Il “cespuglio” dell'evoluzione umana fra il Miocene e la comparsa dei primi Homo

The paleo-anthropological research of the last 25 years has shown, unequivocally, that the evolutionary path that precedes the appearance of Homo sapiens is not made up of a linear succession of species progressively more similar to ours, but can be better described using the metaphor of the 'bush', to underline the presence of numerous branches, often parallel to each other, each of which represents the evolutionary history of one or more fossil species, now extinct. The paleontological record provides evidence of the appearance of numerous species with anatomical features that foreshadow today's humanity (such as some form of bipedal locomotion) already about 7 million years (Ma) ago, in Africa. It is in this time frame, between 7 and 5 Ma, that the human evolutionary lineage began to diversify from the line that gave rise to the extant chimpanzee - the living species with which we share a common ancestor. In the time span between about 7 and 4 Ma the fossil 'record' is extremely sparse, but there are remains of species with anatomical characteristics indicative of upright body posture and the possibility of bipedal locomotion. After 4 Ma the fossil record indicates the existence of many bipedal species, still with the ability to climb trees, some of which coexisted in the same environments. It is among these that, around 3 Ma, under the pressure of marked climate changes, and in changing environmental contexts, there is fossil evidence of a marked diversification event; an "evolutionary radiation" that gave rise to two lines, parallel to each other: on the one hand, the so-called 'robust' Australopithecines, forms with a strong specialization of the chewing apparatus; on the other hand, the line of our most ancient ancestors, with the appearance of the first species attributed to the genus Homo, characterized by a reduction in the chewing apparatus and, at the same time, an increase in brain size, associated with a high manipulative capacity.