Natural selection is the mechanism which acts upon individuals, by which evolution occurs. It is the idea that within a breeding population there are heritable morphological and behavioral variations among individuals which make some individuals more or less able to have a lot of offspring depending on the population’s environment, in other words some individuals are more fit than others because they are better adapted to the environment. Organisms pass along these heritable traits and the ones with greater fitness will come to dominate the population and ones with lesser fitness will fade from the population. Evolution is basically just the aggregate effect of natural selection: it can be called “descent with modification” or a change in gene frequencies but it is a change in a population over time, with speciation a possible but by no means inevitable outcome.
In order to best understand how hominins evolved, I focused only on traits present in all specimens commonly-accepted to be hominins, with an emphasis on the traits of the earliest hominins. The key cranial features of the hominins are non-projecting and more incisiform canines which have lost the honing mechanism, smaller incisors relative to the cheek teeth, a lower-placed and more forward positioning of the foramen magnum, and, although quite modest in the early hominins, increased relative cranial capacity. The key postcranial features of the hominins are a tibia which indicates bipedal motion (the only possible exception is A. ramidus, but only because the remains are not completely conclusive). The later hominins have other features of bipedalism such as the valgus knee and adducted hallux which are assumed but haven’t yet been demonstrated to definitely be features of the earliest hominins. The hominins also retain the adaptations for, albeit reduced, for suspensory tree-climbing such as the olecranon process and the shape of the clavicle.
We cannot point to only one factor for the evolution of hominins. Multiple factors converged at the same time which favored the adaptations seen in hominins. The best estimates say that the Hominins and Panins first began diverging about six million years ago. This coincides with a shift in greater C3 relative to C4 photosynthesis, meaning a shift from forests to woodlands as well as an increase in grasslands. Early hominins were likely living in the woodlands and traversing the grasslands in between. Food was becoming more scarce and the world was becoming drier as more water was tied up in glaciers.
Increasingly incisiform canines made it easier and faster for the hominins to process food, an important adaptation for an animal which had to travel longer distances to find food and were probably having to subsist on food which required more chewing or more efficient chewing. With shrinking and more incisiform canines, hominins were able to use a more efficient grinding cycle. The loss of the honing mechanism was just a side-effect of both having canines too small to effectively grind against each other properly and having a chewing cycle where the canines would have just gotten in the way This chewing cycle also explains the smaller incisors as larger ones would have impeded the hominin chewing cycle.
The more inferior and anterior positioning of the foramen magnum in hominins seems to have evolved alongside bipedalism, not being solely caused by bipedalism but by other factors as well. This positioning requires less musculature to support the head and is therefore already more efficient than a higher and more posterior placing. There might have been some element of both traits reinforcing each other as an animal which doesn’t have to rotate its head to look forward will be more efficient than one that does, and a quadruped with a foramen magnum placed like a hominin’s would be constantly be craning its head down. It is best not to put too much emphasis on it, but in aggregate with the other hominin traits, it would contribute to the explanation of why the hominins evolved fairly efficient bipedalism so quickly.
It is hard to tell why the hominins started to evolve brains larger relative to body size, the explosion in brain size didn’t happen until fairly recently. An increased brain size would have come in handy when being forced to juggle a wide variety in habitat along with scarcer resources which had to be gotten at more intelligently. Increased brain size also would have allowed for greater collaboration, but there is no evidence of hominin collaboration until later. The general trend to relatively larger brains reflects this- a modest increase in relative brain size and then an explosion later on triggering rapid change, an increase which was cut off when it became too inefficient for the brain to grow larger.
It is largely accepted that all of the species designated as hominins had relatively good bipedal locomotion, although the earlier hominins were still fairly inefficient. Primatologists often look to the non-hominin apes to understand how and why the hominins evolved habitual bipedalism, looking at the activities that they do bipedally- feeding, gathering and carrying food, childcare. While we must consider these reasons, it would be a grave mistake to put too much weight on these uses as they obviously do not increase fitness enough to push the non-hominin apes to habitual bipedalism so something else is at work. We know that the earliest broadly-accepted hominins had tibias designed for the balance required of bipedal locomotion. For why the hominins started on the trend to become more efficient bipeds, we must combine the reasons why non-hominin apes use bipedalism with the need to traverse open territory swiftly while expending little energy.
We don’t know from the fossil record if the very earliest hominins had a valgus knee or adducted hallux, but we do know from the Laetoli tracks that A. afarensis had a fully adducted hallux, suggesting that the earlier hominins at least had partially adducted great toes.
Even modern humans with our short arms retain a faculty for suspensory behavior and tree-climbing. The fact that hominins retained this faculty strengthens the argument that hominins evolved in woodlands and made use of the trees for food as well as quick escape routes from predators. There is plenty of evidence that hominins were preyed upon and the ability to rapidly climb up trees would have been a useful trait. Plus, it would have been useful when competing against other tree-dwelling organisms for such scarce resources.
The early hominins don’t have many traits that strongly distinguish them from the chimpanzees. These traits only come into focus and are seen for how important they are when we consider their progeny and view hominin traits as trends, tendencies toward certain traits. The trick is to not view these traits as inevitabilities. We only need to look at Paranthropus with its megadontia to see how today’s fitness-increasing trait could be the next millenium’s energy-sapping liability.