https://ishe.org/volume-38-2023/the-last-common-ancestor/

Human Ethology 38(2023): 002-007
Commentary
THE LAST COMMON ANCESTOR
Colin Hendrie 2023

Humans have no close living relatives and so it is of interest to human ethologists to try and model the characteristics of the 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.

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humans have a nr of other Great Ape-atypical features incl. fleshy noses, subcutaneous fat and lack of body hair, and these have led to the suggestion that part of our evolutionary history was spent in close association with water, since these features
are rather more typical of marine mammals (Hardy, 1960; Morgan, 1972; Verhaegen, 1985; Kuliukas, 2002, Vaneechoutte et al, 2011; Evans, 2019).
There have been a number of objections to this ‘Waterside’ hypothesis (e.g. Langdon, 1997; Foley & Lahr, 2014; Roberts & Maslin, 2016). However, most of the arguments against are
fundamentally in defence of other hypotheses (e.g., Rantala, 2007; Langdon, 2012). One of the most favoured is the ‘Savannah hypothesis’ which posits that the transition from ‘Ape to Ape-man’ (Morris, 1967) took place on the open plains of East
Africa, with, for example, adaptations like bipedalism giving a higher vantage point for spotting potential prey (for review see Bender et al, 2012). Humans are not however well adapted for living in such an environment as we have dilute urine and moist
dung. We also sweat freely, quickly overheat and die of thirst if we go without water for more than 2-3 days (Morris, 1994).
Other theories along the same lines include the endurance running hypothesis (Bramble & Lieberman, 2004). Humans are well adapted to long distance running and our abilities as persistence hunters are well documented (e.g., Glaub & Hall, 2017). However,
the energetic inefficiencies of the intermediate stages of the transition to bipedalism (e.g., Sockol et al, 2007) mean that running for more than a few steps is only really feasible once the adaptations necessary to be fully bipedal have been developed.
Given the inadequacies of the savannah hypothesis and its variants, the Waterside hypothesis remains the most parsimonious when accounting for how our ancestral species came to acquire so many Great Ape-atypical features (Verhaegen et al, 2007). There
has been speculation regarding
locations where these events may have taken place (e.g. Morgan, 1972; Krill, 2020) but the emerging consensus is that the critical environment was aquarboreal (e.g. Verhaegen et al, 2002, 2007). That is, swamp forest, rather than the beach scenario
originally envisaged by Hardy (1960) and we know of course that the period of interest was in the order of 6-10 million years
before present.
Importantly, in that context, geological modelling shows that mantle convection drove the uplifting of the East African plateau and that by 15 million years before present the uplift had reached 500 m and by 10 million years before present it had reached
1km (Moucha & Forte, 2011). The same modelling reveals subsidence to the west of the East African Rift zone, to form what is now the Congo basin. This subsidence, in combination with the high rainfall in this region, would have led to a marked increase
in the area covered by flooded/swamp forest, and this is reflected in the Congo deep sea fan (Anka et al, 2009).
Swamp forest still exists today in the Congo basin and is characterised by high canopies, lush undergrowth, and mud, all of which make moving through them on land very difficult. Travelling through water would have been the easier option in these
conditions and this in turn would have provided a strong incentive for the less water averse amongst our LCA to enter the water and so put us on an evolutionary path that eventually led to bipedalism.
Chimpanzees are by contrast more immersion-in-water averse, and in areas where forests seasonally flood, they tend to concentrate into the dry areas in the high-water seasons (Poulsen & Clark, 2004). This behaviour gives a powerful illustration of how
important a propensity to engage with water may have been for the splitting of lines from the LCA (Niemitz, 2010). This
may also explain the more recent split in the chimpanzee/bonobo line, where the less water averse ancestors of what became bonobos were able to cross the Congo River and exploit the gorilla-free ranges to the south (Hohmann et al, 2019).
Questions about how our ancestral species avoided predation from crocodiles during this period are answered by studies that show man-eating species (Crocodylus niloticus) originated in Australasia and do not show up in the African fossil record until 2-3
million years before present (Oaks, 2011). Hence, the LCA’s only contact with Crocodilians during this swamp forest phase would have been with slender-snouted fish eaters.
Reasons why our ancestral species left the swamp forest are less clear but may have been to do with the trend towards a cooler/dryer climate over the past 4-5 million years (Zachos et al, 2001).
This is associated with a shrinkage in the distribution of African forests (Plana, 2004) and so our ancestral species could have thus found themselves in a different environment without having changed location.
This general trend towards cooling has been accompanied by increasingly rapid (in geological terms) fluctuations in temperature and hence environmental conditions and this has been a driver for Hominin expansion and diversification (Potts, 1998). From an
ethological perspective this makes it even more remarkable that behaviours that have come through from the LCA have persisted over all this time in both humans and chimpanzees/bonobos.

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