Australopithecus boisei - Leakey ancestors

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The Skull Replica
This half scale model is a reconstruction based on a fossil skull discovered in 1959 by Mary Leakey at Olduvai Gorge, and a mandible (jaw) discovered in 1964 by Kamoya Kimeu at Peninj (both sites are in Tanzania). The skull has the accession number OH 5 and was nicknamed "Dear Boy" by Louis and Mary Leakey shortly after its discovery. It is more commonly referred to as "Zinjanthropus" or "Zinj" for short after the generic name that Louis Leakey gave it in his initial publication when he named the skull Zinjanthropus boisei. Zinj is an ancient Arabic word for the East African coast and anthropos is Greek for man. Charles Boise helped to finance the field work that led to the discovery of Zinj. Louis later recognised that the skull's true affinities lay with the australopithecines and that it should therefore be called Australopithecus boisei. Today the skull is kept in the National Museum of Tanzania in Dar es Salaam. The Peninj mandible is temporally housed in the National Museum of Kenya in Nairobi but will eventually be returned to Tanzania.

The OH 5 skull is of particular importance because its discovery proved Louis' theory that East Africa would provide important evidence of early hominids and that Olduvai Gorge had tremendous potential for finding early human ancestors. After the discovery of 'Zinj' the National Geographic Society in Washington D.C. provided substantial funding enabling Louis and Mary to concentrate their work at this enormously important site. Zinj was the first of a series of spectacular discoveries made in East Africa over the succeeding thirty five years.

Dr. Stephen Wagner, a scientific sculptor, created the restoration under the guidance of Dr. Meave Leakey. The missing pieces have been filled in to demonstrate the morphology (shape) of a complete skull for comparison with the other complete skulls in the series. A replica of the facial skeleton of OH 5 (unrestored) will also be available in consistant scale.

Overview
Australopithecus boisei is one of three species included in the 'robust australopithecines' a group of early hominids distinguished by their enormous cheek teeth and very reduced incisors and canines. The large cheek teeth require huge muscles to apply the appropriate forces during chewing. As a result they have large chewing muscles and a distinctive cranial shape which reflects the mechanical requirements of these muscles.

The 'Zinj' skull has a small brain case. Compared to the heavy body, the brain was not much larger than that of an a modern African ape. Because the brain case was small, the large chewing muscles (called the temporal muscles) had only a small surface area on which to attach. The large crest along the top of the brain case serves to increase this area in the muscular males. A similar crest is seen in male gorillas and baboons, species which also have small braincases and large chewing muscles. Females are more lightly built and have smaller muscles so generally lack these prominent crests. (diagram to illustrate the temporal muscle)

The facial region of the skull of A. boisei is very distinctive being short, deep and rather massive. Unlike gorillas and chimps the face probably did not protrude very far. Structures associated with the powerful chewing muscles are emphasised. The boney arches that extends outwards either side of the skull (the zygomatic arches) are large making room for the large chewing muscles which pass through the arch to attach to the mandible.(diagram to illustrate the zygomatic arches, diagram to illustrate temporal muscle)

The jaws are extraordinarily robust. So much so that Louis Leakey often referred to this early hominid as "Nutcracker Man" because he felt the jaws were strong enough to crack nuts! The mandible is thick and deep. The cheek teeth (molars) are large, low crowned and with thick enamel. The premolars are also large and look rather similar in shape to the molars. All the cheek teeth have a large surface area for chewing on tough foods. In contrast, the incisors and canines are very small and must have been rather unimportant for chewing. (diagram to illustrate structure of the teeth and show the thick enamel)

A. boisei is probably the best known Pleistocene hominid in East Africa because the strong, robust jaws are frequently preserved in the fossil record whereas the more lightly built mandibles of other species are more easily destroyed. Many more male specimens than female are known for this reason, and many more mandibles than bones of the skeleton.

Important Finds
The discovery of Zinjanthropus (OH 5) and the Peninj mandible:
Mary Leakey discovered the Zinj skull quite by chance in 1959. A few days earlier one of their field assistants had found a hominid tooth in a fragment of jaw at a site where stone tools had been reported. Louis and Mary decided this would be a good site to excavate in detail and had invited a friend to film the excavation from start to finish. They therefore delayed the start of the excavation for a few days while awaited the arrival of the photographer. It was then that Mary made her discovery.

The following account of the discovery is taken from Mary Leakey's autobiography Disclosing the Past:
"Then Louis got an attack of flu and retired to bed. So it came about that on the morning of 17 July I went myself, with two Dalmatians, Sally and Victoria, to see what I could find of interest at nearby Bed 1 exposures. I turned my steps towards a site not far west of the junction of the two gorges, where we knew that bones and stone artefacts were fairly common on the surface of Bed 1 sediments. The site was known as FLK, one of the two that Louis had named after Frida, before I knew him.

" There was indeed plenty of material lying on the eroded surface at FLK, some no doubt as a result of the rains earlier that year. But one scrap of bone that caught and held my eye was not lying loose on the surface but projecting from beneath. It seemed to be part of a skull, including a mastoid process (the bony projection below the ear). It had a hominid look, but the bones seemed enormously thick - too thick, surely. I carefully brushed away a little of the deposit, and then I could see parts of two large teeth in place in the upper jaw. They were hominid. It was a hominid skull, apparently in situ, and there was a lot of it there. I rushed back to camp to tell Louis, who leapt out of bed, and then we were soon back at the site, looking at my find together."

The Peninj mandible was found in 1964 when Richard Leakey and Glynn Isaac had returned to the Peninj site with a team of field workers to make a more detailed survey. Richard Leakey recalls the discovery of the Peninj mandible in his autobiography One Life.:

"It was while I was away in Nairobi that the excitement began at Lake Natron. Kamoya discovered a complete lower jaw of 'Zinjanthropus' projecting from a cliff face just a few feet from where I had myself been searching before my trip back to Nairobi. What a moment it was! I received the news from Glynn who reached me via the radio telephone at my girlfriend's home. I contacted my parents at Olduvai by radio the next day and flew there to collect them before I returned to Peninj. I had asked Glynn to leave the discovery well alone so that my parents could see it prior to its excavation and also to enable the work to be adequately photographed.

"There it was, 'my' - or more accurately 'our' - first discovery, and at that time the only known lower jaw of the East African species, Australopithecus boisei , previously known only by my mother's find at Olduvai in 1959. "

Where it is found?
A. boisei is found in the East African countries of Kenya and Tanzania. The first specimen was found at Olduvai Gorge in northern Tanzania.

Other sites where A. boisei has been found
Fossil remains of A. boisei have been recovered at sites in Ethiopia, Kenya and Tanzania. Many specimens have been recovered from the Lake Turkana Basin in northern Kenya and southern Ethiopia. The International Omo Expedition, led by Clark Howell, Yves Coppens and initially (in 1967 only) Richard Leakey, recovered many specimens, as did Richard Leakey's expeditions to the Koobi Fora Formation east of the Lake in the late 60's and 70's, and to the Nachukui Formation west of the lake in the 80's. Australopithecus boisei has also been recovered from other sites in Kenya including Chesowanja in the Baringo basin to the south of Lake Turkana.

Classification
Louis Leakey believed that OH 5 was so different from Australopithecus robustus that he named a new genus, Zinjanthropus aswell as a new species boisei . Later he realised that the similarities with A. robustus were close enough that it should be included within the genus Australopithecus, although the differences from A. robustus were large enough that the species name "boisei" was retained. The science of making these distinctions is called taxonomy. Each creature is described in a hierarchy from the broadest catagory to the most narrow, very much like the game "twenty questions" - is it an animal? Does it have a backbone?

The following shows the taxonomic sequence for Australopithecus boisei:

Kingdom Animalia
Phylum Chordata
Subphylum Vertebrata
Class Mammalia
Order Primates
Superfamily Hominoidea
Family Hominidae
Subfamily Australopithecinae
Genus Australopithecus
Species boisei
Notice that the word endings of the superfamily is always "oidea", of the family is "idae" and of the subfamily is "inae".

When we study fossils we are trying to find out as much as we can about the species that the fossil remains represent. What they ate, how they moved, where they lived, and how they lived.

Many different methods are used to study and analyse the fossil specimens. After comparisons have been made with species that are already known and the fossil has been attributed to an appropriate genus and species, detailed studies are carried out. The size of the brain can reflect relative intelligence. The size of the brain can reflect relative intelligence. Zinj's brain was relatively small showing that Australopithecus boisei was not much more intelligent than a gorilla or chimpanzee.The shape (morphology) of the bones of the skeleton can indicate how the animal moved. The shape and size of the teeth indicate whether the species ate meat, insects, fish, fruits or leaves. Zinj had very large teeth, larger than any other hominid. Teeth may also be examined in more detail under an electron microscope: the tiny scratches and pits on the surface indicate the type of food that was eaten.

Studies of A.boisei teeth show many deep pits and grooves showing that the diet was coars and fibrous. Fragments of teeth show the structure of the enamel: enamel thickness can indicate whether the food was hard or soft. Zinj had very thick enamel and the microstructure of the enamel shows crack-stopping structures: both feastures suggest hard objects were eaten and chewed. The enamel micro structure can show whether seasonal fluctuations resulted in food scarcity and times of stress. Other analyses of oxygen and carbon isotopes in the teeth can indicate whether the animal was eating grass or leaves, providing an indication of the habitat. These days many medical techniques are applied to the study of fossils. As well as regular X-rays which show internal structure, more detailed reconstructions of the internal morphology can be made using CAT scans. In the most sophisticated techniques, the digitised shapes of the specimens can be deployed to reconstruct mirror images of the missing parts and even to make computer generated plastic images of the reconstructions. (illustrations to show X-rays, micro wear on tooth surface, enamel micro structure)

Australopithecus boisei remains are often found with animals that live in relatively well vegetated wet habitats.

It is important to put our ancestors into an environmental context. What sort of country were they inhabiting? Were they living in forests, bushland or open grasslands and savanna? Various clues help us to learn about past environments. The fossil animals that we find preserved alongside the remains of our ancestors can tell us alot about the palaeoenvironment. If we find animals that are mostly browsing on leaves, then the environment was forest or woodland, grazing animals largely inhabit more open grassy country. Some animals are found in a variety of habitats but others are much more habitat specific. For example monkeys are usually found where there are trees, hippos and waterbuck indicate the presence of water, zebra and wildebeest are strictly grazers are only found where grass is plentiful. Rodents are particularly helpful in tenvironmental reconstructions, but because they are so small they are often difficult to find. Some animals lived in different habitats in the past and the further back in time that one invistigates the more difficult it is to accurately interpret the habitat from the fauna.

Evidence of vegetation if it can be found is more helpful. The study of fossil pollen gives us a good indication of the habitat. Plant species can be identified from their pollen and if this can be found it provides and accurate indication of the prevailing vegetation. Unfortunately it is hard to find pollen in many deposits. Sometimes fossilised leaves or fruits may be recovered and these again are useful indicators.

Geologists can reconstruct past habitats from the rocks. The types of rocks indicate the presence of lakes or rivers. Intervals when the landscape dried and became desiccated is indicated by altered rocks, mud cracks, and other features. Fossil soils can tell us about conditions prevailing when they were developed.

Environmental interpretation is dependant on a combination of clues from the animals, the plants and the geology.

Links
Links will open new windows.

The Anthropology 365--Human Evolution site was created as a set of lecture notes by Dr. Randy Skelton for his class at the University of Montana. It is an excellent resource. His lecture notes on the robust astralopithecines apply to Zinj.

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