For a long time, Australopithecus afarensis was considered as the earliest known bipedal hominid. This made this species, which was dated to have existed around 3.5 million years ago, the oldest member of the human clan. However, the discovery of the species, Australopithecus anamensis pushed the age of the bipedal hominids further back by around 600000 years (Tomasik, 2002).
History of the Discovery of Australopithecus anamensis
In 1965, the first fossilized specimen of A. anamensis was discovered by a research team from the Harvard University in the Kanapoi region that lies on the west of Lake Turkana in Kenya. Although at that time the fossil, which was a humerus or arm bone, was not identified to be of this particular species, it was assigned to the Australopithecus genus and dated to be about 4 million years old. However, for further evidence to be procured about this early hominid, it took around 20 more years. In 1987, Allan Morton, an archaeologist from Canada, found pieces of the fossil of this species jutting out from the sides of a weathered hillside lying east of Allia Bay near the Kanapoi site (Tomasik, 2002).
Seven years later, in 1994, Meave Leakey, a Kenyan but London-born paleoanthropologist, along with Alan Walker, an archaeologist, excavated additional fossil fragments of this species at Kanapoi and Allia Bay. Among the specimens that Leakey and Walker found, one was of a full lower jaw bone that resembled that of a modern day chimpanzee, although its teeth seemed more like that of humans. A year later, in 1995, after much study and having noted the difference in the features of the fossil from that of Australopithecus afarensis, Leakey and her co-workers assigned the fossils to a new species they named as A. anamensis. ‘Anam’ in the Turkana language means ‘lake’ and thus the name of this species refers to site at which its fossils were discovered (Dory and Blaxland, 2010).
In all, since the first discovery of the humerus of A.amanensis, 21 fossils have been found in and around the Kanapoi site. These fossils include fragments of the upper and lower jaws, upper and lower parts of the leg bone as well as fragments of the cranium (Smithsonian National Museum of History, 2014).
Much later, in 2006, more fossil finds of A.anamensis were announced. These discoveries were made in Ethiopia and they included one big canine tooth and a femur or thigh bone (Dory and Blaxland, 2010).
Place of Australopithecus anamensis in Human Evolution
Fossilized evidence of A.anamensis was found between two sediment layers. A lower jaw was discovered in the older layer and was dated to be around 4.12 to 4.17 million years old. In the upper layer of the sediment, the bones of the limbs were found which are said to be around 3.5 million years old. Based on the dating of these fossils, done using the 40 Argon/39 Argon method, A.anamensis has been dated to have lived around 4.2 to 3.9 million years ago (Tomasik, 2002).
Since A.afarensis is said to have survived around 3.5 million years ago, many scientists opine that the fossils of A.anamensis and A. afarensis are representative of a single species that had perhaps evolved over time. Even though up till now the mid-section fossils of A.anamensis have not been uncovered, the number of specimens unearthed over a period of 40 and more years is enough to show that A.anamensis was more primitive than A. afarensis. The jaw fossils of this species also indicate that it probably descended directly from Ardipithecus ramidus, which has been dated to have survived around 5.8 to 4.4 million years ago (Tomasik, 2002).
Important Features of Australopithecus anamensis
The important physical features of A.anamensis that distinguish it from its predecessors and successors are its body size, brain size, jaws, teeth and limbs. The body size of this species is thought to be similar to that of the modern day chimpanzees. One interesting feature is that within A.anamensis, there was a clear difference between male and female body sizes and up to an extent shown by the present day orangutans and gorillas. This implies that sexual dimorphism existed in this human ancestor (Dory and Blaxland, 2010). A male A.anamensis is estimated to have weighed around 47 to 55 kilograms while the females are thought to have weighed much lower at 27 kilograms. Additionally, it is postulated that the males of this species were probably 1.5 meters tall, whereas the females were shorter at just around 1 meter (Tomasik, 2002).
As far as the brain size of A.anamensis is concerned, fossil specimens indicate that this species had a relatively small brain. However, the exact cranial capacity of the species is not yet known (Dory and Blaxland, 2010).
The fossils of the jaw bones of A.anamensis clearly show that this species had a prognathic or an elongated, ape-like face (Ward, Leakey and Walker, 2001). The jaw was shaped such that the rows of teeth were placed straight back and in parallel lines. Further, in the lower jaw, the bone beneath the front teeth sloped backward instead of protruding in front into the chin. Additionally, the upper jaw of A.anamensis had a shallow palate (Dory and Blaxland, 2010).
The teeth size of this hominid was intermediate to that of apes and humans. The tooth enamel was thicker and the canine teeth were shorter and less pointed than that found in the ancestors of A.anamensis (Ward, Leakey and Walker, 2001). With respect to the canines, a male-female difference is clearly evident. As compared to the females, the males of A.anamensis had larger canines. This species also had a gap between its rows of teeth, called as diastema, which allowed it to close its mouth comfortably (Tomasik, 2002).
Fossil evidence of A.anamensis suggests that to a great extent, its limbs were more human-like than resembling that of apes. As found in humans, the shin bone of A.anamensis was concave in shape and thick enough to support a bipedal gait (Ward, Leakey and Walker, 2001). The condyles of the tibia were also similar in size to that found in humans. The hand tendons of this species are predicted to be strong, based on the marks found on the wrist bones. Also, instead of rigid locking elbows that are commonly found in apes, the elbow joint of the A.anamensis was more flexible, like that in humans (Dory and Blaxland, 2010). Two other human-like features of the limbs of this species include absence of a grasping toe and a stable knee joint that would have helped the hominid to walk upright. At the same time, a few ape-like features of the limbs of this human ancestor are that it had long forearms and curvy fingers that were probably useful for tree climbing (Tomasik, 2002).
Behavior of Australopithecus anamensis
Based on the site where the fossil specimens of A.anamensis were found, the habitat of the species is thought to be a warm floodplain, which could have been a forest, open grassland or a bushland situated around lakes. The area would fall somewhere between what is now southern Ethiopia and northern Kenya (“Origins of”, 2001).
Fossil evidence, particularly, the strong hand tendons, suggest that this species might have been an expert climber. Tree climbing might have helped it to hunt for food and to escape predators. Fossilized bone fragments of A.anamensis also show that the creature was built powerfully and was able to walk upright when on the ground (Tomasik, 2002).
Since the jaws of A.anamensis were strong and had thick enameled teeth, it is predicted that the species might have eaten hard, coarse foods, such as tough to chew fruits and nuts. On the whole, the diet pattern of A.anamensis is thought to be ape-like, with the species being an herbivore (Dory and Blaxland, 2010).
Although no implements made by this hominid have been unearthed yet, it is suggested that the species might have used simple tools such as those used by modern day chimpanzees. Some such tools include plant materials such as sticks and twigs that can be quickly modified. These tools might have been used for procuring food. Stones whose shapes were unaltered might also have been employed by A.anamensis as tools to process nuts and other hard foods before consumption (Dory and Blaxland, 2010).
Very little is known about the culture and lifestyle of A.anamensis as there is limited evidence pointing to these (Tomasik, 2002). However, existence of sexual dimorphism suggests that polygamy might have persisted among the males of the species (Dory and Blaxland, 2010). While a male-to-male competition might have existed to catch the attention of females, decreased canine size implies that this tooth might not have been used in such a competition (Tomasik, 2002).
On the whole, fossilized evidence of A.anamensis shows that this species predates many others that have been assigned to the same genus (Ward, Leakey and Walker, 2001). This species is also easily distinguishable from A.afarensis and existed before it. Given this human ancestor’s social behavior and eating patterns that co-existed with the evolution of bipedality, it is postulated that these three must be correlated.
Dorey, F., & Blaxland, B. (2010). Australopithecus anamensis. Retrieved from http://australianmuseum.net.au/Australopithecus-anamensis
Origins of Humankind. (2001). Retrieved from http://www.pbs.org/wgbh/evolution/humans/humankind/c.html
Smithsonian National Museum of History. (2014). Australopithecus anamnesis. Retrieved from http://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-anamensis
Tomasik, B. (2002). Australopithecus anamensis. Retrieved from http://www.sccs.swarthmore.edu/users/09/btomasi1/australopithecus-anamensis.pdf
Ward, C.V., Leakey, M.G., & Walker, A. (2001). Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya. Journal of Human Evolution, 41, 255-368. doi:10.1006/jhev.2001.0507