Between 4 million to 1 million years ago there was a a very diverse group of bipedal hominins in East and South Africa called the australopiths. There are currently 9 specious of australopiths discovered and several of them lived concurrently at a time. They can be divided into two groups, the more gracile generally having a smaller build and a more generalized diet. While the robust have a tank of a skull that was very specialized for the food consumption in their niche. And based on there teeth between the two groups they had a very diverse diet that most likely varied between species. But what changed in Africa gave rise the the first hominin and would eventually lead to us?
Before 4 million years ago in Africa there were prevalent rainforests with species similar in morphology in the skull and diet to modern day African apes, gorillas, and chimps. Chimps and gorillas spend half of there waking hours chewing on mostly fruits and high fibrous plants. The shape of there molars with thin enamel are used for sheering there food and large canines used to peel the hard exterior of fruit. But by the epoch of the Pliocene(5.3-2.6 million years ago) there was this ongoing trend of the earth becoming cooler which made Africa drier. During the australopiths era, the cooling made wide spread open wood lands and grasslands. Which greatly diminished and scattered limited fruit. This would lead australopiths to a diet of less nutritious diet of leaves, stems, seeds, and also tubers and roots, and bulbs(which were year round and can survive droughts). With the change in the environment and a change in the opportunity of less nutritious foods would have strong selective pressure for adaptations to obtain the nutrients from there varying diet. Also because they had such a varied diet compared to the their ape counter parts, and lived in varying environments creating new niches creating a very diverse (textbook adaptive radiation) radiating genus separated into the two groups of gracile or robust Australopithecus.
Because of the hard, less nutritious, and fibrous foods they where primarily eating, unlike chimps in the rain forest, natural selection acted strongly on the components of mastication (chewing) to breakdown foods into smaller particles to easily get more nutrients. The components mostly changed for the adaptations to eat the new African environment are the shape, size, and thickness of teeth, jaw muscles, jaw bones, the maxilla and mandible.
There was big push for big grinding posterior teeth for there type of chewing that could withstand the endlessly repetition of powerful chewing and be able to bite forcible with out damaging the teeth. The forceful chewing also created thick molar enamel that could withstand the chewing. There was also a trade off for smaller canines as they weren’t being used as usefully as their ape counter part who used them for peeling tough fruit. You can see this progressive trade off from earlier australopiths who had bigger canines then the later specious of the genus. Apes also have thinner molar enamel and higher cusps in there molars for sheering there food unlike australopiths who grinded their food with thick enamel and flat molars.
They needed large jaw muscles to be able to chew so forcefully on their food. The robust australopiths are the extremists in this category for they had such large temporalis muscles they had a boney crest called a sagital crest for more muscle. They also had widely expanded zygomatic arches that some robust austslopiths faces were as wide as it was long. The expansion was for more room for the masseter chewing muscles.
There jaw bones, maxillas, and mandible, were also under selective strain to be tougher so that they didn’t fracture under strain which would be game over if you could noteat. So their jaws developed to be thicker, taller, and wider to lower the stress of chewing forcible to obtain the nutrients in there diet.
Gracile australopiths would eventually lead to the genus Homo and the robust became extinct probably because their more specialized adaptations lead to a less flexible diet as climate change continued in Africa. Even today we still have a reflection of what the australopiths left as we became there decedents. Like a more diverse diet less dependent on fruit, adaptations that was continued as we developed into our more specialized bipedal moment, grinding chewing as we chew with thick molar enamel, and small incisor-like canines.
For more information
The Story of the Human Body by Daniel Lieberman