Without a fossil representation, how do we know that placental mammals share a common ancestor?  Well, today we're going to talk about mammalian diversification!

Sixty-five million years ago, a mass extinction event occurred that wiped out 70% of all life on the planet.  Until recently, evolutionary biologists believed that it was because of this mass extinction event that placental mammals were able to go on to evolve, diversify, and become a globally dominant vertebrate group.  Because there was suddenly a lack of predators and no longer competition for major food sources, the world essentially became a major "all-you-can-eat" buffet.

Mmmm...buffet...

So, mammals were able to diversify and occupy new niche little areas of the environment that had previously been occupied by Little Foot and Ducky.  Before then, the only mammals that we had walking the planet were monotremes, primitive egg-laying mammals similar to the echidnas and platypus that we have today.  This old theory has been refuted in a recent paper published by the journal Science.  Now researchers claim that major mammalian groups diversified before this major mass extinction event.  This means that placental mammals lived alongside their monotreme friends, and it was after the mass extinction event that they continued to break apart even more and diversify.  But there's still somewhat of a divide between those scientists that work with fossil morphology and those that work exclusively with genetic data.

Mmmm...genetic data...

The controversy arises when attempting to determine when exactly the placental mammal began to show up on Earth.  Many geneticists contest that placental mammals were already present during the Cretaceous period, but if we don't have any fossil evidence of this hypothetical placental mammal, how do we know it existed, or even what it looked like?

The new study took relevant information from the gigantic online databases like Morphobank and GenBank and used them to try to answer this question.  Through this database, researchers from all over the globe were able to analyze animals that we do have evidence for.  They looked at both morphological, or physical, characteristics as well as gene sequences in order to paint a more cohesive picture of our common ancestor.

So, not only now are we able to see when in the evolutionary timeline placental mammals split from the monotremes, we can also form a shrewd idea--get it?  Shrewd?  Shrew?

[Image of mounted shrew]

...but we can also form a shrewd idea of what this early placental mammal may have looked like and even how its DNA was structured.  It's like a big game of connect-the-dots, but with really intense molecular phylogenetics instead of the ABC's.

So, what did this hypothetical placental mammal really look like?  Well, researchers were able to determine everything about its physical appearance from how its skull would've been shaped to how many teeth it would've had and even how its sperm would've swum.  They determined that it probably looked a lot like shrews that we still have today:  small and quick with lots of tiny, sharp little teeth in order to eat all of the food that the dinosaurs couldn't because they were dead.

In 200 thousand relatively short years after this major extinction event occurred, this species was able to diversify into many of the major mammalian groups that we still have today--everything from bats to whales to bunnies and kitties and puppies.

So using a combination of different types of data contributed by dozens of researchers from all around the globe, in addition to some really well-engineered computer algorithms, we've been able to build and date the tree of placental mammal evolution.  Where a few decades ago we would've only been able to guess at ancient species and their relationships between other ancient animals and even living animals that we have today, today our reconstructive methods are so good that we're able to fill in the gaps of the fossil record.

[Echoing voice] Hashtag science!

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...it still has brains on it.