The Last Common Ancestor

Abstract

Humans have no close living relatives and so it is of interest to human ethologists to try and model the characteristics of the last common ancestor (LCA) of humans and chimpanzees. This can be done by examining similarities in the behaviour of these species and also by considering their behavioural differences. This analysis indicates that the LCA was a self-aware, tool-using, hunter-gathering, hand-assisted arboreal biped. It is suggested that the human line's most likely point of origin was in the flooded/swamp forests of what became the Congo basin following the uplifting of the East African Rift region during the mid-to-late Miocene. It is proposed that this subsidence created the conditions for the LCA line to divide based on propensity to engage with water and that this is still reflected in the behaviour of chimpanzees and humans today. COMMENTARYTinbergen's four questions of Form, Function, Ontogeny, Phylogeny, the 'what does it look like, 'what does it do' , how does it change within an individual's lifetime' , 'how do other closely related species solve homologous problems' questions (Hendrie, 2022, after Tinbergen, 1963 are undoubtedly Ethology's most powerful tools. They provide guidance for systematic study that is missing from disciplines like Psychology and elevate what would otherwise be an interesting technique for analysing behaviour into a scientific discipline.Whilst answers to the first three questions can be obtained by direct observation, answering questions about phylogeny is less straightforward for those studying humans as there are no close relatives of our species still in existence. Comparisons with chimpanzees (Pan troglodytes) are commonly made on the basis of there being a 96% overlap between genomes (The Chimpanzee Sequencing and Analysis Consortium, 2005). Whilst this is clearly of importance for biomedical studies etc, it is of lesser relevance when considering behaviour as behaviour is not predicted by genetic relatedness. There is for example, significant genetic overlap between humans and simple animals like sponges (Gaiti et al, 2017). A significant proportion of the human genome also derives from endogenous retroviruses (Nelson et al, 2003) and other hominins, such as Denisovans/Neanderthals (e.g. Malaspinas et al, 2016) and whilst bonobos (Pan paniscus) are