What are some of the key structural osteological (bone) changes in Hominids over time beginning with Australopithecus and ending with modern Homo sapiens. Consider feet, knees, hips, hands, and crania (skull). What is the significance of these changes?
Hominids have displayed a steady progression of evolution both genetically and physically leading up to the modern Homo sapiens. Some of these developments led to "dead ends" but others led to species such as our own.
Cranial changes provide the greatest amount of information and are the most easily identifiable to certain species and their place on the evolutionary timescale.
Earlier hominids such as Australopithecus anamensis and Australopithecus afarensis display primitive hominoid cranial characteristics such as a prognathic face (in which the lower cranial features project outwards), postorbital constriction, and a nuchal crest. Prognathic facial construction was due to the V-shape dental arcade you often see in current primates. Australopithecines would have had a dental arcade somewhere between the extreme V-shape of a modern Chimpanzee and the rounded U-shape of a modern human. Postorbital constriction is a narrowing of the cranium behind the eye sockets. This allows for limited frontal lobe growth in the brain. Modern humans no longer have this constriction, and the degree of postorbital constriction is a good indicator of how far along a specimen is on the evolutionary path - primates have the highest degree and early modern humans the lowest until the ultimate lack of postorbital constriction in modern Homo sapiens sapiens. The nuchal crest is a bony structure on the back of the head to which muscles attach that are fundamental in holding the head upright. You would find this feature in almost all early hominids. What is progressively less frequent later into the hominid evolution are sagittal crests. These are bony structures found on the top of the skull to which muscles for the jaw attach. This crest was important when primates and very early hominids ate very tough foods. As our diets changed, so shrank the sagittal crest until it no longer existed as seen in Homo sapiens sapiens.
Later Australopithecines had more "gracile" cranial structures. Simply put, this means the skull was more lightly built than those of the "robust" Australopithecines. In gracile Australopithecines such as Australopithecus africanus, there is no sagittal crest, the mandible is smaller, and there are larger anterior teeth than posterior teeth which makes the dentition overall a lot smaller and lighter.
Australopithecines often overlapped with the Paranthropus species in robust and gracile forms. Some robust Paranthropus are Paranthropus aethiopicus, P. boisei, and P. robustus. P. aethiopicus featured a compound crest (where sagittal and nuchal crests meet) suggesting extreme musculature, extreme prognathism, and prominent postorbital constriction. P. boisei had a very large and thick mandible and extreme megadonty (extremely large post-canine teeth). P. robustus had broad zygomatics, sagital crests, postorbital constriction, large temporal fossa, and large premolars and molars.
Later hominids such as Homo erectus, H. heidelbergensis, H. neanderthalensis, and H. sapiens had features more and more similar to those of modern humans as time went on (as compared to early hominids and primates).
- The supraorbital torus shrank: Originally meant to reinforce the skull due to pressure put on it by mastication muscles, as our diets changed to softer foods this shrank as did the sagittal crest.
- The postorbital constriction shrank/disappeared: This bone limited growth in the frontal cortex of the brain. This part of the brain allows for higher process thinking and decision making which is why these processes are more prevalent in those species with out postorbital constriction.
- Prognathism lessened: as our dentition changed to that of larger anterior teeth and smaller posterior teeth, we did not need the sharp V-shape or projection of the mouth to allow for ripping with canines. In comparison, later hominids had extremely flat faces when compared to their earlier cousins such as the Australopithecines. An exception to this would have been the prognathism seen in Homo neanderthalensis. However, rather than dentition it was sinuses that caused this. Enlarged sinuses allowed the species to warm the air of the cold climates they lived in before the air hit their lungs.
Modern humans have a relatively vertical and flat face with a forehead, the occiptal region of the skull is rounded, and they have chins. These changes may mirror cranial growth with brain changes or may reflect the decreased use of teeth as tools as compared to the next closest cousin, the H. neanderthalensis.
Other osteological changes mostly correspond to the increased rate of bipedalism. Compared to modern humans, early hominids such as Australopithecus africanus had very bowl shaped pelvic regions which are much more similar to modern human structures. This made the pelvis strong and more stable for the weight put on it in bipedalism. Long, thin pelvises such as those in chimpanzees are associated with quadrupedal locomotion. Pelvic structures suggest that many Australopithecines were both efficient bipeds and tree-climbers. As time goes on, later species become exclusively bipeds.
Their feet were longer and narrower and had arches, like ours, to support the intense weight focused on them. Toes became non-divergent and less like fingers.
Their legs became longer than their arms (which are more or less even in length in primates) and the muscles bigger to propel them forward more easily. The length of the leg may have come about to give muscles greater surface area for attachments.
Knee joints expanded to take on more weight and knee extension became more limited. This keeps the knees straight when walking to reduce energy loss on two legs.
In the same way, hip joints expanded to take on more weight. This brought the spinal column in closer and that, combined with a more lateral illium, creates more surface area for the glutius maximus muscle to grab onto and take some of the stress from the ball-and-joint socket. Hip, pelvic, and knee structure also focus in the center of gravity to increase balance.
Modern human hands have a greater trapezium and trapeziometacarpal joint to allow for great muscles stresses associated with gripping and squeezing. A short thumb compared to finger length ratio allows for more precision gripping. A broad distal phalangeal tuberosity (fancy name for fingertip) compared to the base of the finger allows maximum surfaces area for gripping and grabbing.