Results and discussion
1. Cultural drive in an undivided population with
‘Machiavellian’ culture. First, we explored the dynamics of the
simulated population under the following simple conditions: all
population is a single group (no between-group competition), only TrE
and Useless memes are allowed, social learning is limited only by MC,
brain volume = 20 + MC (which means that parents have to spend two
additional resources for each additional unit of the child’s memory
capacity), initial MC = 0, MC gene is the only gene that can mutate and
therefore evolve, LE=1 and does not evolve (perfect congenital copying
ability), TE = 0 (see Suplementary Table 1 for further details).
Under such parameters, population stabilizes at 450-500 individuals, a
new meme is invented approximately each 5-6 years. Population is quite
viable from the start and quickly reaches the bearing capacity of the
environment. There is no need for change, but the changes are forced by
within-group competition and ‘cultural drive’. As soon as any individual
invents and remembers an efficient TrE meme, she is at advantage. One
needs MC to learn the meme, and so selection for MC starts. The process
is autocatalytic: the higher is the average MC, the higher is the
probability of preservation and spread of newly invented memes. The
larger is the number of useful memes in the meme pool, the more
beneficial it is to have large MC. As group members become more
sophisticated tricksters, it becomes harder for individuals with low MC
(and small brain) to survive and reproduce in such a group.
The runaway brain-culture coevolution forced by cultural drive is
reminiscent of Baron Munchausen pulling himself out of a swamp by his
hair: there are no external incentives and no reason to change, but the
growing brain and developing culture continue to push each other further
and further.
The initial stages (first 1500 years) of brain-culture coevolution under
these parameters are shown in fig. 2. At first, there is a ‘dormant
phase’: no memes are preserved because individuals do not have enough
MC. Initially, MC=0, but MC gene mutates, and mutant individuals with
MC>0 appear. As long as there are no useful memes in the
meme pool, MC is a slightly deleterious trait (more resources are
required to produce offspring). Thus the average MC tends to
mutation-selection equilibrium which in this case corresponds to
MCavg≈0.3. Average size of invented memes is 1, but
variance is high. So sooner or later a small enough meme is invented by
an individual brainy enough to remember it. Efficient TrE meme can
promote selection for MC and thus initiate the ‘cultural drive’.
Useless memes cannot initiate cultural drive. Moreover, if the first
meme to spread happens to be a small but efficicent Useless meme, then
large MC may become considerably more deleterious, and
mutation-selection balance may shift to a lower equilibrium value of MC.
This can prolong the ‘dormant phase’. As the cultural drive proceeds,
Useless memes infiltrate the meme pool (in the example shown in fig.2,
three Useless memes have spread, marked by arrows). However, under the
current set of parameters parasitic memes cannot stop cultural drive.
Littering of memory by Useless memes results in continued brain/memory
expansion so that it can accommodate TrE memes despite being littered.
Further events (up to the year 70000) are shown in Fig. 3.
Cultural richness (total number of unique memes in the meme pool)
follows an S-shaped curve. Average number of memes known by an
individual decelerates somewhat faster because this parameter is limited
by lifespan and learning speed. Average individual tends to know only
about 4% of the total knowledge of the population.
Average brain volume grows driven by more and more efficient
Machiavellian culture, but then it stops growing and even declines. The
limit to growth is explained by the fact that at some point the benefit
from the ability to learn more TrE memes stops to exceed the cost of
further brain expansion. The cost grows linearly with brain volume (the
larger the brain, the more resources are spent by parents for each
child). The benefit, however, increases with deceleration, because it
takes time to fill a large memory. As culture and brain expand, the
‘juvenile’ period of incomplete use of memory becomes longer.
Individuals start to fully exploit their large memory at progressively
later age, and the benefit from further brain/memory expansion
decreases. At some point the benefit stops to exceed the cost, and the
brain stops growing.
The reasons for further decline in brain volume are more subtle. They
stem from the gradual decrease in the average size of memes stored in
memory (fig. 3), a process observed under virtually any parameters,
unless we make the sizes of all invented memes equal. This process ,
which we call ‘meme simplification’, is driven by meme selection:
smaller memes spread faster irrespective of their phenotypic effects,
because there are always individuals in the population whose free MC is
insufficient to learn a large meme, but is still large enough to learn a
small meme. Meme simplification was observed by Gavrilets and Vose in
their model (Gavrilets, Vose, 2006) and is clearly reproduced in our
study.
For the first ~15000 years the average size of the memes
stored in individuals’ memories was increasing (fig. 3). This is because
meme’s size is positively correlated with its efficiency, and average MC
was growing rapidly, thus making it possible to remember larger (and, on
average, more useful) TrE memes. Later, however, selection for smaller
memes led to meme simplification. Accumulation of simple (small) memes
in culture makes large MC less beneficial, because learning takes time,
and at some point individual lifespans become insufficient to fill large
memory with progressively smaller memes. The efficiency of memory usage
decreases, and the brain starts to shrink.
The integral efficiency of Machiavellian culture (average phenotypic TrE
of individuals, fig. 3) continues to grow despite the decrease in MC,
because memes not only become smaller: they also become relatively more
efficicent (efficiency-to-size ratio increases) due to the interplay of
meme and individual selection.
Interestingly, the proportion of memory occupied by Useless memes
decreases only slightly with time, and the proportion occupied by TrE
memes does not increase (fig. 4, bottom right diagram), although TrE
memes are highly beneficial and Useless memes are slightly deleterious.
This reflects the fact that individual selection has only a very limited
ability to remove maladaptive memes from the culture (Enquist,
Ghirlanda, 2007) or, more generally, to control the content of the meme
pool. This is because memes, unlike genes, spread horizontally. If a
maladaptive meme does not kill its host immediately, which Uselless
memes do not, the host has time to disseminate the meme. Conversely,
between-group competition and group selection can purge the culture very
efficiently (see below).
The evolution of the simulated population can be divided into three
stages (observed under other sets of parameters as well).
1. Start of the runaway brain-culture coevolution: ‘rough’ culture
stimulates rapid brain expansion. At first, the brain starts to expand
to accommodate the first small but efficient memes. As the brain grows,
the average size of the memes also grows, while their efficiency-to-size
ratio decreases (‘extensive’ development of culture). The culture formed
during this stage is ‘rough’: it consists of large memes with low
efficiency-to-size ratio. It is this culture, however, that stimulates
the fastest expansion of the brain.
2. The potential for ‘extensive’ development is exhausted. Meme
selection results in gradual meme simplification. As memes become
smaller, brain expansion slows down and stops; this results in further
selection for smaller memes. The integral efficiency of Machiavellian
culture continues to increase, as seen from the increase in average
phenotypic TrE of individuals. The efficiency-to-size ratio of the memes
starts to increase again.
3. ‘Intensive’ cultural development. Further meme simplification
promotes the decrease in brain volume. ‘Childhood’ (the period needed to
fill one’s memory with memes) becomes longer. The integral efficiency of
culture is still growing; the culture becomes more ‘sophisticated’.
Perhaps it is not too far-fetched to note that the three stages are
somewhat reminiscent of the Early, Middle and Late Paleolithic, although
there are prominent differences as well. Most importantly, in our model
the cultural development is generally decelerating (rather than
accelerating as in the real history of Homo ), and the relative
durations of the stages are reversed. We believe that the descrepancies
stem from the fact that the simulated culture is not cumulative: memes
cannot be modified, improved or used as the basis for further
development. Modeling cumulative culture is a task beyond the scope of
the current study.
2. Cultural drive in a population consisting of competing
groups; ‘Machiavellian’ culture. Next, we asked how between-group
competition (BGC) influences the brain-culture coevolution. We used the
same set of parameters with one exception: we varied the value of
G (maximum group size), which was initially set to 800
so that the population always consisted of a single group. We tried two
other values: 40 and 15, which correspond to approximately 20-25 and
55-65 groups in the population, respectively. This results in moderate
to strong BGC and group selection.
The results are shown in fig. 4. BGC alters the pattern of brain-culture
coevolution. The outcome of BGC generally depends on cultural
differences between groups (cultural group selection) and on brain
volume (large brain hinders reproduction and group propagation). In this
case, culture per se is useless for competition with other groups,
because only TrE and Useless memes are allowed (TrE memes only influence
the distribution of resources within the group and have no significant
impact on group competitive ability and propagation). Still TrE memes
initiate cultural drive. In a way, Machiavellian culture harms the group
by enforcing brain expansion which, in turn, results in reduced
fecundity. Those groups that can somehow curb cultural drive will be at
advantage. As seen from fig. 4, what really happens is that BGC promotes
stronger selection for smaller memes and effective purging of culture
from Useless memes. Both processes help the brain to remain small
despite the cultural drive.
BGC results in lower cultural richness (fig. 4), mostly because of
cultural drift: groups compete, often die out (with their culture), or
split and propagate. This results in lower overall meme diversity.
However, average individual knows approximately the same number of memes
as in the previous case (without BGC). Overall, the culture is less
diverse and more stereotypic.
BGC also results in lower MC and brain volume. When there is no BGC
(G =800), only individual selection is working against
brain expansion. When BGC is present, individual selection is aided by
group selection. Consequently, the limitations on brain expansion become
more stringent. This, in turn, results in stronger selection for smaller
memes, more radical ‘meme simplification’ and higher efficiency-to-size
ratio for TrE memes. Smaller memes, once again, reduce selection for
larger brains. We call this feedback mechanism ‘the vicious circle of
meme simplification’ (fig. 5).
The integral efficiency of Machiavellian culture is, somewhat
unexpectedly, even higher than without BGC. This is because TrE memes
which accumulate in the meme pool are extremely small and efficient, and
Useless memes are effectively removed. BGC cannot stop ‘selfish’
brain-culture coevolution driven by the TrE memes, but it is fully
capable of removing Useless memes from the culture. Groups with
additional costly MC filled by maladaptive memes loose the competition.
Expectedly, such efficient cultural group selection is possible only if
between-group migration rate is comparatively low (in the simulations
discussed so far, individuals move to another group with probability
0.001 per year; this means that only 2-3% of individuals migrate in
their lifetime). The higher the migration rate, the more similar is the
pattern of brain-culture coevolution to that observed in the absence of
BGC (see below).
3. ‘Machiavellian’ culture vs. ‘cooperative’ culture. Next, we
simulated brain-culture coevolution in a population with ‘cooperative’
(rather than ‘Machiavellian’) culture. The same parameters were used as
before, with the only exception that ‘Hunting efficiency’ (HE) memes
were allowed instead of TrE memes. The parameters for HE memes (mean
efficiency, standard deviation of efficiency, C, R [see Supplementary
Table 1 for explanations]) were 4, 6, 0.25, 2. This means that an
average HE meme increases the phenotypic value of HE by 4 units, has
size 1, there is a weak positive correlation between the efficiency and
size, but the variance of both values is high. The situation is thus
completely symmetrical to the previous one, except that TrE memes are
beneficial for the individual but useless for the group, while HE memes
are beneficial for the group rather than for the individual. The cost of
having larger brains is paid by the individual, while the benefit from
remembering more HE memes is shared between all group members.
The results are shown in fig. 6 (middle row of diagrams). For
comparison, simulation results for Machiavellian culture (discussed
above) are shown in the top row of diagrams.
When brain expansion is driven by cooperative culture, the intensity of
BGC is the major factor promoting cultural drive. Contrary to
Machiavellian culture that provides maximum brain expansion under
minimal BGC, cooperative culture results in larger brains under strong
BGC. If BGC is absent (G =800, all population is a single
group) or low (G =300, population consists of 3-5 groups,
data not shown) cooperative culture fails to initiate cultural drive,
and the brain remains small. Thus Machiavellian culture appears to be
able to initiate cultural drive under a wider range of BGC levels than
cooperative culture.
If BGC is strong (G =15), cooperative cultural drive is
very efficient, and brain volume reaches even higher values than in the
case of Machiavellian culture and no BGC. Moreover, under strong BGC and
cooperative culture, both meme simplification and decrease in brain size
are less pronounced and start later. This is because every HE meme,
regardless of its size, is very important for group survival when
competition with other groups is high. Cultural group selection strongly
favours groups with the most efficient cooperative (hunting) culture.
The resulting culture is ‘rough’ and uniform on the population scale,
but very efficient.
BGC appears to be a powerful mechanism to prevent the spread of the
Useless memes. This is true for cooperative culture as well as for
Machiavellian culture. Strong BGC generally results in a uniform,
efficient culture with a minimum amount of unnecessary elements.
The comparison of the top and the middle rows of diagrams in fig. 6
shows that the average meme size covaries with the average brain volume
(MC). This is apparently because smaller brains (or stronger constraints
on brain expansion) promote stronger selection for smaller memes, and
saturation of culture with smaller memes results in weaker selection for
larger brains (fig. 5).
4. Complex culture vs. specialized culture. Next, we simulated
brain-culture coevolution in a population with a complex culture, that
is, with all three types of memes allowed (TrE, HE, Useless). All
parameters were as in previous simulations, except that creativity was
set to 0.0004, in order to retain the fixed rate of invention of memes
of each type (0.000133 per individual per year, see Supplementary Table
1).
The results are shown in fig. 6 (bottom row of diagrams). The complex
culture appears to be a more powerful driver of the runaway
brain-culture coevolution than both types of specialized culture
discussed above. The differences are as follows:
a) The complex culture promotes strong cultural development and brain
expansion at all levels of BGC (contrary to Machiavellian culture which
works best at low levels of BGC, and to cooperative culture which
initiates cultural drive only at sufficiently high levels of BGC);
b) The complex culture results in a slightly richer meme pool and
slightly lower levels of memory clogging by the Useless memes. Under
strong BGC, ‘cooperative’ (group-beneficial) HE memes outcompete
‘selfish’ (individually beneficial) TrE memes (fig. 7, bottom row of
diagrams).
c) The complex culture makes it possible for some hunting skills to
develop even when HE memes are not supported by group selection. E.g.,
the average phenotypic value of HE under complex culture and
G =800 is much higher than under specialized cooperative
(hunting) culture and G =800. When G =800,
there is no BGC and the HE memes are virtually useless, because the
initial (congenital) phenotypic value of HE is high enough for the
individuals to survive and reproduce. The population quickly grows up to
the bearing capacity of the environment, after which all available
resources are effectively extracted and used. Thus in the absence of BGC
the HE memes provide no benefits to either individuals or the group.
This is why cooperative culture is unable to initiate runaway
brain-culture coevolution in the absence of BGC. However, when
G =800 and the culture is complex, cultural drive is
initiated by the TrE memes, the brain and MC expand, and the HE memes
begin to spread as ‘parasitic’ memes despite their uselessness under the
circumstances.
d) The complex culture prevents the extreme meme simplification.
The dynamics of memory loading (fig. 7) shows that cultural group
selection (which is efficient when BGC is high) is capable of shaping
the content of the meme pool, while individual selection is not. When
G =800, the proportions of the meme types stored in
memory are similar to the proportions of meme types invented, whereas
under G =15 cooperative HE memes prevail in the meme
pool.
The advantages of the complex culture discussed above imply that the
chances for a large-scale runaway brain-culture coevolution may improve
when the population finds itself in a situation where the individuals
have the opportunity to invent many different, cognitively demanding and
highly beneficial memes of different types, e.g. cooperative andMachiavellian. In this case, cultural drive is possible irrespective of
the intensity of BGC.
This situation is conceivable for early Homo. Chances to invent
valuable and cognitively demanding ‘cooperative’ memes could have
increased due to changes in foraging behaviour and new feeding
strategies, e.g., cooperative scavenging or hunting for large prey in
savannah habitats (Rogers et al., 1994; Braun et al., 2010; Patterson et
al., 2019). Early hominin scavangers and hunters are thought to have
relied heavily on within-group cooperation in order to effectively
compete with large carnivores and other hominins (Flinn et al., 2005;
Moll, Tomasello, 2007; Bickerton, Szathmáry, 2011; Gavrilets, 2015).
Stone tool production and use is an example of behaviour which probably
was highly beneficial for the group because fast and effective
butchering of large carcasses, along with other types of cooperative
behaviour, could have been essential for avoiding direct aggressive
encounters with stronger competitors and predators (Rose, Marshall,
1996; Plummer, 2004). At the same time, even the production of seemingly
simple and primitive Oldowan tools appears to be cognitively demanding
and requiring high-fidelity social learning (Morgan et al., 2015). We
suggest that socially transmitted skills needed for stone tool
production and use by early Homo may be comparable with the
‘Hunting efficiency’ memes in our simulation. Confrontational
scavenging, hunting and butchering were most probably collective
endeavors whose success benefited the group rather than an individual
hunter or butcherer.
Chances to invent highly beneficial and cognitively demanding
‘Machiavellian’ memes could have increased due to the proposed
evolutionary shift towards lower within-group aggression, social
monogamy, increased social conformity, parental care and paternal
investment in offspring (Lovejoy, 2009; Stanyon, Bigoni, 2014; Raghanti
et al., 2018). In a society where direct physical aggression against
group memebers is not encouraged, ‘Machiavellian’ skills and tricks may
become the main way to achieve higher status and reproductive success
(Humphrey, 1976; Byrne, Whiten, 1988).
5. High-fidelity, costly social learning is a powerful driver of
brain expansion. Selection for high-fidelity social learning is the
cornerstone of the ‘cultural drive’ hypothesis (Lewis, Laland, 2012;
Laland, 2017). In the simulation experiments described above, social
learning was limited by memory capacity which was costly in terms of
brain expansion (each unit of MC required one additional unit of brain
volume). This cost is nearly optimal for achieving the largest brain
size: under the current parameters, it is not possible to achieve much
larger brain volumes by making it higher or lower. If the cost is lower,
the extent of cultural development ultimately will be similar (limited
in the long term primarily by learning speed and lifespan), but the
brain will be smaller. With higher cost, the ‘vicious circle of meme
simplification’ will be more intense, the average meme smaller, and
selection for larger brains weaker. The resulting culture will again be
similar, and the brain volume smaller.
In an attempt to find additional factors promoting brain expansion, we
varied the parameter LE (‘learning efficiency’) which determines the
probability of success when an individual with sufficient MC attempts to
learn a meme. In the experiments discussed above, LE was set to maximum
(LE=1), did not evolve, and did not impose any costs. In the next
experiment, we set the initial value of LE to 0, made it very costly (30
units of brain volume per unit of LE), and allowed it to evolve
genetically (LE gene mutation rate 0.04, mean mutation effect 0,
standard deviation 0.1). All other parameters were left unchanged. This
means that brain-culture coevolution was now limited not only by the
costly memory capacity, but also by the very costly learning efficiency.
In other words, much higher price must be paid now for the same level of
cultural development.
This is different from just making MC more costly, because there is one
major difference between MC and LE as the limiting factors of
brain-culture coevolution in our model. Costly MC limits the spread of
large (complex) memes to a much larger extent than the spread of smaller
memes, thus providing selective advantage to simple memes and
facilitating ‘the vicious circle of meme simplification’. Conversely,
costly LE limits the spread of all memes irrespective of their size
(complexity). Thus costly LE is not expected to promote meme
simplification. Of course, LE can also be programmed to select for
smaller memes (as was done, e.g., by Gavrilets and Vose [Gavrilets,
Vose, 2006]), but we aimed to explore a complexity-insensitive
limiting factor (LE) along with a comlexity-sensitive one (MC).
Rather unexpectedly, we found that very costly LE does not prevent the
runaway brain-culture coevolution. Under some combinations of parameters
([complex culture, G =15] and [cooperative culture,
G =15]), average LE evolves up to 0.89 – 0.91 within
70000 years despite the fact that the cost of such evolution is 26.7 –
27.3 additional units of brain volume. The results are summarized in
fig. 8 (compare with fig. 6 to see the effects of evolvable, costly LE;
note different scales on the brain volume diagrams).
Costly, evolvable LE (fig. 8) results in cultural development similar to
that observed under free, fixed LE (fig. 6). Runaway brain-culture
coevolution starts under all combinations of parameters except
[cooperative culture, G =800] and [Machiavellian
culture, G =15]; under [Machiavellian culture,
G =40] it starts in approximately half of model runs.
Total cultural richness is somewhat lower than in the case of free,
fixed LE, but the integral efficiency of the resulting culture
(phenotypic values of TrE and HE) are comparable. Meme simplification is
slightly less pronounced (average meme size is larger). Parasitic memes
are more efficiently removed from the meme pool. The most striking
differences are in the brain volume: when LE is costly and evolvable,
brain volume reaches much higher values, especially if the culture is
cooperative or complex and between-group competition is high.
Why does the costly learning efficiency result in evolution of larger
brains than the costly memory capacity? The apparent reason is that LE
is insensitive to meme complexity, and so the constraints on LE do not
result in the ‘vicious circle of meme simplification”, whereas the
constraints on MC do.
The results imply that the runaway brain-culture coevolution can be
strongly facilitated by the development of mechanisms of social learning
that are (1) costly in terms of brain expansion or structural
optimization, and (2) tolerant to the complexity of knowledge. In other
words, these mechanisms must be neurologically demanding (e.g., rely on
sophisticated neuronal circuits), but they must make it possible to
transfer complex information relatively easily. Human language seems to
fit this definition accurately.
TribeSim program allows to simulate genetic and cultural evolution of LE
and TE (teaching efficiency). Evolution of high-fidelity teaching is
thought by some scholars to be essential for human evolution (Morgan et
al., 2015; Laland, 2017). We found that LE can evolve both genetically
and culturally (by propagation of the LE memes) with similar success.
Interestingly, TE ‘prefers’ to evolve culturally rather than
genetically. This is because TE memes have viral properties: when an
individual learns a TE meme, she becomes a more efficient machine for
meme dissemination. In other words, TE memes are the only memes that
help themselves to propagate. When TE is allowed to evolve culturally,
TE memes tend to occupy disproportionally large portions of memory (fig.
9) . We tentatively suggest that this may have some relation to the fact
that human ability to acquire language appears to be mostly congenital
(analogous to the LE gene in our model), while the language itself is
learned. By learning words and grammar, people acquire the ability to
transfer their knowledge to others, that is, to teach; some components
of language thus can be regarded as analogs of the TE memes in our
simulation.
6. Additional factors that affect the dynamics of brain-culture
coevolution. Finally, we tested the effects of four additional factors:
population size, meme invention rate, life span, and between-group
migration (fig. 10).
Larger population size enhances brain-culture coevolution: the
values of brain volume, learning efficiency, hunting efficiency,
cultural richness were higher after 50000 – 70000 years of evolution
(fig. 10).
The effect of higher creativity (higher rate of meme invention)
is much smaller. This means that the positive effect of larger
population is not explained by higher number of potential inventors, but
rather by higher number of brains that can store and disseminate
knowledge.
Increased life span resulted in a much more powerful runaway
brain-culture coevolution, apparently because longer-lived individuals
are more efficient ‘machines for meme accumulation and dissemination’.
Doubling of lifespan resulted in only 20% increase of population size,
because the latter is limited primarily by resources
(R ). Lower death rate automatically results in lower
birth rate, because it becomes more difficult for the individuals to
accumulate sufficient resources for reproduction.
Absence of between-group migration (isolated groups) generally
hinders brain-culture coevolution, especially the development of
Machiavellian culture is compromised (average phenotypic TrE remains
low). Conversely, higher migration rate is good for
Machiavellian culture, but does not support the development of efficient
cooperative culture (HE remains low). This emphasizes the advantage of
complex culture, which can ensure powerful brain-culture coevolution at
different levels of between-group migration as well as at varying levels
of between-group competition (see above).