattern, and that this species is thinking differently than the one before it, showing more and more of these “human” biological and cultural traits. The next species is Homo neanderthalensis, emerging 130 kya, these hominins were extremely robust. Their average cranial capacity was in fact larger than Homo sapiens averaging 1520 cm3. This larger brain size is thought to be associated with better metabolic efficiency in the colder climates this species is found in. All of these robust adaptations, large muscles and barrel chested included, are hypothesized to be adaptations to the harsh and colder climates (Lewis 263). There is also clear evidence that Neandertals and humans were living in close proximity during the middle and upper paleolithic periods, sharing stone tool knowledge and methods (267). There are also archaeological sites involving Neandertals burying their dead, and also depicting cave art, as is the site of Cueva de los Aviones in Spain, where researchers found Neandertal cave art and beaded artifacts dating to at least 65,000 years ago (Greshko). This shows advanced thinking and expressive capabilities that are clearly human.
Having human biological characteristics such as a higher cranial capacity, greater encephalization, a taller height, an occipital bun, and smaller dentition are all defining traits of humans, these combined with technological achievements and cultural developments are what define what it means to be human. With that being said, the species of the genus Homo, beginning with Homo erectus and ending with Homo sapiens sapiens, are considered to be human based upon these traits; all other hominins before, while containing some of these traits, do not possess enough of them to be considered fully human.
Works Cited
Lewis, R. Barry., et al. Understanding Humans: Introduction to Physical Anthropology and Archaeology. Wadsworth, 2013.
Greshko, Michael. “World’s Oldest Cave Art Found-And Neanderthals Made It.” National Geographic, National Geographic Society, 22 Feb. 2018, news.nationalgeographic.com/2018/02/neanderthals-cave-art-humans-evolution-science/.
