Hi guys, today I want to discuss my thoughts on human self domestication.
Scientists have discovered a surprising reversal in our species: that our brains are now the
smallest they have been at any time in the past 100,000 years, over 10% smaller. At the same time,
our encephalization, brain size relative to body mass has also declined. This decrease is
in contrast to the general evolutionary tendency of the human cranial capacity to increase.
Why did our brain size start to shrink? There have been different theories, which have tried
to unfold the alarming reduction in our brain size and encephalization. The more talked about
conjecture is self-domestication, which proposes that people became tamer and less aggressive
in order to work with each other. I have a few questions on this hypothesis and I
want to present three of them to you. I would love to know what you think after hearing them.
According to Richard Wrangham, a primatologist at Harvard University
and Brian Hare, a professor of evolutionary anthropology at Duke University, our brain volume
decrease trend is selection against aggression, in short, that we domesticated ourselves.
Domesticated animals like sheep, pigs, cats, and dogs have smaller brains; their brain sizes are
10-25% smaller than their wild ancestors and wild counterparts. For instance, a wolf-sized
dog has a brain around 10% smaller than its wild ancestor. Domesticated animals also have lower
EQ/ Encephalization Quotient which presents as a generally reduced brain volume relative to
body size. Studies have found a close relationship between intelligence and EQ. So, the domesticated
animals are likely to be less intelligent than the feral ones. Reduced brain size,
reduced encephalization and tamer temperaments are all parts of the “domestication syndrome."
Such phenomenon has been observed by multiple researchers.
Paleontologist Ana Balcarcel and her team studied the average brain size relative to body size,
for wild versus domestic cattle. They found that domesticated cattle had brains about 25% smaller
than their wild forebears. On top of that, they discovered that bullfighting breeds,
which are bred for aggression, have brain sizes nearly as large as those of wild aurochs.
In essence, compared to domesticated animals, feral and wild counterparts have significant
larger brains and higher brain to body mass ratio. Wild animals are smarter, more aggressive and need
to fend for themselves. The domesticated ones have smaller brains; they are tamer,
compliant and depend solely on humans for feeding and management. .
From that point of view, if we humans domesticated ourselves,
then our average brain size would probably become smaller just like other domesticated animals.
Our current brain size is about 10-17% smaller than people from 40k years ago. This percentage
decrease is in the same ballpark as the ones we see across the board for domesticated species.
Modern people might be less aggressive and more social than our predecessors.
Therefore, self-domestication seems to be a reasonable proposal to explain our
diminishing brain size. But I have a few questions, in order to consider and further
accept the self-domestication hypothesis, these questions would need to be discussed and answered.
1st, is self-domestication the only way for humans to work with each other?
The prominent scientists who support the self-domestication hypothesis
argue that our brain size getting smaller is because we domesticated ourselves
to be less aggressive, tamer and docile so humans can work and cooperate together as a group.
As we know, human genuses from the beginning are social animals. Primates have always lived
within communities. Today’s hominins besides us modern humans are chimpanzees and bonobos.
They are our distant cousins. Chimpanzees live in groups which consist of up to 150 individuals.
Bonobos’ groups are also fairly large with up to 120 members. This is interesting because 150
is the Dunbar's number which was established by British anthropologist Robin Dunbar. He identified
a correlation between primate brain size and average social group size. The claim is that
an average person can only really maintain about 150 connections such as friends and acquaintances
at once. This rule of 150 remains true for early hunter-gatherer societies as well as our
modern groupings: offices, communes, factories, residential campsites, military organizations and
more. Exceed 150, a network is unlikely to last long or cohere well. In this sense, the social
group sizes of chimpanzees, bonobos and humans are almost identical. These primates have been
working and relying on one another for millions of years. Have they domesticated themselves?
Professor Hare suspects that bonobos are the domesticated version of chimpanzees. Wrangham and
Hare both see parallels between bonobo development and our own possible self-domestication.
While bonobos and chimpanzees are both genetically and behaviorally very similar,
they also differ in significant ways. Bonobos are more cautious, peaceful and socially tolerant
while chimpanzees are more aggressive and can be violent. Hare highlights how bonobos have a
build and disposition that resembles domesticated species and they also have smaller brains. But how
about the EQ or encephalization quotients between these two animals? Let’s not forget that even
though bonobos have smaller brain volume compared to chimps, their body sizes are smaller too. A
bigger individual with more muscles naturally requires more brains to function. If bonobos are
really domesticated chimps. then their EQ should be smaller as well. But is that actually the case?
From a published 2012 paper, I found information, which points to the opposite direction.
In this paper, I read that the average chimpanzee brain volume is 398 cubic centimeters or cc.
The average bonobo brain volume is 358 cc, 10% smaller than that of the chimps.
This study listed the cranial sizes and body weights of the animals. We can see the average
bonobo weighs 37.73 kg and chimps 52.25 kg. If we take the body size in this case body weight into
consideration, then this turns out that bonobos have larger encephalization than chimpanzees.
Studies with large samples are definitely needed but based on this report,
the docile and social bonobos have higher encephalization than the more aggressive
chimps – which indicates that bonobos might not be the domesticated versions of chimpanzees.
On another note, many wild animals, such as lions, hyenas, elephants,
whales and dolphins work in groups - generations after generations. The survival of their species
depends on the pack for food and safety. Have these animals domesticated themselves? I don’t
think so. These wild animals are not considered to be "self-domesticated" yet they rely on teamwork
and cooperation as well. Where do we draw the line between self-domestication and the natural animal
instinct of group work? Lionesses in the same pack have strong bonds. They hunt together, care for
all the cubs even if not their own and share their food; at the same time, they are feral as feral
can be. This means that maybe we don’t need to be self-domesticated or tame to work as a group.
Now for my 2nd question. As I mentioned earlier,
studies have found domesticated animals have smaller brains and lower encephalization.
Feral animals are smarter and more aggressive with larger brains. But is this the case with humans?
Are more aggressive people in general smarter than tame and docile people?
I assume that scientists on average are less aggressive.
They are brainy and likely possess higher EQ. They are not in need of great physical strength
to do their work and earn a living; and they are typically calm and mild-tempered. While really
aggressive people or hard-core bullies usually use fists or physical strength to do their talking.
How about psychopaths? They are aggressive and really smart, right? Psychopaths
being highly intelligent is a very common misconception; probably created by pop culture
and Hollywood with their fictional portrayals of “evil genius” or super-villain characters.
In reality, scientists found there is no evidence
that psychopaths are more intelligent than people who don’t have psychopathic traits;
and the correlation between psychopathy and intelligence is nearly zero. In addition, some
studies discovered that, in general, psychopaths seem to have below-average intelligence.
For example, criminologist and associate professor Brian Boutwell and his colleagues
analyzed the results of 187 published papers on intelligence and psychopathy. These papers
included research on psychopaths in prison as well as those enjoying high-flying careers.
They found in fact, that psychopaths, on average, scored notably lower on intelligence tests.
In another study, after scanning nearly 700 volunteers from birth,
researchers noticed that life-long criminals have smaller brains.
Small-brains might be a link to long-term antisocial behavior.
Earlier we talked about domesticated animals being tame with smaller brains; while their
wild counterparts are aggressive with larger brains. But this seems to be contrary with humans,
because generally, the most aggressive, anti-social people,
such as long-term criminals, might have smaller brains.
Psychopaths overall are not highly intelligent and might even have below average IQs. On the other
hand, scientists and people with advanced degrees are normally mild-mannered and not aggressive.
This is a 180 reversal from the domesticated animal syndrome. Bigger brained humans are not
particularly aggressive like bigger brained feral animals. That makes me question whether we can use
the current animal domestication observations as guides to project human self-domestication.
Hominins didn’t evolve in the same path as most animals.
The evolution process of most if not all other animals
are mainly physical. Some gain amazing speeds; some have incredible biting power. However,
hominins, instead of growing claws, sharper teeth, or stronger longer legs to help us run faster,
have instead became highly encephalized, whereby we developed much bigger brains relatively to our
body size. This is one of the main characteristics that set us apart from other animals.
Modern humans are the last surviving human in Homo genus and we dominate the world.
Humans got to this position not because we grew physically stronger or faster. It has everything
to do with the increased brain size. The human evolution is not really a physical one but a
cranial one. As our brains grew, we developed tool making and complex planning abilities,
and established societies. Even though the human brain is only about 2% of our body weight;
it consumes almost a quarter of our energy input. With the increasing brainpower and the caloric
intake it required, our muscles shrunk; while other mammals remain much more muscular than us.
Our survival and dominance depended on our increased brain size and cognitive
capacity but not brute strength. Otherwise. our more robust and now extinct relative,
the Neanderthals, would probably have been living here today instead of us. The Neanderthals were
stronger and more muscular than modern humans, and they had even larger brains.
Though with a bigger body, their EQ is smaller than ours.
Thus. Neanderthals were probably less intelligent than us. It’s conceivable that brain size changes
in hominins and other mammals are not really comparable. Therefore, using animal
domestication syndrome to compare human’s brain size decline might not be appropriate.
That leads to my 3rd and final question today – If the animal domestication model might not
work on humans, then what about our relatives, the Neanderthals, Denisovans and other archaic
hominins? Did they get self domesticated? Hominins all lived in communities and worked in groups.
The Neanderthals emerged in Europe as far back as 400,000 years ago.
They co-existed with modern humans for hundreds of thousands of years before eventually becoming
extinct about 28,000 years ago. Multiple archeological findings have proven that the
Neanderthals were quite smart and loving – they painted, cared for their wounded and buried their
dead, very much like the early modern humans. If there is self domestication in archaic humans,
then this phenomenon likely has had a much longer history and should be observed from
other hominins like the Neanderthals as well. Then did the Neanderthals domesticate themselves?
A 2019 study suggests that modern humans domesticated themselves after they split from
Neanderthals and Denisovans, approximately 600,000 years ago; which in a way suggests that these
researchers didn’t consider Neanderthals having domesticated themselves. According to this study,
Homo sapiens started self domestication about 600k years ago and domestication leads to smaller
brains and docile temperament. Then we should have seen a brain size decline starting 600k years ago.
But this is not the case – because human brain size and encephalization continued to grow bigger
for 500,000 years and peaking around 100k years ago. It’s only during the last 100k
years that our cranial size and encephalization decreased. How come the domesticating process
caused two opposite effects? At 1st with an increased brain size and then decreased?
Some learned professors argued that self-domestication resulted in smaller human
brains and tamer people so they could work in groups. But the human species have always worked
in groups and lived within a community. Hominins evolved for over 3 million years and the volume
of the human brain has nearly tripled during this long period of time. They lived and worked with
each other from the get go and have been doing do for millions of years. They were capable of social
communication and collaboration through the time when their brain sizes kept growing. They didn’t
need to be domesticated, to have a reduced brain size to be tame enough to work with one another.
So, there you have it - my three questions, which conflict with if not challenge the
human self-domestication hypothesis. Is self-domestication really the reason to
explain our declining brain size? I would like to hear your thoughts on this subject.
I think it’s likely that more than one factor led to the recent reduction in brain size. Human self
domestication might have occurred during certain time frames and influenced brain size changes
but it’s far from conclusive. If you have any insights, please leave me a comment. If you like
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support ` . This is Curious Being, I am Tina. Thanks for watching and see you next time.