If you’ve ever visited a natural history museum, you’ve probably seen timelines like this,
or maybe branching ones like this, showing you the evolutionary chronology of familiar mammal
species. And in most of these timelines, if you all the way back to the beginning, you’ll find a
pint-sized version of mammals you know today. This isn’t a coincidence. Modern mammals originated
from species tiny enough outlast the extinction event which wiped out the dinosaurs, and then grew
in size and diversity to fill the ecological niches the dinosaurs left in their wake. As
result of this shared origin, looking at the ‘first drafts’ of mammals, so to speak, kind of
feels like looking at someone’s baby photos. And most of these miniature ancestors of the species
we know today emerged during the Eocene period, a geological epoch which gets its name from Greek
terms meaning “New Dawn,” an appropriate moniker considering the Eocene represented the ‘dawn’ of
modern mammals. So, let’s take a look at the tiny mammals who populated this period, and better
understand their connection to the mammals we know today. Beginning with Moeritherium,
this extinct genus of rotund, stubby Eocene mammals is a distant relation of mammoths and
elephants. The size and body shape of the animal makes the Moeritherium look more like a modern pig
than an elephant, standing only 70 centimeters or 2.3 feet high at the shoulder. The shape of their
skulls suggests they didn’t have a fully-formed trunk like an modern elephant, but more of a
stumpy upper lip for grasping aquatic vegetation. Although maybe an offshoot instead the direct
ancestor to modern elephants, it’s still amusing to think that other species in this diminutive
taxonomic order would one day become the largest land animals on the planet. Here’s another genus
from the early Eocene which looks quite different from it’s gigantic modern ancestors. This is the
Ambulocetus the four limbed ancestor of the modern whale. Their name means "walking whales” because
scientists once belied they were amphibious and could walk on land as well as swim. Although
most modern research suggests they were fully aquatic, the concept of a sharp-toothed, furry
whale with legs is still pretty incredible. Most modern whale species actually have trace remnants
of vestigial leg bones in their tails however from this early land to ocean transition period:
pretty interesting. What’s also interesting is Ambulocetus, as a predator, might have preyed
on some early genus of horses due to a temporal overlap. If this is indeed the case, the idea of
a whale which ate horses is… quite jarring, to say the least. Speaking of early horses,
Hyracotherium is an extinct genus of very small odd-toed ungulates basal to the Equid family, a
taxonomic classification which eventually gave us the horse — as well as the donkey and the zebra.
Although not technically Equid themselves like they were once believed, Hyracotherium have quite
the evolutionary lineage, which is impressive considering just how tiny these species were.
These proto-horses maxed out at about 9 kg or 20 pounds, about the weight of a Miniature Schnauzer.
Taking to the skies we have the Onychonycteris, a creature from the early Eocene which
looks somewhat alarming from fossils alone, but more familiar in re creations. — it’s the bat,
or the most primitive taxonomic ancestor of the modern bats we’re familiar with.
Analysis of the bone structure suggests the Onychonycteris could already fly,
and probably echo-locate. Indeed, compared to other species, it seems. early bats already had
it all figured out… evolutionary speaking. Up next is a middle Eocene animal I felt I had to mention.
The Andrewsarchus is an extinct genus which might have been the largest predatory mammal
ever to walk the earth. The three foot-long wolf like skull we’ve uncovered alone suggests an
animal even larger than a modern grizzly bear, for example. So, what is this early mammal the
ancestor of? Well, not much, actually. Most scientists think the Andrewsarchus has no
direct evolutionary descendants whatsoever, and was an offshoot which died out, proving
in biology as in life, you can never tell what who will succeed long-term based on early signs
of promise. Ending our adventure through first drafts of mammals, we have the Omomyidae, a group
of early primates with massive eyes, similar to the startling peepers on a modern-day Tarsier.
So what are these perpetually shocked animals the ancestor of? You and I. Well, maybe. It’s
difficult to say for certain if Omomyidae are the evolutionary ancestors of humans and other apes,
but it’s certainly possible. If so, our ancestors weighed around .5 kilograms, or
just over one pound. Pretty tiny. Despite this, they had some early versions of features your
might recognize on yourself, like grasping hands with complex digits. It’s difficult to know much
else about these organism dye to scattered remains, but if they are an early branch
in our familial tree you might be looking at your evolutionary baby photos, to put it a certain way.
And so, our journey through early Eocene versions of mammals you might recognize today comes to an
end. I hope you enjoyed this video, and learned a thing or two along the way.
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