I came from That??!?

Last weekend I visited the Field Museum in Chicago.  It may come as no surprise to some of you that this museum completely blew my mind.  My friend Willard and I spent almost the entire time in their "Evolving Planet" exhibit, which walked us from the formation of the earth through to the present day.  Afterwards we talked about asteroids and looked at some wicked crystals.

Anyhow, the most surprising discovery from this visit concerned Dimetrodon, which when I was a child was one of my favorite dinosaurs from my extensive collection of plastic dinosaur toys.  My Dimetrodon toy looked vaguely like the one below, minus the "Dino-Strike Clamping Jaws".

What an awesome sail on his/her back, right?  Coolest dinosaur ever, or so I thought until my weekend stroll past the "Synapsid vs Diapsid" section of the evolving planet exhibit.  It was in this section that one of my childhood dino dreams died, and out of the ashes rose what I like to call the phoenix of discovery!  I discovered that Dimetrodon was not a dinosaur, but was instead our ancestor!!!!!  While that sinks in, here are some cool Dimetrodon pictures.

Right?!!?

The reasons scientists classify Dimetrodons as not being dinosaurs have mainly to do with the number of holes found in their skull behind their eyes (Dimetrodons had one, dinosaurs had two).  But it still blows my mind that one of our ancestors* (see note below) looked so much like a dinosaur but was not a dinosaur.  When did this split occur between "actual dinosaurs" and "not dinosaurs" happen?

To help answer that question, here is an evolutionary tree of life, known as a cladogram.

image courtesy of this Nature article

FYIs helpful in looking at this cladogram:
   1 - Dimetrodon was alive mainly during the Permian period (~275 million years ago)
   2 - We are Eutherians and dinosaurs were Archosaurs
   3 - You should learn more about monotremes, coolest order ever.  Here's a cool link to a platypus genome blog post.

Aside from the fact that Dimetrodon and dinosaurs began their evolutionary march apart 315 million years ago, what you might also realize from that cladogram is that whatever species it was that diverged in two at that point, that animal was our last common ancestor with birds (WE ARE ALL ONE!!).

Whew!  The Universe is a lot to take in sometimes....

All of this craziness makes me think of the Long Now Foundation podcast I heard by David Eagleman (currently my #2 science crush, right behind Carl Sagan), which he ended by reading a short story.  In it he describes a thought experiment where he spends five seconds pondering each woman in the line of matriarchs that led to him, starting with his mother and working his way back.  Read the story, but here is a tiny excerpt to ponder:

Three and a half years after I have begun, my great^22,075,801 matriarch is amphibious. Her features are duplicated in thousands like her. She follows the mesmerizing smells to find males in beams of moonlight. She finds irresistible the rush of the cold water after dropping in from the basking-rock, and she enjoys this ritual most of the days of her four years.

* Dimetrodon might be our ancestor in the way that your grandmother's sister is your ancestor, you know.  Difficult to prove that we are descended from Dimetrodon, but when we're talking about 275 million years ago, it doesn't matter much in my mind.