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We mammals are very proud of being the  big brains of the vertebrate world,   with the largest brains relative  to body size of any group.

Our big, complex brains mean that mammals  are frequently counted among the best   problem-solvers, planners, team workers,  and inventors that nature has to offer.

Of course, it tends to be us doing the  counting, so there might be some bias there.

And it’s tempting to think of this  feature as the key to our success.

Surely we live in what's sometimes called The  Age of Mammals for a well-earned reason... …It’s the reward that we won in part by  being, well, smarter than the relatively   small-brained reptiles that had  previously dominated the planet.

We prioritized brains over brawn, which  is always the winning strategy, right?

Well, maybe not.

It turns out that the journey  toward braininess in mammals   was far from straightforward or inevitable.

In fact, during one of the most pivotal  moments in our evolutionary story our   brains actually shrank relative to our bodies.

Now, when thinking about brains,   it’s important to remember that size  isn’t the only thing that matters.

Lots of other factors, like structure and density  of the neurons, also determine how brains work.

Take Neandertals and us modern humans, for  example.

They had similar sized brains to us,   or even a little bigger in absolute terms.

Yet, evidence suggests that differences in shape,  as well as neuron development and organization,   probably led to meaningful cognitive and  sensory differences between us and them.

But because those aspects of  the brain are often hard to   examine or compare between species -  especially extinct ones - scientists   generally just measure what's  called encephalization instead.

This is essentially a calculation  of a species’ relative brain size,   based on the ratio between their observed vs  expected brain mass for a given body mass.

Having a bigger brain than expected by body size  alone equals a higher level of encephalization.

And in vertebrates, encephalization seems   to broadly correlate with what  we recognize as intelligence.

Modern placental mammals are the most  encephalized vertebrates, with us humans,   the other great apes, whales, and  dolphins generally coming out on top.

But the evolutionary road  mammals took toward becoming   so brainy has been pretty mysterious for a while.

This is partly because well-preserved   fossil skulls from key periods of our  evolution have been hard to come by.

So we’ve been left with a lot of unanswered  questions, like was it a process that started   early in our evolution and continued  gradually, but inevitably, over time?

What ecological pressures  might have driven it?

And   how was it shaped by key moments in our evolution?

We know that the K-Pg extinction  for example - the one that wiped out the   non-avian dinosaurs around 66 million years ago  - was a major event in the story of our rise.

And one common idea was that this period of  upheaval may have encouraged the evolution   of bigger brains in our ancestors by  rewarding ecological street smarts.

But ideas like this had actually been really tricky to test.

So, in 2022, a team of paleontologists  published a new study of brain-size   data from 124 species covering much of  the evolutionary timeline of mammals, from ancient shrew- and badger-sized species that  lived alongside dinosaurs in the Mesozoic Era,   to the first members of many modern groups  that emerged in the mid-Cenozoic Era.

And they high-resolution CT scanned  some newly discovered fossils from   the crucial window in the middle of  this timeline: the Paleocene epoch,   the time period that includes the  direct aftermath of the K-Pg extinction.

But instead of the intertwined chaos and  opportunity of that situation rewarding   mammals that could problem-solve and adapt, thus  pushing us inevitably towards ever-bigger brains,   the data told the complete opposite story.

See, mammals from the end of the  Mesozoic, just before the K-Pg,   were generally small-bodied and small-brained.

Their encephalization was much lower  than that of modern mammals - although   the proportion of their brain  responsible for smell was higher.

Otherwise, though, their sensory capabilities and   overall intelligence were probably  nothing to write home about.

Yet, mammals that came after them in  the Paleocene, in the wake of the K-Pg,   didn’t increase in relative brain  size from this low base level,   it turns out.

They shrank even  more as the planet recovered.

Now, this isn't to say that their  brains shrank in absolute terms   compared to their Mesozoic ancestors and  relatives - they actually got bigger.

But mammal body sizes grew at a much  faster rate over the 10 million years   post-K-Pg, which left them less encephalized  overall than both later and earlier mammals!

So what happened here?

Well, the answer goes back to one of the most  basic things we know about evolution: it’s a game   of constant trade-offs, and in some ecological  situations being ‘smarter’ isn't an advantage.

Having a big, powerful brain is really expensive  in terms of energy requirements.

Pound for   pound, brain tissue uses almost an order of  magnitude more energy than other body tissues.

Despite providing greater cognitive, behavioral,  and sensory capabilities, in some situations,   having to fuel a big brain can reduce an  organism’s ability to survive and reproduce.

And the immediate aftermath of the K-Pg  seems to have been one of those times.

With the demise of the non-avian dinosaurs,  mammals that survived the catastrophe were faced   with an opportunity that would define the rest of  the Cenozoic and usher in the ‘age of mammals.’ As ecosystems recovered and new food webs emerged,   previously unavailable ecological  niches were now open to them.

Take the pantodonts, for example, an  ancient extinct group of medium- and   large-sized herbivorous mammals  with proportionally tiny skulls.

They included some of the earliest  recorded Paleocene mammals to get   bigger from the much smaller Mesozoic  ancestors, filling gaps in the   ecosystem that had previously been occupied by  medium- and large-sized herbivorous dinosaurs.

And it seems that getting  brawny rather than brainy was,   quite literally, the bigger priority  for radiating into those niches.

The CT scanning the researchers did also  revealed how the size and shape of different   brain regions changed over this period - or  more specifically, how little they changed.

Because the brains of the pantodonts  and other Paleocene mammals grew only   as much as necessary to control their larger  bodies, which scaled up at a much faster rate.

So, during this time, the increase  in size did not include any major   changes in the brains’ structure  or the addition of new features.

This left them with proportionally  smaller brains and no new sensory or   cognitive regions, compared to Mesozoic mammals.

In this new post-apocalyptic  world with lots of empty niches   and little competition, thinking was overrated.

In fact, the researchers found that mammal  brains only really started to catch up with   their bodies as the Paleocene gave way to the  Eocene epoch, around 56 million years ago.

Multiple mammal lineages independently  experienced shifts toward much greater   encephalization around this time, with body  growth slowing down and brain growth picking up.

And that was probably a result  of ecosystems fully recovering   by this point and reaching  a high level of saturation.

In such rich and competitive ecosystems, bulking  up was no longer enough to secure success.

This new world instead rewarded the ability to  think on your feet…or paws?...hooves?, so being   brainy finally became a major advantage for many  mammal lineages simultaneously - including ours.

The researchers found that encephalization really   took off in certain early mammals  that emerged around this time:   the ones that were ancient members of  living groups that are still around today.

They included the first horses, whales,  dogs, bats, and primates, which all saw   huge increases in encephalization compared to  both their Paleocene and Mesozoic relatives.

And their brains didn't just scale up,  they changed radically in structure, too.

Their olfactory bulbs, which are  responsible for sense of smell,   became proportionally much smaller.

And the regions of their brains  involved in vision, eye movement,   and balance became proportionally  larger, along with the neocortex.

This is a highly complex outer region  that helps to integrate processes and anticipate sensory information, and  is responsible for higher-order thinking.

Growth in these regions gave those  more-encephalized mammals the edge over   more ancient small-brained critters like the  pantodonts, who still relied mostly on smell.

And that might be a big part of what led to  those ancient mammal lineages declining   over the Eocene epoch as the newer  groups radiated and outcompeted them.

So, when all is said and done, maybe  we mammals should be proud of our big,   complex brains, as they’ve served us  well over our more recent evolution.

But if you trace our story back far enough,  they actually dropped a place on our list   of evolutionary priorities -  and at a pivotal moment, too.

And all this shows that there’s no inevitable,  linear, evolutionary progression towards bigger   brains and higher intelligence - or  any other traits for that matter.

Evolution is about trade-offs,  contingency, and ecological context.

And that’s something we  should always bear in mind.

Thank you to Brilliant for supporting PBS.

Brilliant is an online learning  platform for STEM with hands-on,   interactive lessons.

Brilliant is for  curious learners, both young and old,   professional and inexperienced.

Brilliant courses  teach you how to think (via interactive lessons   and problem-solving activities or exercises) and  solve problems with interactive lessons in STEM.

For example, Computational Biology was written  in collaboration with quantitative biologists   and biophysicists.

This course merges  the algorithmic thinking of the computer   scientist with the problem-solving approach  of physics to address the problems of biology.

Since the year 2000, an ocean of sequencing  data has emerged that allows us to ask new   questions.

Here you’ll develop an intuition  for a selection of foundational problems in   computational biology like genome  reconstruction, sequence alignment,   and building phylogenetic trees to  look at evolutionary relationships.

You’ll also address certain problems of molecular biology like RNA folding.

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Thanks to this month’s brainy Eontologists!

Gale Brown, Juan M., Jacksy Weiss, Melanie Lam  Carnevale, Raphael Haase, Annie & Eric Higgins,   John Davison Ng, Jake Hart, and Colton.

By becoming an Eonite at patreon.com/eons   you can get fun perks like submitting a  joke for us to read.

Here’s one from Todd.

What's the most polite prehistoric reptile?

A PLEASE-iosaur!

And as always thanks for   joining me in the Adam Lowe studio.

Subscribe  at youtube.com/eons for more fabulous fossils.

I'm gonna take a sip of my Chai.

I'm from LA, so I'm like, iced Chai is a  February drink, I don't care where I am.

It's too hot to have warm Chai in LA, so, I've never wanted Chai to be warm... and now I just, that's who I am now.

They're like, "Oh, you want coffee  that's the heat of the outside world?"

See, I'm in equilibrium with  the outside world right now.

It's cold outside, I'm cold outside...