The archaic hominins
Time has moved on a bit since last week, both in the real world and in the world of this blog. Now, the archaic hominins are coming in to their own and the date is between 2.4 and 4.5 million years ago.
The archaic hominins are fossils that are regarded as being almost definitely more closely related to humans than chimpanzees, alhough it is very difficult to tell whether any of them were our direct ancestors. The majority would have been evolutionary offshoots which ultimately did not stay around to give rise to any modern species.
The earliest fossils we call an archaic hominin are assigned to the species Australopithecus anamensis (southern ape of the lake – fossils are from near Lake Turkana in Kenya) dating from 3.9 – 4.5 million years ago (Mya). The main bipedal features we know about in Au. anamensis are in the tibia. The top of the tibia has two articular surfaces separated by a ridge (see fig.1) which articulate with the condyles of the femur to form the knee joint. In Au. anamensis the tibial articular surfaces are concave in order to provide a large contact area for the femoral condyles, helping to decrease stress at the knee joint during upright standing and walking. Furthermore, the lower or distal end of the tibia is more expanded than in the putative hominins, suggesting a greater weight bearing role.
Australopithecus afarensis (southern ape from Afar, in Ethiopia) is probably one of the more famous species thanks to the remarkably complete (40%) example known as Lucy (fig. 2). Along with Lucy, the diverse fossil remains of the ‘first family’ from Hadar, Ethiopia provide some excellent examples of bipedal-adapted morphology (Johanson & White, 1979) dating from between 3.0 and 4.0 Mya.
If anything, the pelvic anatomy of Au. afarensis suggests an exaggerated adaptation to bipedality, with a very expanded sacrum (base of the spinal column), a small font-to-back dimension of the pelvic cavity and very highly flared iliac blades. A marked bicondylar angle shows that Lucy’s knees were held close to her body’s midline for efficiency in bipedal locomotion, while tibial morphology is very similar to that of Au anamensis described above.
There are also remains of foot bones for Au. afarensis from which suggest that the lateral (outside) bones of the foot would have formed a stiff lever to allow efficient weight transfer during walking. Despite, this medial (inside edge) foot bones and those closest to the ankle (proximal) most resemble human bones when subjected to detailed shape analysis. Despite this, the best evidence that Lucy and her contemporaries were bipedal comes from the set of bipedal footprints uncovered at Laetoli, Tanzania (fig. 3) (Leakey & Hay, 1979). These footprints made by 2 adults and a juvenile suggests that the pattern of weight transfer through the foot was quite modern, with the heel contacting the ground first and the centre of mass passing from the lateral to the medial side of the foot with each step.
Despite this almost unanimous evidence for a modern bipedal gait, it is likely that Lucy and co. still spent quite a lot of their time in the trees and were not habitual or obligate bipeds. The glenoid fossa of the scapula was still quite superiorly orientated, and relative sizes of the attachments for the rotator cuff muscles were intermediate in size between modern humans and chimpanzees. Vertebrae of Au. afarensis also show patterns of damage repair similar to those caused by the stresses of arboreal locomotion in modern chimpanzees.
The final transitional hominin that is informative on the evolution of bipedal locomotion is Australopithecus africanus (southern ape of Africa), all fossils of which have been recovered limestone formations in South Africa. The nature of limestone makes reliably dating these fossils very difficult, althoguh the best estimates suggests that Au. africanus walked the Earth between 2.4 and 3.0 Mya.
The foramen magnum of Au. africanus is again positioned more forward on the base of the skull than in the other archaic hominins, and anticipates the condition seen in the robust australopiths, of which more next time. Au africanus is the first species in which we can reliably detect a lumbar lordosis to help balance the bodies centre of mass over the base of support, although the relatively small size of the vertebrae again suggest that Au. africanus would not have been a habitual biped with constantly weight-bearing vertebrae. There is also evidence of highly flared iliac blades in this hominin species, while various foot bone fossils including Stw 573 ‘Little Foot’ fossil suggest that the lateral side of the foot possessed a human-like stiff arch and the medial side remained flexible and ape-like, while the distal bones including the metatarsal heads and phalanges are also human-like.
It’s definitely worth noting that the lateral and distal foot is most similar to modern humans in Au. africanus, while Au. afarensis above had proximal and medial foot bones most similar to the modern pattern. This could come in useful in later posts if we try to work out where the modern form of bipedalism came from.
Next time, prepare yourself for the weird and wonderful phase of hominin evolution as we explore the robust australopithecines.
eFossils, 2008. Australopithecus africanus: Sts 5. Available from http://www.efossils.org. Accessed: 05/10/2014
GIRLintheCAFE: The Laetoli Footprints. Accessed 10/05/2014.
Johanson, D., & White, T. (1979). A systematic assessment of early African hominids. Science, 203, 321–330.
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.
Wikimedia Commons. Image available from http://upload.wikimedia.org/wikipedia/commons/3/31/Lucy_blackbg.jpg. Accessed 10/05/2014.