These two skulls are noticeably different in size. On the left is a skull called Oasa 2 from what is
now Romania. They are believed to be a male, lived about 40,000 years ago, and had a cranial capacity
of between 1525 to 1600 cc, cubic centimeters. The other is the skull of a modern European male,
the average modern capacity being 1411 cubic centimeters plus or minus 113. This is measuring
the volume of the skull so it takes into account the thickness of bone, we can't blame that. The
smallest estimate of the Oase 2 skull would place them amongst the biggest brains of today. For some
reason it seems that since the end of the ice age ,about at least 30,000 years ago, our brains
have shrunk, which is an extremely curious and fascinating example of modern, recent evolution.
This is so odd because the general theme of human evolution over the past three million years has
been a gradual increase in brain size, on average gaining about 70 cubic centimeters every 100,000
years. Apart from a couple notable exceptions, of course, shout out to homo floresiensis.
Yet recently our brains began to shrink. Similarly to the male skulls I just showed, in the past
10,000 years the skulls of European women decreased from an average of 1502 ccs to 1241cc's,
a decrease of 240 cubic centimeters. That's basically a lemon's worth of brain just gone. You
wouldn't want to lose a lemon's worth of brain right now, you would consider that an absolute
disaster. This hasn't just affected European populations it's been observed in China, southern
Africa, Australian aboriginals, other regions as well. A huge geographic spread, a diverse
cross-section of humanity. Though whether it is truly a global phenomenon is still up for debate.
Bizarrely this decrease in size has occurred at the same time that human
civilization achieved its greatest artistic, scientific, and cultural achievements. There's no
obvious sign of a cognitive decline at all, the exact opposite in fact. So what's going on here,
how do anthropologists explain this phenomenon? Well, to be clear, no one knows why it's happened
but there are some ideas. Today i'm going to discuss three hypotheses,
the last one having extremely interesting implications for the future of humanity.
The first hypothesis is quite a contentious idea, human self-domestication, also phrased as
selection against reactive aggression. By reactive aggression anthropologists mean in-group fighting,
lashing out at those around you. Definitely not intergroup fighting which we still sadly excel at.
Domesticated animals exhibit much less aggression than their wild counterparts.
Naturally this was a trait that early farmers selected for, early farmers desired.
An animal has to be willing to live alongside you, to be handled by you.
If you're constantly having to fight it, it's not gonna make for an easy life for the farmer.
When people in the upper paleolithic and the neolithic selected animals based on this increased
sociability and reduced aggression certain physical changes also appeared in their bodies.
Reduced size, shortening of the face including a reduction in teeth size, reduced sexual dimorphism
males and females are more similar in size and appearance, importantly for this video reduced
cranial capacity, smaller brains. These changes are called domestication syndrome and there are
lots of examples of this. The brains of dogs are approximately 29% smaller than those of wolves,
pig brains are 34% smaller than the brains of wild pigs, cow brains are 25% percent smaller than
auroch brains and this decrease in brain size does seem to go hand in hand with reduced aggression.
Cows bred for bull fighting who have very little contact with humans until their "debut" fight and
are bred for aggression not other attributes like milk production, have the largest brains of all in
a study of 71 different cattle breeds. So amongst cattle more human contact, more sociability,
seems to result in smaller brains, less contact, more aggression equals bigger brains.
Now these features might not manifest uniformly across all species and there are of course
other changes that happen to domesticated animals like more variation in fur color and increased
lactation etc etc. But there's lots of evidence for these four changes occurring and arguably
these are the changes that have also occurred in homo sapiens compared to other archaic humans.
Look at our skull compared to a neanderthals.
Our faces have moved further below the skull, our brow ridges have almost disappeared,
our teeth are smaller, our jaws are thinner, we developed a chin, and arguably our brains
are slightly smaller. These are extremely similar to the differences between dogs and wolves. There
are even signs in our DNA that homo sapiens have this domestication syndrome. Isn't that crazy,
isn't that so crazy. So have we also had this pressure against reactive aggression,
have we somehow domesticated ourselves by perhaps slowly removing the most violent amongst us from
the gene pool? Well the jury is out basically, there are still loads of unanswered questions.
The first problem is that these changes seemingly have not occurred at the same time.
Judging from the Jebel Irhoud skull, the earliest archaic homo sapiens we have found so far,
our faces had moved further below the brain case 300,000 years ago. Suggesting this process
had already began at the very beginnings of our specific lineage. Yet this reduction in brain size
has only occurred since the end of the ice age, maybe even more recently than that. FORESHADOWING.
So that's a bit of a head scratcher. The second major issue is that we have no
great way to explain this mechanism. These changes probably occur in domesticated animals because the
farmer has removed many selective pressures that are found in nature. The pressure for large males
to compete against each other for reproductive success, gone the farmer decides who mates. The
pressure for animals to be large, fast, agile in order to better fend off predators, gone
farmer and his dogs will protect you. When we're talking about humans, what was driving
this change? We don't have farmers deciding who we do and don't boink. Although you could do,
i'm not here to kink shame anyone. If you do, that's totally fine it's just not typical.
Perhaps the development of lethal weapons allowed our prehistoric ancestors to safely, shall we say,
remove more aggressive people from the gene pool. Very possible, however lethal weapons
are probably not just restricted to homo sapiens. Archaic humans had spears at least,
but true projectile weapons do seem to have only been produced by homo sapiens so
maybe the bow and arrow literally altered our evolution. Perhaps more cooperative breeding
and the raising of children, living in tighter communities, selected against aggression. But
why did this only affect us? Other archaic humans almost certainly raised children collectively.
Food sharing and things like this almost has to take place in any human society because
our calorie requirements are so great, and then also why did this only affect homo sapiens after
the ice age? We were certainly raising children collectively long before that. Maybe the
development of language allowed people to scheme and team up on the bullies in their communities?
Again, entirely possible but testing when language developed in the archaeological record
is extremely difficult. It almost certainly occurred before the ice age
and archaic humans again probably had some way to communicate. No doubt this was a really long,
complicated and drawn out process and there was probably many factors that affected this.
So to summarize hypothesis number one, human self-domestication. It genuinely
does a very good job of explaining a lot of our physical features and it could very well have
happened, and of course our modern society would not be possible if we were constantly
lashing out at everyone around us, but it seems to have begun much earlier than our brain size
reduction has been detected and it's not easy to understand why this process of self-domestication
affected homo sapiens so greatly but not our close relatives like neanderthals.
It seems that there were other factors at play, other causes of this brain size reduction.
Hypothesis number two, we've just generally gotten smaller. Humans have genuinely decreased
in size slightly in the last 50,000 years 30,000 years. A couple of factors might explain this.
First, after the ice age, a shift away from hunting huge animals like mammoths
and wooly rhinos into a more broad spectrum economy. Hunting smaller animals, eating a
diverse array of plant foods, using technologies like bows and arrows and nets that
don't require us to getting close and stab a rhino.
These just may have resulted in us needing to be less physically big than our ice age ancestors.
Secondly, later a shift to an agricultural diet. Around 12,000 years ago possibly the
most significant change in all of human history occurred. For 1000 years prior the earth had
suddenly plunged back into temperatures of the last glacial maximum. However, 12,000 years ago
the climate warmed and more importantly became a lot more stable. This new climate allowed humans
to experiment generation after generation with the same plants, same animals and in the next
few thousand years agricultural communities sprang up all around the world. The population
grew massively, trade specializations developed. It's really the foundation of our modern world,
truly. But life wasn't easy for these farmers. Crops fail, animals die, their diet may have been
lacking certain key nutrients, and we seem to have a lot of evidence of malnutrition in these early
farming communities. Probably as a result of this malnutrition, our bodies did get smaller, and if
our bodies are smaller our brains are smaller too. Our brains are by far the most expensive organ in
our body, consuming twenty percent of our calories despite the fact that they only account for two
percent of our body mass. So If you're living in an environment of nutritional stress, there's
probably some advantage to having a smaller brain, to requiring less calories, and obviously
less calories in your diet means less calories are going to your brain anyway. It's estimated that in
the last 10,000 years we lost about 5 kilograms of body mass. The relationship between brain size
and body size is very well understood so how much brain should we have lost if our bodies decreased
in mass by 5 kilograms? According to John Hawkes, this body mass reduction should have resulted
in a 22 cubic centimeter decrease in our brain size, but the average decrease in
brain size in this time period has been between 100 and 150 cc's. More than five times what you
would expect to see. So to summarize hypothesis number two, just general body size reduction,
it definitely happened and of course that is going to affect our brain size, but our brains
have shrunk much more than is expected and it seems that there must be other factors at play.
Hypothesis number three, the one with the really intriguing implications. So key to this whole
debate obviously is the timing of brain size reduction. Human self-domestication seems to have
started at least by 300,000 years ago, though some argue it really kicked off with the development of
more modern human behaviors, say 100,000 years ago. Agriculture being to blame would obviously
depend on this brain size reduction happening around 12,000 years ago 10,000 years ago.
In order to understand this problem better, a team of scientists set out to determine
exactly when this decrease occurred. They analyzed 985 skulls from throughout
human evolution try and pin down the timing and uh these are the results. Ba da ba da boo.
Starting roughly 10 million years ago with the miocene hominins through to the australopiths
at around 3 million years ago, there was a slow and steady increase in noggin capacity.
Then around 2.1 million years ago they spotted the first huge spike in brain growth, this coincides
with the evolution of homo erectus and probably technological and social developments. Better
hunting strategies, more efficient digestion of food through cooking, more social complexity,
things like that. This rapid brain growth continued until around 1.49 million years ago.
From this point on growth slowed but there was still a steady increase in brain size all
through the pleistocene, not decreasing until around 3000 years ago! That's extremely recent,
that's super recent, talk about modern human evolution, 3000 years. At least from the skulls
selected in this study and remember any study is only as good as the material provided for it,
this decline is 50 times faster than the preceding brain growth of the last 3 million years.
Incredible! What to make of this? Decreasing brain size without any evidence of a declining
cognitive ability does suggest that there is perhaps some pressure towards efficiency for
our brains and smaller brains probably result in easier births, so the smallest brain you can
probably have and still function in human society is probably an advantage. But an efficient brain
and easier childbirth is an advantage in any time period, why in the last 3000 years have our brains
seemingly decreased in size so rapidly? The authors of this paper wonder if this
drive for efficiency was compounded by new human developments, new human inventions.
Throughout human history, but especially within the last five, six, seven thousand years,
human society has become increasingly complicated. This was not because we had certainly become so
much smarter than our prehistoric ancestors, far from it, but because a growing population just
necessitated more complex and dynamic systems of interaction. We started living truly urban lives,
so many different craft specializations. I mean we literally don't need to understand how anything
works in this society to function as a modern human. I have no idea how you are watching this
video right now, absolutely none. But that doesn't stop me from taking advantage of that technology,
just so long as someone else knows how to use it. We've externalized information hugely through
writing, so we're not required to remember absolutely everything and if it falls from
our memory it doesn't matter it's not lost to us as long as we wrote it down somewhere. As
a result of these developments perhaps you could argue that decisions, really important decisions,
that affect our chances of survival, our group survival are being taken
really collectively. When we consider how expensive our brains are to maintain,
maybe now that we've externalized all this information and are taking decisions so much
more collectively, perhaps there is an advantage in them decreasing in size a little bit?
Any one of these hypotheses could be right or wrong. It's entirely possible that they have all
played a part in the evolution of our brains, but what is so intriguing about that last hypothesis
is the implication it has for the future of our evolution.
Which is entirely speculative and impossible to predict so don't hold me to these predictions
in any way whatsoever. But if social complexity and externalization of information
and collective decision making has reduced the size of our brains in the last 3000 years,
then what will happen in the next 3 000 years now that our ability to externalize
information and communicate and take decisions globally has increased dramatically infinitely?