A bat fluttered through the sky above whatâs
now Wyoming some 52 million years ago. But it wasnât like the bats you and I know. It was small, like most modern bats, but it
had claws on the tips of all five digits that supported each wing. And its wings were a bit shorter, while its
hindlegs were a little longer. With those claws and long hindlimbs, it was
a better climber than most modern bats. And its teeth suggest that it ate insects,
but it probably didnât use echolocation to find and catch them. This little flying mammal was Onychonycteris
and it was definitely a bat - the most primitive bat that we have good fossil evidence of,
and also one of the oldest. And on the Mammal Family Tree, it sat on the
next-closest branch to all other bats, living and extinct. But when you trace that branch back...well,
itâs a mystery. Bats pretty much appear in the fossil record
as recognizable, full-on, flying bats. And they show up on all of the continents,
except Antarctica, around the same time, in the early Eocene Epoch. However, the very earliest fossils consist
only of teeth and limb bones, which donât give us many clues about what the ancestors
of todayâs bats looked like. So where did bats come from? And which of the many weird features that
bats have, showed up first? Like, did these mammals learn to fly first? Or echolocate first? Or ... both? And how do they fit into the mammal family
tree? We used to think that -- based on their skeletal
structure -- bats were closely related to primates: like...us! Which is kind of strange, when you think about
it. But once we were able to study their genetic
history, their DNA revealed something much weirder. Instead of being closely related to the mammals
that they kinda look like, bats turn out to be much more closely related to the ones that
they ⌠donât. The fossil record of bats is both great and
terrible - which is why weâve waited so long to do this episode. Itâs great, in that, several of the earliest
fossil bats we have are exquisitely preserved. Theyâre basically complete, because they
were buried in an ancient lake deposit. The best bat fossils come from these types
of lagerstatten, or fossil sites with exceptional preservation. On the other hand, the fossil record of bats
is also terrible, because itâs either that or nothing. Most bats are small and have thin, fragile
bones. This keeps them light, which makes flight
easier, but it also means they donât preserve well. But we are lucky enough to have a few specimens
of very old bats. Flying around at the same time as Onychonycteris,
and likely sharing the skies with it, was Icaronycteris index. This species also comes from Wyoming, and
for a long time it held the title of earliest known definitive bat, since it was first described
in 1966. Like Onychonycteris, which was published in
2008, Icaronycteris dates to around 52.5 million years ago. And because these two were already distinct
from each other, we know that bats mustâve evolved sometime before this. Both bats were clearly capable of powered
flight and also looked a lot like most modern insect-eating bats. But modern bats only have claws on one or
two of their digits -- and Onychonycteris had claws on all five digits, while Icaronycteris
had a well-developed claw on its second digit and bony tips on three others.. And this ancient bat was probably capable
of echolocation, because it had specialized features on one of the inner ear bones -- called
the malleus -- and in the base of its skull that are linked to echolocation in living
bats. Also early bats werenât restricted to North
America. There are at least four more spectacularly
preserved taxa of bats from just a bit later, around 47 million years ago, from the Messel
Pit in Germany, a site weâve talked about before. These are also complete or nearly complete
skeletons, and in some of them you can even see the outlines of their soft tissues! And, like Icaronycteris, theyâve got all
the hallmarks of modern bats, and several of them seem to have been able to echolocate,
too. But all of these fossil bats had already mastered
powered flight: So how did their ancestors make it into the air in the first place? The answer to that question is tied up with
the evolution of another one of the defining features of most bats: echolocation. And among the experts, there are three competing
hypotheses for how bats evolved: either echolocation came first, or flight came first, or the two
developed together. All three hypotheses start with the same basic
assumptions based on the traits that are most common in bats today. These are that the bat ancestor was arboreal,
or lived in trees; it was insectivorous, or ate insects; and it was probably nocturnal. In the echolocation-first hypothesis, the
bat ancestor wouldâve already had ultrasonic capabilities to start off with. This seems like it could be a possibility,
because there are modern insectivores, like some species of shrews, that use ultrasound
for communication or navigation. So the thinking here goes that this ancestral
bat might have reached out from tree branches to snatch passing insects. Over time, the super high-pitched calls wouldâve
evolved into sonar that it could use to track its prey. And, being arboreal, its digits wouldâve
been selected to get longer, with a membrane stretched between them, to more effectively
capture food. Those long, webbed hands wouldâve then been
co-opted for gliding, when the animal started leaping to get at insects that flew further
and further from its perch. And eventually, it acquired adaptations for
powered flight. But the main problem with this hypothesis
is that this kind of hunting behavior -- catching insects that just happen to fly where you
can reach -- hasnât been observed in the wild. Plus, weirdly enough, it turns out that it
takes a lot of energy to echolocate -- especially when youâre stationary - so this whole feeding
strategy seems pretty inefficient. So what if they flew first, instead? In this model, powered flight evolved from
a gliding ancestor, which had originally started by leaping between trees or branches. This arboreal creature wouldâve developed
longer digits and a membrane between them as part of its gliding phase, eventually transitioning
into powered flight. Once this proto-bat was flying around, it
likely encountered insects, possibly scooping them up with its wings or catching them out
of the air. And from there, an energy-efficient form of
echolocation developed, with bats exhaling - and squeaking - in time with their wingbeats. Some researchers have argued against this
idea, saying that a leaping or gliding nocturnal animal without specialized senses - either
vision or echolocation - wouldnât be able to see where it was trying to land. But here at least, the fossil record can tell
us something: the skeleton of Onychonycteris shows that it was definitely capable of powered
flight, but it didnât have the cranial features that are linked to echolocation. Because the skull was partially crushed, we
canât tell if it had large eyes like many nocturnal gliding and leaping creatures do,
such as flying squirrels, but itâs still good evidence that flight likely came first. But thereâs still the third option to consider:
that maybe echolocation and flight evolved in tandem. In this hypothesis, the bat-ancestor originally
used ultrasound to communicate and was able to start using it like basic sonar to help
it plot its nighttime leaps between branches. As its ability to echolocate evolved in power,
so too did its ability to make longer jumps, which eventually turned into gliding and powered
flight. And those two adaptations -- stronger echolocation
and powered flight -- made bats the stealthy aerial predators of insects that many still
are today. The problem with this model is: Onychonycteris
didnât echolocate, but it did fly. Okay, so it seems that the fossil evidence
we have is in favor of the flight-first hypothesis. So thatâs one piece of the puzzle of bat
origins that we can kinda snap into place. But it still doesnât tell us where bats
came from! To figure out where bats truly fit in the
mammal family tree, paleontologists have teamed up with geneticists to study the DNA of living
bats. For a long time, bats were thought to be part
of the superorder Archonta -- the group that includes treeshrews, colugos, and primates,
because those are the mammals they look the most like. Now, superorders are, by their very nature,
incredibly diverse. But members of Archonta do share a lot of
the same skeletal features, from the presence of a tiny bone in the inner ear to the particular
way their ankle bones fit together. And some studies even suggested that bats
and colugos were more closely related to each other than to the rest of the group, based
on some features of their hands, elbows, and feet. Colugos are nocturnal, arboreal, and they
glide using a membrane of skin stretched between their limbs - like the transitional pre-bat
is hypothesized to have done. So itâs easy to see why scientists thought
they were closely related. And in the 1980s and â90s, an Australian
neuroscientist even suggested that the fruit bats evolved from primates, based on similarities
in the pattern of connections between the retina and the brain. But in more than two dozen molecular studies
carried out since the early 1990s, bats have never grouped with Archonta. Instead, all of these studies put bats into
a totally different superorder, one known as Laurasiatheria. This includes a number of the placental mammals
that are thought to have originated on the supercontinent Laurasia during the Late Cretaceous
Period. And this group is also very diverse, including
the orders containing moles, camels, horses, whales, pangolins, and bears - most of which
look /nothing/ like bats. So thatâs right. It turns out that bats are more closely related
to whales than they are to us. Within this group, analyses usually place
bats with a clade that contains pangolins, carnivores, and ungulates, or as the sister
group of shrews, moles, and maybe hedgehogs. But itâs still really unclear who within
Laurasiatheria bats are most closely related to and how. They seem to have come from some very primitive
mammal near the base of Laurasiatheria that also gave rise to one or more of the other
groups in the superorder. Another benefit of all these genetic data
is that they can give us a sense of when bats became a thing. According to studies based on that model known
as the molecular clock, bats seem to have originated around 65 million years ago, just
after the extinction of the non-avian dinosaurs. So where does that leave us in understanding
bat origins? Well, we seem to be getting closer to figuring
out the order in which bats evolved their most distinctive traits. It looks like flight came first, followed
closely by echolocation. And weâre still digging up wonderfully preserved
early bat fossils. As genomics continues to grow as a field,
we will hopefully be able to zero in on exactly what group bats are closest to. And this might be able to tell us what kinds
of traits to look for in a bat ancestor. It may end up looking totally different from
what we expected. After all, itâs happened before. But for the moment, the lack of enough evidence
-- both in the ground and in their DNA -- is keeping the true origins of bats in the dark. So what do you think? Did bats evolve flight or echolocation first,
or did it evolve in tandem? Let us know in the comments which hypothesis
you support and why! Also thanks to this monthâs Eontologists:
Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve! Be sure to go to patreon.com/eons and pledge
your support! And thanks for joining me in the Konstantin
Haase Studio. If you like what we do here, then subscribe
at youtube.com/eons.
If they've got whale relatives, could it be echolocation just went dormant for a while before reemerging? I wonder how similar those ear bones are!
I just want to say that I found out about this sub yesterday, and ever since, Iâve only been using Reddit solely for all things Bats vĂa this sub, and all of the other Bat subs I discovered yesterday.
And very quickly I began appreciating you more and more for your efforts on this sub/for bats in general!
Youâre doing a profound job, and I canât fully express my thanks or even begin to describe the amazing work and education youâre doing for bat lovers, bat haters, those uneducated about bats, and the immense impact on bats as a whole/batâs rights!
You have more dedication in your pinky finger than most people have in their whole bodies keep up the great work! :)