Homo habilis is a well-known, but poorly defined species. The specimen that led to the naming of this species (OH 7) was discovered in 1960, by the Leakey team in Olduvai Gorge, Tanzania. This specimen and its designation was the subject of much controversies up through the 1970s. The material was found in the same region where A. boisei had previously been found, and many researchers of the time did not fully accept that the material was sufficiently different from that material (or maybe A. africanus) to denote a new species. Louis Leakey was convinced that this was the Olduvai toolmaker he had spent his life looking for, and placed this as a direct human ancestor, with H. erectus a dead-end side-branch.
The specimen was subjected to intense study by the multidisciplinary team of Louis Leakey, John Napier, and Phillip Tobias. They placed the material as different from penecontemporary australopithecines due to the teeth, which fell outside the known range of A. africanus, with very large incisors. Also, the large brain size and shape of the hand suggested a closer affinity with Homo. In January 1964, the team announced the new species Homo habilis. The name was suggested by Raymond Dart, and means “handy man,” in reference to this hominids supposed tool making prowess.
Leakey believed that habilis was a direct human ancestor, with erectus out of the picture. While H. habilis is a generally accepted species, they opinion that it was a direct human ancestor seems to be in question. There are now at least two species of early Homo (whether habilis and rudolfensis or an undescribed species) living prior to 2.0 myr. In addition, H. erectus (which is almost universally accepted as a direct human ancestor) continues to be pushed further back into the paleontological record, making it possible that it is the first Homo ancestor of modern humans.
Other problems include that some people see KNM-ER 1813 as a near perfect erectus, except for its small brain and size. It could be an erectus that was at the small scale of a wide variation of traits, or it may belong to ergaster, which some believe to be the ancestor of erectus. The questions are far from solved, and new specimens are needed. Homo habilis may be a direct human ancestor, a dead-end side-branch that leads nowhere, an invalid species whose designated examples belong in other species, or Wolpoff may be right, and all these species are basically part of one highly variable widespread species.
It is particularity hard to list the features of Homo habilis, because the specimens attributed to habilis (and the reasons the material was placed there) vary widely. The species is a mishmash of traits and specimens, whose composition depends upon what researcher one asks. The simplest way to describe the general features is to describe specimens that are generally considered habilis by most people, and list their relevant traits.
OH 7 is the type specimen of habilis, and the first material attributed to the species. The specimen consists of a nearly complete left parietal, a fragmented right parietal, most of the mandibular body (including thirteen teeth), an upper molar, and twenty-one finger, hand, and wrist bones. The remains belonged to a 12 or 13 year old male. The brain size attributed to this specimen varies, ranging from 590–710 cc. P. Tobias and G. von Koenigswald used three traits to set habilis apart, as a transitional species between A. africanus and H. erectus:
- Expanded cranial capacity (relative to africanus).
- Reduced postcanine tooth size.
- The presence of a precision grip (determined from the hand bones present in OH 7), which provides the anatomical basis for tool-making.
General features of the specimen seems to support these three traits (whether or not it is transitional from africanus to erectus):
- Larger cranial capacity (though very problematic). Tobias gives an estimate of 647 cc, Holloway gives an estimate of 710 cc, and Wolpoff has estimated it at 590 cc.
- Molar megadontia is gone, with molars longer than they are wide.
- The P3 is smaller and more asymmetric.
- The P4 is much more similar to the P3.
- Metacarpal 1 and trapezium is much less interlocked, which allowed more movement.
- The distal phalanges have apical tuffs.
OH 8 – a fairly complete foot – was found nearby OH 7, and was initially determined to be from another individual. This was due to the fact that OH 7 was known to be from an adolescent around age 12, and the foot seemed to be of a more advanced age, due to the presence of arthritis in the specimen. However, the partially gnawed remains have arthritis due to a sustained injury, and the actual age runs close to OH 7, making it likely they are from the same individual (many researchers consider this part of OH 7 now, rather than OH 8). The remains show clear signs that this was an obligate biped, including:
- Presence of digital shortening.
- Enlargement of the hallux, as well as being fully abducted.
- Alignment of digits 2–5.
- Thickened metatarsal shaft with a humanlike cross-sectional shape.
- A fully developed double arch to the lower surface.
- Mechanically set up for efficient weight transmission at the ankle.
While it shows definite obligate bipedalism, the specimen also has a marked tubercule for the tibialis posterior muscle, an invertor of the foot that could be useful for climbing. So it is possible that while this individual was an obligate biped, it still spent some time in the trees (which goes well with paleoecological evidence that suggests that various hominid species spent most of their time in marginal woodland environments). From the talus, H. McHenry calculated an estimated weight of 31.7 kg. Using the various estimated brain size, one gets brain/body weight ratios of:
Wolpoff: 590 cc brain = 1.86%
Tobias: 647 cc brain = 2.04%
Holloway: 710 cc brain = 2.24%
Even using the smaller brain estimate, this is one of the largest relative brain size for any male hominid up to the time period this individual lived (1.75 myr). When compared to primate allometry, the OH 7 brain size is at the top of the allometric expectations within non-human primates. This is a large brained specimen relative to its body size.
Another relatively complete habiline specimen is OH 13 – “Cinderella”. This is a poorly preserved and fragmentary specimen of a 15–16 year old female habilis, dating to a little younger than 1.66 myr. This makes it one of the most (if not the most) recent habilis specimens known. The material consists of the mandible and the maxilla, several teeth, pieces of the cranial vault, and some postcranial elements, including a small piece of proximal ulna. This specimen (along with OH 16) were the object of much inaccurate brain size estimations, which originally lead to the two being classified as H. erectus. More recent estimates put the brain size at around 500 cc, and along with an estimated body size near that of AL 288–1, gives this specimen a relative brain/body weight ratio similar to OH 7.
The case of OH 16 is a tragic one. The specimen was discovered nearly complete near the end of a field day, so the position of the find was marked and roped off. The next morning the researchers were horrified to discover that a herd of cattle had charged through the area, and completely crushed it. Some of the specimen’s features include:
- Very large teeth (close to australopithecus in size).
- An uncertain brain size, but probably larger than OH 7.
- The individual was age 15–16 when they died.
- The individual had very bad caries on one side of its jaws (very unusual in ancient specimens), which lead to differential chewing on the other side, causing it to develop a huge temporalis muscle on that side.
- Cranial bone markedly thinner that erectus.
- Dramatic differences in the supraorbital torus and the nuchal torus that distinguish it from erectus.
These features (the last two shared with OH 13) seem to indicate that the specimen is a habiline, and not an erectus specimen, as was attributed by J. Robinson.
OH 24 (“Twiggy”) is the most australopithecine-like of specimens attributed to H. habilis, and may be more highly correlated with A. africanus. The specimen was found completely fractured, and cemented together in a coating of limestone. R. Clarke was the researcher who went through the long and painful process of reconstruction, but over 100 small fragments could not be placed in the reconstruction. Hence, the specimen is extremely distorted, making an accurate estimate of its brain size very difficult, though Holloway has given an estimate of 590 cc (many researchers believe that number is too high). Several features caused this specimen to be placed in habilis, including:
- Increased cranial capacity over australopithecines (though some doubt this estimate).
- Less postorbital constriction.
- Elongated molars.
- Absence of postcanine megadontia.
- Large front teeth relative to the postcanines.
- A broad and short cranial base.
- Anteriorly positioned foramen magnum.
- Less convex and bulging zygomatics, and more vertically oriented.
- A distinct maxillary notch.
While these features seem to support the notion that it is not an australopithecine, several other features do not support the habilis distinction. For example:
- Lacks a salient anterior nasal spine.
- Lacks broad nasal bones.
- Lacks nasal bone peaking caused by the internasal angle.
- Lacks the projection of the middle and top of the nose away from the face, shown by expanded and outward projecting maxillary bones to its side (i.e., maxillary pillar eversion).
While this specimen does not seem to be an australopithecine, it also does not seem to fit perfectly into the classic habilis mold. Perhaps it fits more closely with rudolfensis, or an undefined penecontemporary species. Another option may be that the distorted reconstruction is blurring a clear species designation. For now, a clear designation is up in the air.
The last to discuss is OH 62. Publicized widely as “Lucy’s Child” by Johanson, it is a very scrappy collection of 302 bone fragments. Portions of the maxilla (which permitted identification as habilis), parts of the femur, and upper limb bones. The entire specimen is problematic, and raises many questions as to sexual variation and behavior. The most controversial aspect of the specimen was the Johanson et al. calculation of a humero-femoral index of 95%. The material was far too sparse to calculate such an index, but even using their own estimated range of possible lengths for the incomplete femur, they should have computed an index quite close to the A.L. 288–1 value of 83.8%. It is unclear why Johanson et al. calculated the index in the manner they did, and it is generally not accepted in any form at all.
Homo habilis is a very complicated species to describe. No two researchers attribute all the same specimens as habilis, and few can agree on what traits define habilis, if it is a valid species at all, and even whether or not it belongs in the genus Homo or Australopithecus. Hopefully, future discoveries and future cladistic analyses of the specimens involved may clear up these issues, or at least better define what belongs in the species.
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