Paranthropus Robust Australopiths

It is becoming more popular to refer to the robust species of the australopiths with their own genus Paranthropus. According to the rules of Linnaean classification, groups must share a single ancestor, but it is possible that robust australopiths evolved independently in East and South Africa from the australopiths in each region (A. afarensis and A. africanus respectively), meaning they could have separate roots. But for clarity and for continuity with current trends, here the robust australo-piths are referred to as Paranthropus.

Although morphological links between the australopiths and Paran-thropus are evident, there is no mistaking Paranthropus with their distinct skull and tooth morphology. Paranthropus, which includes Paranthropus aethiopicus, Paranthropus boisei, and Paranthropus robustus, makes up an evolutionary dead end in the hominin phylogeny. It is an extinct lineage that branched off from australopiths around 2.5 Mya and then hung around long enough to coexist with early Homo in East and South Africa until about 1 Mya.

Paranthropus cranial capacities range from 410 to 530 cc. Although they shared an enlarged brain size and bipedal capability with A. africanus and A. afarensis, species of Paranthropus had markedly larger teeth and jaws.

Paranthropus used their large flat molars in their large jaws to process fibrous foods. Four of a modern human's molars would fit on one

Paranthropus molar, yet Paranthropus was only half as tall as average adult male humans. Their skulls boast prominent attachments for the chewing muscles (temporalis), including a sagittal crest along the top of the skull (similar to a small bony mohawk). Their large, flared cheekbones and postorbital constriction make room to accommodate those same muscles running from the jaw to the sagittal crest, which would have been as big as a pound of steak on either side of the head. Based on the cranial and dental morphology and also on isotopic analysis of the composition of tooth enamel (see Chapter 5), we know that Paranthropus ate a diet of tough fibrous foods and hard seeds, nuts, and fruits. Average adult body masses were about 33 kg for females and 45 kg for males (range of about 70-110 lbs) with a stature comparable to that of A. afarensis.

P. boisei (pronounced boy-zee-eye) is probably the most well known of the genus as it boasts the complete cranium (OH 5) that Louis Leakey found at Olduvai Gorge in 1959. (OH stands for "Olduvai Hominid".) Leakey originally placed OH 5 in a new genus Zinjanthropus and it earned the nickname "Zinj." The OH 5 skull was the first hominin to be accurately dated (1.75 Mya) by the potassium/argon method. Out of the three Paranthropus species, P. boisei has perhaps the most exaggerated molars compared to the size of the small incisors, and has "hyper-robust" cranial crests for the chewing muscles, which led to the nickname "Nutcracker Man" (Figure 3.7). The species ranged from 2.5 to 1 Mya and specimens of P. boisei are also found in Tanzania and at Koobi Fora, Kenya.

P. robustus fossils come from the South African sites of Sterkfontein, Swartkrans, and Drimolen and have skulls and teeth that are like less exaggerated versions of P. boisei. They spanned from about 2.0-1.5 Mya. P. robustus is affiliated with a record of bone tools at the Swartkrans cave site. In the first half of the 20th century, Raymond Dart interpreted these tools as weapons that were part of an "osteodontokeratic" (bone-tooth-horn) culture used by "killer" ape-men. However, since Dart's time, much more observations of chimpanzee behavior have been made and new microimaging techniques for artifacts and bones have become available. With modern knowledge it is clear that the bone tools from Swartkrans were used for digging tubers out of the ground and some were also used as wands for fishing termites out of their mounds, similar to the way chimpanzees obtain the insects with twigs.

The beautifully complete skull that Alan Walker discovered in the early 1980s on the west side of Lake Turkana, Kenya, is the best known specimen of the earliest species, P. aethiopicus (2.7-2.3 Mya). It is known as the "Black Skull" (KNM-WT 17000) for the color it became as it fossilized. (KNM stands for Kenya National Museum and WT stands for West Turkana.) The cranium resembles an aerodynamic, road-hugging sports car because of its prognathic face (like the hood) and its sharp cranial crests, particularly the horizontal one across the back of the head (like the spoiler). P. aethiopicus brain size is estimated to have been very small at about 410 cc and it is considered to be ancestral to P. boisei and P. robustus.

There is no clear explanation for why Paranthropus went extinct about 1 Mya. It is intriguing that a group of small bipedal vegetarians would go extinct just as taller, carnivorous H. erectus was spreading across the Old World. What a striking parallel to the present-day vanishing of great apes in the presence of humans.

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