Hey, I have an announcement! I wrote a book! It’s called “Tales of the Prehistoric World.”
It features over 60 stories of paleontological discoveries with
illustrations by Becky Thorns. It’s rated for ages 8 - 10 but also anyone young at heart.
You can order it through your local bookseller or wherever else books are sold.
On December 6th, 1834, a young Charles Darwin was
visiting San Pedro island off the coast of southern Chile.
It was the latest stop on his long voyage around the world – the one where he began
to develop his ideas about the origin of species.
On the island, he noticed a tiny fox-like creature - a canid of some
sort - sitting on the rocks, completely absorbed in watching the men working.
The species was unknown to western science and
different from the other canids Darwin had seen in South America.
So, as the group’s naturalist, he decided to collect a specimen to send back to England.
The fox was so distracted that he was able to sneak up behind
it and knock it on the head with his geology hammer – not cool, Darwin.
He’d later describe that poor fox as “...more curious or more scientific, but less wise,
than the generality of his brethren.” In other words, curiosity killed the canid.
Today, we call this species ‘Darwin’s Fox’, and it’s one of ten very different canid
species that call South America home - the most diverse canid community found anywhere on Earth.
But while Darwin collected the fox for science, he’d never know the real scientific story it,
and those other canids, represented – though he probably would have loved it.
Because, while South America today has the most canid species,
until relatively recently it had none.
And the origin of these species? Well,
just like the finches of the Galapagos that would inspire him later on,
it turns out that South America’s canids are the result of an adaptive evolutionary radiation…
One driven by great migrations,
insurmountable geological barriers, and new ecological opportunities.
These canids were, in a sense, the finches that Darwin missed.
It would take nearly two centuries of further research, and development of
entirely new fields of science for us to finally figure out how South America got its canids.
And the reason that it was so mysterious for so
long is because the whole situation is kind of odd.
See, despite South America having the greatest canid diversity of any continent,
it was clear that they must have only arrived fairly recently.
After all, North America - not South - was the cradle of canid evolution.
That’s where the fossil record shows canids originated and evolved for tens of
millions of years, as far back as the late Eocene epoch, around 40 million years ago.
Some groups rose and fell in North America, and, in the late Miocene epoch, around 6 million years
ago, some groups migrated into Eurasia, giving rise to all modern Eurasian and African canids.
But canid migration into South America could only have become possible more recently,
within just the last 3 to 4 million years,
following the formation of the isthmus of Panama in the late Pliocene epoch.
This land bridge enabled an exchange of many species between the two continents
in a series of events called the Great American Biotic Interchange.
And for a long time, biologists have been trying to figure out
why the canids of South America are so diverse when they only arrived recently.
Did they diversify in North America and migrate south separately?
Or did they suddenly radiate from a single common ancestor
into radically different species once they arrived?
Well, the fossil record of canids in South America is pretty patchy,
so it can’t give us many clear answers to those questions.
But if you can't trace a story forward in time from ancient evidence,
then you have to trace it back in time from modern evidence.
And today, we can do this by digging into and comparing the genomes of
living species to reconstruct their evolutionary history.
It’s an approach that, I think is fair to say, would have blown Darwin’s mind.
See, he knew nothing about DNA, and, even today,
genomes are sometimes mistakenly just thought of as molecular recipe books for building organisms.
But they’re also historical records, as the pattern of mutations and genetic
diversity they contain reflect an organism's evolutionary history…
From the migrations of their ancestors, to the population expansions and bottlenecks
they underwent, to the selective pressures they faced, and more.
And with enough genomic data, sufficient computing power, and the right algorithms, it’s now possible
to reconstruct the history of populations far back into deep time, even with few fossils available.
So in 2022, scientists published the results of a
study applying this approach to solve the mystery of South America’s canids.
Using a mix of new and previously sequenced genomes, they compared
all ten living species from across the continent.
And the family tree they built showed that,
despite their many physical and ecological differences,
all ten descended from a single common ancestor that lived just 3.9 to 3.5 million years ago.
And, based on patterns of genetic diversity, they estimated that this
ancestral population was fairly small, with only around 11,600 breeding individuals.
This suggests that all South American canids stem from a single dispersal event of one
ancestral species from North America over the developing isthmus of Panama.
This single species gave rise to the most diverse canid community on Earth in just a few million
years - a staggeringly fast and large-scale radiation for a group of mammalian carnivores.
But that ancient canid lineage would have first been faced with a major geological barrier.
See, the Andes Mountains had reached their present-day elevation by 6 million years
ago and stretched the length of the continent.
And using a statistical approach called biogeographic modeling,
the researchers estimated that, while canids today are found on both the eastern and
western sides of the mountains, the ancestral species actually only entered via the east.
This suggests that they came from Central America
to the east side of South America by way of a natural corridor of grassy woodland.
We know from fossil evidence that many other North American mammal
groups also used this route during the Great American Biotic Interchange,
so it seems like the Andes were a major barrier to western dispersal.
And the researchers found the genetic signal of a massive increase in population size once those
early canids arrived - a result of the enormous ecological opportunities they found there.
They faced very little direct competition in their new environment, as the few South American
carnivores, like the marsupials and terror birds, played very different ecological roles.
So the canids were unlike anything else on the continent, which allowed them to thrive
and radiate into a range of unoccupied niches. Based on their genomic analysis, the researchers
also found that the first lineage to branch off was a group containing the bush dog and
maned wolf, which, despite their obvious differences, are actually sister taxa.
Much like the finches of the Galapagos, the distinctiveness of these canids from
each other is the result of wildly different niches that they fill.
The maned wolf became adapted for navigating through tall grasslands
and developed an omnivorous diet including lots of fruit,
while the bush dog specialized as a small, but powerful, hypercarnivore.
Despite having a common ancestor just 3.1 million years ago - around half the
divergence time of us and chimps - these two sister canids adapted
so differently to their environment that they lost almost all family resemblance.
Tracking the radiation of the rest of the canids, the researchers found they
continued to diversify on the eastern side of the Andes as they expanded farther south.
This gave rise to a second group that includes the crab-eating fox and short eared dog.
Then, starting around 1.4 million years ago,
the 6 fox-like species in the genus Lycalopex began to diverge.
And by 1 million years ago, a lineage of this group finally made it west of the Andes.
The researchers’ modeling suggested they went down and around the mountains, a journey of
thousands of kilometers along the southern edge of the range. Talk about taking the scenic route!
And this probably happened during dry periods of the Pleistocene epoch
that drove the expansion of more grassy woodland corridors that they could cross.
This lineage gave rise to three species on the western side of the range,
including our friend Darwin’s fox, in a final burst of speciation.
When he encountered that fox on San Pedro Island, Darwin had no idea that it, too,
had only recently made it there as part of an epic, trans-continental journey of its own.
And he never found out that this radiation of ten canids from one
small ancestral population was actually an unusually powerful example of the
evolutionary process he would later propose in The Origin of Species…
The process by which, in his own words, '...from so simple a beginning,
endless forms most beautiful and most wonderful have been, and are being, evolved.”
Don't forget! Our 2023 calendar featuring 14 reconstructions of
some of our favorite main characters of paleontology from classic Eons episodes,
are available now at ComplexlyCalendars.com.
And thanks to this month’s foxy Eontologists! Jacksy Weiss, Colton, Melanie Lam Carnevale,
Chase Archambault, Annie & Eric Higgins, John Davison Ng, and Jake Hart.
By becoming an Eonite at patreon.com/eons you’ll get fun perks - like submitting a joke for us
to read. Here’s one from Andrew M. Did you hear about the species of
skunk that evolved to lose their stink gland? They went ex-stinked!
That was actually...that, that was good, that was good, good job.
And as always thank you for joining me in the Adam Lowe studio. Subscribe
at youtube.com/eons for more prehistoric stories.
This is so...
We'll try one with [the hammer], we'll try one without!
The fox was so distracted that he was able to sneak up behind it and
knock it on the head with his geology hammer – not cool, Darwin, not cool.
It's so bad. It's so bad! Ohhhhhhh... it's so bad!
Ok, one without.