Human Evolution: A Description of Fossil Hominids and Their Origins
The primate family hominidae consists of two commonly accepted genera, Australopithecus and Homo. However, two other genera are proposed. The genus Ardipithecus has recently been described and placed as the earliest common ancestor to all hominids. Also, others accept the genus Paranthropus that I will here include as a member of the genus Australopithecus. I will outline the hominid family as having three genera, Ardipithecus, Australopithecus, and Homo.
When considering human origins it’s important to keep in mind the nature and structure of the primate evolutionary tree. Primates are the product of significant mammalian adaptive radiations which occurred throughout the early Tertiary, ~ 70–50 mya (millions of years ago). Within the development of extant (living) primates, there have also been several adaptive radiations (~ 50 mya-present). The emergence of hominids came from a common ancestor of extant apes and humans from approximately 6–7 mya. Modern apes, (chimpanzees), are not our immediate ancestors or progenitors but rather more appropriately, our siblings. We both exist in the temporal present and have only morphologically distinct parent taxa to serve as a common ancestor. According to this view, the australopithecines, like the ardipithecines, are hominid radiations. One species-level taxa from the ardipithecines gave rise to the australopithecine radiation. Likewise, from this radiation came one Australopithecus species that gave rise to the Homo lineage.
The latter family includes species such as H. habilis, H. erectus and H. sapiens. The relationship between the three commonly accepted species of Homo is subject to some controversy as recent analyses have concluded australopithecine affinities of H. habilis. Some view the emergence of H. erectus as the hallmark of the modern human race. This view is attractive for those wishing to argue regional continuity of H. erectus in Eurasia with modern human populations. This regional approach posits that the various species of hominids out of Africa are members of the same lineage, only separated by time.
The single species from this genus, A. ramidus, dates to approximately 4.5 mya. This earliest hominid has been identified from deposits in the locality of Aramis, Ethiopia. Most likely, this individual was part of a radiation that included other hominids and/or hominoids. Because of its recent discovery, age (and thus contentious value), descriptions remain unpublished. The specimen’s hominid affinities have been verified and we can thus infer morphological adaptations required of bipedalism. These include a valgus knee angle, anteriorly placed foramen magnum and most importantly, a short, broad pelvis.
This genus is made up of several species that fall under two general types, robust forms and gracile forms. Both of these types are known from fossils found in South Africa and East Africa. The robust forms persist from ~ 4–1 mya while the gracile forms appear ~ 4 mya but disappear ~ 2 mya. (It has been suggested that the latest australopithecine species gave rise to the earliest members of Homo.) It is likely that these hominids enjoyed a pan-African dispersal which we are unaware of due to the preservation biases of such great fossil hunting locales as are found in South and East Africa. The differences in type between the robust and gracile australopithecines is viewed as differences in adaptive profiles. The cranial and facial morphology of these two types indicates different diets and thus separate niches. This helps explain the co-evolution of these two groups on the same landscapes for 2 million years.
The adaptive profile of the robust species indicates reliance on a diet of fibrous, gritty vegetation. The extremely large cheek teeth, robust mandible and heavy bony architecture (i.e. zygomatic arch) which supports strong chewing muscles serve as convincing evidence for a diet requiring intense mastication. These forms appear ~ 4 mya and persist until ~ 1 mya. Throughout their existence, the robust australopithecines are sympatric with the gracile forms and later with early Homo. Robust species include A. aethiopicus, A. boisei and A. robustus.
The adaptive profile for the gracile australopithecines includes the ‘humanesque’ generalist advantage. That is to say that rather than specializing in hard-to-process herbivorous food, the graciles’ specialization is in generalism. The absence of bony crests for heavy chewing muscles, the presence of smaller cheek teeth with an emphasis on the anterior dentition and the generally ‘lighter’ appearance of the gracile species indicate this hominid as having a more opportunistic approach to carving out a niche for one’s self. These forms appear ~ 4 mya, the same time as their robust counterparts, and persist until ~ 2 mya. Gracile species include A. anamensis, A. africanus and A. afarensis. The recent discovery of A. garhi introduces a likely candidate for the origin of Homo. The discovery of this specimen associated with stone tools has forced the anthropological community to question the formerly held notion that lithic technology did not appear until the origin of Homo.
The emergence of the genus Homo, ~ 2.5 mya, precedes the later H. erectus species which is most likely the first hominid to leave Africa. The genus Homo includes species such as H. habilis, H. erectus, and H. sapiens (including Neanderthals). I will posit the view of the regional approach mentioned earlier. At the time of the dispersal of H. erectus out of Africa ~2 mya, the sapien human lineage was established. The establishment of separate populations of H. erectus in Eurasia created insular, regional groups that traded genes with neighboring regions. This model of gene flow argues that the regional variations we see in modern humans (i.e. racial differences), are the products of 2 million years of limited inter-populational gene flow as well as much intra-populational gene flow. This model can be viewed as several gene pools being established soon after the Out-of-Africa hominid dispersal. These ever-growing gene pools began to establish ever-growing connecting streams, which maintained enough genetic homogeneity, that a single, inter-breeding species remains. We, as modern humans, are the products of this pooling model.
It is during the origin of the genus Homo when we can be assured of the manufacture and use of stone tools. I will argue further that it is this rudimentary technology which allowed hominid dispersal out of Africa and the establishment of separate gene pools in Eurasia without causing geographic speciation. Geographic, i.e. allopatric, speciation results from the isolation of a gene pool coupled with the selection of certain individuals according to the new environment. By utilizing this early technology during out-of-Africa radiation, early hominids were able to buffer the environmental factor inherent in allopatric speciation. Thus genetic homogeneity was conserved.
According to many in the field the above model is insignificant to explain modern human variation. This subject of variation due to a 2 mya radiation out of Africa or a much more recent 100,000 ya radiation is the subject of extreme contention. It is difficult to accept either arguments without scrutinizing their evidence. I have chosen to approach the subject of modern humans from a regionalist model. However, there are fossil specimens, which are argued to be separate species from modern H. Sapiens. These fossils argued to have been separate species are H. heidelbergensis and H. neanderthalensis. These hominid specimens are aberrant and prove difficult to place in the above existing hominid categories. It is their uniqueness, I’ll argue, that support a model of linear continuity between species of Homo after the out-of-Africa dispersal.
This specimen has been recorded at localities spanning Africa, Mediterranean Europe and Eastern Europe at times ranging from ~ 600,000 ya – 200,000 ya. This specimen is unusual for it displays a mosaic of features, which are most similar to H. erectus. However the non-erectus features and temporal and spatial distribution make this specimen a likely candidate for the progenitor of the Neanderthal stock.
These popular specimens range in time from ~ 130,000 ya – 30,000 ya. Their spatial distribution is predominantly found throughout Western Europe. H. neanderthalensis have perhaps provided the most fuel for the out-of-Africa/ multi-regionalist debate. Their robust appearance has been argued by the out-of-Africanists to be the hallmark of a distinct species, reproductively isolated from modern humans. However, it is the contention of the multi-regionalists that the Neanderthals did not die out. Instead, the influx of anatomically modern Homo sapiens assimilated this aberrant population of Neanderthals. Modern Europeans would thus carry with them the genes of H. neanderthalensis.
Anatomically modern H. sapiens:
This group of hominids share the anatomically modern traits associated with us, humans. Their origin in the fossil record ~ 100,000 ya marks the time when modern traits first appear. The earliest modern H. sapiens have been found in South Africa and within a few thousand years these modern features are found throughout Eurasia. It is the contention of the out-of-Africanists that these modern humans at 100,000 ya spread rapidly throughout the Old World, competing with and ultimately leading to the demise of all other Old World hominids. The out-of-Africanists contend that old world hominid populations existing before the spread of modern human characters were not assimilated by modern features but rather disappeared as the result of extinction. The multi-regionalists contend that the disappearance of Old World hominid populations such as H. erectus, H. heidelbergensis and H. neanderthalensis were not the result of extinction. Modern human features and most likely culture, coming out of Africa, assimilated these Old World hominid populations.