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.
