9.4: Very Early Hominins

Most notable, our ancestors and their relatives became increasingly more intelligent. Our brains have increased in size more than four-fold, from a more chimp-sized brain (<400 cc) in the earliest hominins to a mean of ~1400 cc. This likely occurred in response to environmental stresses as well as competition with other hominins for resources. Skull size and shape changed in response to encephalization, i.e., increasing brain size.

Brains are very costly organs and researchers believe that in order for brain size to have increased, there would have had to have been a corresponding decrease in some other costly organ system. It is hypothesized that a higher quality diet allowed the hominin gut to shrink and, in turn, the brain to expand. Marked encephalization in the hominin lineage began with the first members of our own genus: Homo. While there is some evidence that earlier species (e.g. australopiths) manufactured tools, there is solid evidence that early Homo did, and the archaeological record suggests an increasing reliance on meat in their diet. 

Many of the early hominins had pronounced, forward-oriented jaws, termed prognathism (pro = forward; gnath = jaw). Over time, hominins became more flat-faced, or orthognathic. While extant African apes retain primitive prognathism and the shearing/honing dental complex, hominins lost those pronounced canines, as well as the gaps in the corresponding tooth rows—termed canine diastema (singular) or diastemata (plural)—that allow apes to close their jaws. 

Figure \(\PageIndex{1}\): Temporalis muscle: Originates on frontal, parietal, and temporal bones and inserts on mandible. (Zygomatic is shown as having been cut to reveal underlying muscle. Plate 382 from Gray’s Anatomy. “The temporalis” by Henry Vandyke Carter is in the public domain.

The size of jaw and neck muscle attachment sites on the skull became reduced in the hominin lineage over time, along with a reduction in the size of the teeth and craniofacial robusticity. The action of the powerful temporalis muscle (a muscle of mastication) changed from primarily acting on the front of the jaw, allowing apes to clamp their jaws powerfully shut during fighting, to acting on the molar region for grinding food (see Figure). The origin of the temporalis muscle moved over time from the midline of the top of the skull to a more inferior position on the lateral aspect of the frontal and parietal bones, due to the reduction of the sagittal crest and decrease in temporalis power in hominins.

Hominin fingers became shorter and lost their curvature over time. By the time of the australopiths, hands had become more dexterous. There is evidence that Australopithecus africanus possessed a “power” thumb, giving them increased abilities for holding objects in one hand while manipulating or working them with the other hand. This was necessary for our ancestors to have made and efficiently used tools. 

The hominins can be divided into three groups, based on shared characteristics and/or phylogenetic affinity:

1. Earliest bipeds: Orrorin,  Sahelanthropus, Ardipithecus
2. Bipeds that exploited a more open and drier niche with thick molar enamel: Australopiths, such as Australopithecus africanus and afarensis. Also the related Paranthropus
3. Hominins that retained the gracile masticatory apparatus of their australopith ancestors and exhibited a trend for encephalization and increasingly complex culture: Homo species.

Contributors and Attributions

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