Introduction
One of the most intriguing features of hominin evolution is the rapid
increase in brain volume, both absolute and relative, which was
especially pronounced during the last two million years in the Homo clade. Within this time interval, brain volume increased
threefold, from approximately 400-500 cm3 in the
ancestral australopiths to 1300-1500 cm3 in the Late
Pleistocene species such as Neanderthals and early modern humans
(Rightmire, 2004; Schwartz et al., 2004; Roth, Dicke, 2005; Sherwood et
al., 2008; Leigh, 2012; Neubauer, Hublin, 2012; Holloway, 2015). This
striking pattern of directional and accelerating evolution towards
larger brains appears to be quantitatively unique among primates (Miller
et al., 2019). Given that a large brain is very expensive metabolically
and imposes other costs, e.g., increased load on the cervical region of
the spine and difficulties at giving birth to larger-headed offspring
(Mink et al., 1981; Leonard, Robertson, 1992; Gavrilets, 2015), its
continued expansion during two million years of human evolution implies
unusually strong and long-lasting selection pressures in favour of
larger-brained individuals (or groups containing such individuals).
This, in turn, almost inevitably implies a positive feedback loop in the
evolution of the human brain: its expansion must have been promoting
further expansion (Holloway, 1967; Crespi, 2004; Miller et al., 2019).
Several kinds of hypothetical feedback mechanisms have been suggested in
this context including sexual selection for intelligence accelerated by
Fisherian runaway process (Miller, 2000), within-group competition for
social status which boosted the evolution of increasingly elaborate
‘Machiavellian intelligence’ (Humphrey N.K., 1976; Byrne, Whiten, 1988),
and ever-increasing between-group competition in a higly social,
ecologically dominant species which resulted in accelerated evolution of
cognitive abilities needed for effective within-group cooperation and
group-beneficial behaviours (Alexander, 1989; Gavrilets, 2015).
More recently, another type of hypothetical positive feedback mechanism
termed ‘cultural drive’ started to receive increasing attention among
researches (Laland, 2017). The term was originally coined by Allan C.
Wilson (Wilson, 1985) who hypothesized that enhanced cognitive
abilities, especially the abilities for social learning and cultural
transmission of adaptive behaviours (Laland, Galef, 2009), can
accelerate biological evolution. Smarter animals invent new adaptive
behaviours more often and transmit them across generations more
effectively; new cultural traditions create new selective environments
(‘cultural niche construction’ [Laland et al., 2001; Laland, O’Brien,
2011]) in response to which animals evolve faster (Wilson, 1985). The
‘cultural drive’ hypothesis was further elaborated by Kevin N. Laland
(Laland, 2017) and other researches who suggested that the coevolution
of social learning, cognitive abilities and culture can be
self-sustaining (Whiten, van Schaik, 2007; Heyes, 2012; Laland, Rendell,
2013; Whiten et al., 2017). In its simplest form, the positive feedback
mechanism of the ‘cultural drive’ can be described as following: better
social learning and cognition → more behavioural innovations become
fixed as cultural traditions; richer culture → more skills available to
be learned from conspecifics; increased usefulness of learning
abilities; more sophisticated and flexible behaviour results in new
cognitive challenges → selection for still better social learning and
cognition (Fig. 1).
Additional positive feedback loops are conceivable, e.g., via longer
lifespan (elaborate culture → enhanced survival → longer lifespan →
better prerequisites for intergenerational transfer of knowledge → still
more elaborate culture → stronger selection for enhanced social
learning) or via better nutrition (elaborate culture, including enhanced
food acquisition strategies → enhanced nutrition → relaxed constraints
for the evolution of larger brains) (Kaplan, Robson, 2002; Crews, 2003;
Caspari, Lee, 2004; Laland, 2017).
The cultural drive hypothesis has received some empirical support (e.g.,
Kopps et al., 2014; Foote et al., 2016; Whiten, 2017), including
recently described positive associations between social learning
proclivity, absolute and relative brain volume, longevity, social group
size and technical innovation in primates (Navarrete et al., 2016;
Street et al., 2017) and cetaceans (Fox et al., 2017).
Importantly, the ‘cultural drive’ hypothesis does not contradict other
ideas such as ‘Machiavellian intelligence’, ‘social brain’, ‘cooperative
brain’, ‘brain for stone tool production’ or ‘brain for mate
attraction’. In fact, the ‘cultural drive’ hypothesis can embrace a wide
variety of such ideas, because it does not particularly rely on any
specific type of culturally transmitted skills or behaviours.
Potentially, any skills will do, given that they are cognitively
demanding, require high-fidelity social learning and provide
reproductive benefits, thereby enhancing the spread of the good
learners’ genes.
This versatility is vividly illustrated by a computer model designed by
Gavrilets and Vose (Gavrilets, Vose, 2006). Although their paper is
titled “The dynamics of Machiavellian intelligence”, the model
actually simulates cultural drive in one of its simplest forms. In fact,
it accomodates the theories of Machiavellian intelligence and sexual
selection within the framework of cultural drive. A brief description of
this model is necessary for understanding the theoretical context of the
current study. Simulated males in a polygynous, promiscous population
compete for mates. Males sporadically (and very rarely) invent
‘Machiavellian memes’, that is, behaviours that improve their
competitive ability. Memes can be acquired by other males via social
learning. The chances to successfully learn a meme depend on the meme
size (‘complexity’) and the male’s memory capacity and learning ability.
Both characters are ‘costly’, that is, they decrease survival (it is
assumed that they require larger brains, although brain volume is not
modeled expicitly). Genes for memory capacity and learning ability
mutate at a specified rate. Meme size positively correlates with its
fitness effect, but the correlation is weak. Initially, all males have
zero memory capacity and learning ability. The evolution of the
simulated population starts with a more or less prolonged ‘dormant
phase’ during which both memory capacity and learning ability remain low
(as slightly deleterious traits at the mutation-selection equilibrium),
and only newly invented memes are present in the meme pool. But sooner
or later a self-accelerating process starts which the authors call
‘cognitive explosion’. During this phase cognitive abilities,
population’s cultural richness (meme count) and Machiavellian fitness of
individuals all increase in a runaway fashion. The cognitive explosion
is fuelled by cultural drive (although the authors do not use the term):
the more memes there are in the meme pool, the more it pays to have good
memory and learning ability (Gavrilets, Vose, 2006).
Here we elaborate on this approach by designing a more complicated
model, TribeSim, aimed to explore the impact of different factors on the
dynamics of the brain-culture coevolution in a highly social species.
The most important of the studied parameters include the extent of
within- and between-group competition, different types of memes
(individually beneficial, group-beneficial, useless or maladaptive) and
their combinations (specialized culture vs. complex culture). We set out
to elucidate the possible prerequisites for the most extensive brain
expansion observed in the Pleistocene hominins, and we argue that the
early representatives of the Homo clade, but not the other apes,
probably found themselves in a situation suitable for an unprecedentedly
powerful ‘cultural drive’.