Our Evolutionary History,

Beginning with Australopithecus afarensis, despite it likely not being a species that we homo sapiens can directly trace lineage to, there existed many important characteristics within this species. The Australopithecus afarensis existed from about 3.9 to 3 million years in Eastern Africa—probably survived for almost one million years [1]. In this species, distinct human and ape-like features are expressed however, evidenced by Lucy, looked very different from the more modern and related species that are found after afarensis. Afarensis had a flat-nose and strongly projecting lower jaw as well as a rather small brain—around 500cm³—a third of the average human size [2]. Furthermore, afarensis had small  canine teeth like that of the modern human and most likely regularly walked upright while also retaining arboreal ability. Stanley Rice notes that bipedalism was a major adaption afarensis had due to the location of the opening for the spinal cord and the pelvis was found suitable for walking; the feet, however, were only partially adapted for walking upright. Furthermore, Stanly Rice writes that this suggests that afarensis lived mostly on the ground but frequently scrambled into the trees when danger was present [3]. The arms of the afarensis substantiate the possibility of afarensis retaining the ability to still be able to be arboreal. According to the Savanna hypothesis, bipedalism evolved—or further evolved—due to some common ancestor beginning to live within savanna biomes. Sexual dimorphism is quite overt in afarensis as well; the average male is found to be about 151cm while females average at 105cm. Dental microwear seems to support that afarensis most likely had a plant based diet that could likely include small vertebrates [4]. So, essentially, the afarensis is important due to quasi-human and ape characteristics as well as it being very related to the modern human despite the probability of the afarensis not being on our direct evolutionary lineage. 

   In a sense, our direct roots come more recently with the Homo habilis which is one of the earliest members of the Homo genus existing 2.4-1.4 million years ago [5]. The most notable differences from habilis and afarensis are that habilis had larger brain and smaller face but, however, retained an ape-like face and arms. The habilis is probably most known as the first tool-maker despite some earlier species also having made some tools a bit before habilis is known to have existed. Also, during the lifetime of the habilis' lifetime, there existed other similar species that would also be capable of creating tools; to claim that habilis was the first is rather difficult. However, it is most probable that the habilis was able to use stone tools with a decent proficiency. Regardless of which species was the first to make tools, tool-making increases efficiency, namely, in food gathering—flora and fauna. The hard enamel of the habilis suggests that the species' diet consisted of rather hard foods like nuts, tree roots, woody plants, animal tissue and so forth. It is also quite probable that habilis practiced marrow eating as evidenced by fossils of marked bones that suggest this [6] [7]. The importance of the practice of consuming the bone marrow substances is due to the nutritional benefits that may have boosted the performance efficiency of habilis [8]. The overall ability and anatomy of the Homo habilis allowed for its species survival until the species branched off into, probably, Homo ergaster or Homo erectus. It is in this sense, that habilis succeeded within its species' lifetime.  

   After Homo habilis comes Homo ergaster which is sometimes synonymous with Homo erectus but is also used to differentiate between the early and late Homo erectus; the former is sometimes named as the Homo ergaster but, for simplicity, the Homo ergaster and erectus will be referred to as Homo erectus [9]. Homo erectus, upright man, comes into existence sometime around 1.9 million years ago and moved out of Africa into lower Asia—Middle East, Indian subcontinent, lower China [10]. The Homo erectus' important contribution towards to evolutionary history is that it may have been able to make fire, advance hunting skills and tools as well as possibly having some semblance of a language and culture [11]. The important further development of tools the erectus is credited with is the development of Oldowan or Acheulean tools which are a bit more complex than the tools made by previous homo species[12]. Important anatomical developments include the species' growth heights comparable to the modern human and increased cranium size compared to earlier species viz. Homo erectus' average brain size was about 69% the size of the modern human but 50% larger than the Homo habilis[13] [14].  Furthermore, Homo erectus was likely to have been able to run or walk great distances sufficient enough to be able to hunt small and medium size game—an ability still retained by the homo sapiens[15]. The combination of both the further development of hunting skills and brain development may have been due or led to erectus being able to learn the patterns of vultures and scavenge more meat than the previous species [16]. This begins a positive feedback loop; the more meat consumed, the better the brain development and the better development, the more potential to consume more meat. In regards to appearance, essentially, the Homo erectus physically appeared much like a modern human from the neck downwards with a somewhat ape-like face and large muscles. In regards to the dental aspects of the Homo erectus, the teeth—size and molars in particular—clearly resemble the modern homo sapiens. This being so, it is quite probable that their diet included softer foods than previous species as well as an increase in meat consumption that contributes to the success of the Homo erectus. Due to the various developments like the ones stated above, Homo erectus, with its foundation based upon Homo habilis, further succeeded its predecessor and later evolved into, namely,  Homo sapiens and Homo heidelbergensis—quite likely the predecessor of homo neanderthalensis. 

   Lastly, we have the Homo heidelbergensis. The heidelbergensis evolved from Homo erectus partially within Africa and out of Africa [17]. This species existed from about 700,000 to 200,000 years ago in eastern and southern Africa as well as parts of Europe and possibly Asia [18]. Very related to the modern homo species genetically and physically, heidelbergensis had a large browridge and braincase as well as a flatter face compared to those of the previous species [19]. On average, heidelbergensis maintained a shorter height than the previous species which most likely served the purpose to conserve heat in order to survive in the less tropical and more colder climates out of Africa [20]. Apart from the change in physical characteristics, the heidelbergensis further developed its tools and possibly began to practice the burying of their kin as well as build small huts and mastered its control of fire [21]. The further development of tools resulted in the making of the first spears that could be used to thrust or throw—some being two meters long [22]! This could be the reason as to why heidelbergensis is credited with being able to hunt large game. It is suggested that the European branch of heidelbergensis later evolved into the neanderthalensis whilst the African branch evolved into the Homo sapiens.  

   In summary, the evolution of the later Australopithecus and the Homo habilis, erectus, and heidelbergensis features/demonstrates slight change over time. The slow transition from the more ape-like ancestors slowly began to appear much more human. These slow transitions are often correlated with changes within climate, biome, diet, tool development, and so on which, in turn, slowly begin to alter the species' evolution. The ability of the hominid species to have adapted to their environment enabled the species to live long enough to evolve and survive natural selection. Millions of years of the evolutionary struggle is what our species, Homo sapiens, are the result of so that we could be here—fantastic! 


  

1: "Australopithecus Afarensis." Australopithecus Afarensis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
2: "Australopithecus Afarensis." Australopithecus Afarensis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
3: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print. 
4: "Homo Habilis." Homo Habilis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
5: "Homo Habilis." Homo Habilis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
6: "Homo Habilis." Homo Habilis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
7: McPherron, Shannon, and Et Al. "Evidence for Stone-tool-assisted Consumption of Animal Tissues before 3.39 Million Years Ago at Dikika, Ethiopia." Nature. Nature, 12 Aug. 2010. Web. 5 Nov. 2015. 
8: “Bone marrow adipose tissue is an endocrine organ that contributes to increased circulating adiponectin during caloric restriction,” Cell Metabolism, 3 July. 2014. Web. 5 Nov. 2015. 
9: O'Neil, Dennis. "Early Human Evolution:  Homo Ergaster and Erectus." Early Human Evolution:  Homo Ergaster and Erectus. Behavioral Sciences Department, Palomar College. Web. 5 Nov. 2015. 
10: O'Neil, Dennis. "Early Human Evolution:  Homo Ergaster and Erectus." Early Human Evolution:  Homo Ergaster and Erectus. Behavioral Sciences Department, Palomar College. Web. 5 Nov. 2015. 
11: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print. 
12: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print. 
13: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print. 
14: O'Neil, Dennis. "Early Human Evolution:  Homo Ergaster and Erectus." Early Human Evolution:  Homo Ergaster and Erectus. Behavioral Sciences Department, Palomar College. Web. 5 Nov. 2015. 
15: O'Neil, Dennis. "Early Human Evolution:  Homo Ergaster and Erectus." Early Human Evolution:  Homo Ergaster and Erectus. Behavioral Sciences Department, Palomar College. Web. 5 Nov. 2015. 
16: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print. 
17: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print. 
18: "Homo Heidelbergensis." Homo Heidelbergensis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
19: "Homo Heidelbergensis." Homo Heidelbergensis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
20: "Homo Heidelbergensis." Homo Heidelbergensis. Smithsonian Institution, 5 Nov. 2015. Web. 5 Nov. 2015. 
21: Rice, Stanley A. Encyclopedia of Evolution. 1st ed. Vol. 1. New York: Facts On File, 2007. 468. Print.
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