Thank you to Brilliant for supporting PBS! 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. To learn more about Brilliant,
go to brilliant.org/eons. 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...
