Translator: Rhonda Jacobs Reviewer: Ellen Maloney
So here's a question for you:
Has a human ever ridden a dinosaur?
My answer, as an evolutionary biologist, may surprise you.
Well ... an ostrich is a bird,
and we now recognize that birds are dinosaurs,
so by riding ostriches, yes, indeedy, humans have ridden dinosaurs.
Why anyone would want to ride an ostrich is a completely separate question.
(Laughter)
But what's wrong with the panel on the left?
Time. Time is what's wrong.
This sort of non-avian dinosaur went extinct tens of millions of years
before humans ever came on the scene.
Time and its relation to evolution
is one of the things I'll be talking to you about today.
But before I go any further, I'm going to define evolution,
so we're all on the same page.
Evolution is when a living thing sustains a change,
generally a genetic change that it passes on to its offspring,
and that change allows all those living things
to better adapt to and survive their environment.
For those of you who struggle with evolution,
and 40 to 50 percent of Americans do,
hopefully this makes it a little bit more clear.
So, when it comes to time,
evolution can occur on a huge range of timescales.
You may not be as familiar with this,
but evolution can occur on a short timescale,
on the order of minutes to months.
Examples of this are cancer, and bacterial antibiotic resistance.
If you think of cancer, a cancer cell is a living thing
that undergoes one or more genetic changes that allow it to escape its confines,
migrate to distant sites, reproduce a bunch of times,
thereby passing on those changes to its offspring,
and ultimately form a tumor.
I'm not saying all living things are tumors,
but we are like them in some ways.
So the evolution you are probably more familiar with,
is evolution on a long timescale - years, even billions of years.
It's the exact same process as with cancer,
but instead of just a couple of changes,
we're really looking at millions of changes.
There are millions of genetic changes, for example,
between our ancient ancestors and modern humans.
It takes a really long time,
not just for all of these changes to occur,
but for them to be selected for and to spread throughout a population.
What I study is the long, deep evolution of birds from Mesozoic dinosaurs.
I've been working with Jack Horner on this project, the DinoChicken Project,
and it was indeed inspired by the Jurassic Park movies.
Jack is famous for saying, "Why don't we turn a chicken into a dinosaur?
Why not?"
He surmised that we could learn a heck of lot
about dinosaur-to-bird evolution, and as usual, he was right.
But there's a lot to study here
because we're looking at about 150 million years of evolutionary change
between what is generally regarded as the first bird, Archaeopteryx,
my favorite dinosaur, and, say, a modern chicken.
Because there's a lot to study, we had to narrow it down.
We decided to start with bird tail evolution.
In the Jurrasic and Cretaceous, the most primitive birds
had long, reptilian-like tails.
But about 130 million years ago,
the fossil records show a sudden appearance of short-tailed birds.
Long- and short-tailed birds co-existed for a couple of million years,
but since then, all birds have had this modern tail configuration -
there are fewer vertebrae in the tail, and the vertebrae at the end
are all fused together into a structure called the pygostyle.
This modern tail has been hugely advantageous for birds,
with improvements to flight lift and aerodynamics,
and concomitant changes in tail feathers likely impacted sexual selection.
Female birds seemed to think
that male birds with striking tail-feather displays are pretty shiny.
(Laughter)
So, how do we study bird tail evolution
or really any event that occurred millions of years ago?
For this we use a strategy called evo-devo -
evolutionary developmental biology.
We are currently at an incredible crossroads in science,
where multiple fields are converging together
to give us unprecedented understanding of biology.
Paleontology is no exception.
We can now combine paleontology and developmental biology
to understand not just what happened and when,
but now we can understand how something happened.
So using evo-devo, where do we start?
Well we start with the fossil record.
The fossil record shows us that about 130 million years ago
there was an abrupt transition of long to short tails,
and despite lots and lots of searching,
no one has ever found a Cretaceous intermediate form,
like a bird with a short tail without a pygostyle,
without that fusion of vertebrae at the end.
So now that we know the what - tail shortening and tail fusion -
and the when - 130 million years ago,
what steps do we take to get the the "how"?
We look at DNA, at genetics.
We don't have Mesozoic dinosaur DNA ... yet,
but we do have modern dinosaur bird DNA.
We also have the DNA for a whole slew of different vertebrates,
and since we're all vertebrates,
and we all share the same basic set of genes,
we can look far and wide in living vertebrates
to see how changes in DNA cause particular physical changes.
We can also look at development.
The way a structure forms in an embryo
can provide significant clues as to how that structure evolved.
Combining DNA analysis and development
gives us a powerful double-punch strategy to look at deep-time evolution.
When we looked at mouse mutants that had short and-or fused tails,
and at chicken embryo tail development,
we found some really interesting things.
The evidence so far for how the bird tail went from long to short and fused,
points to somites.
Somites are the embryonic precursors of boney vertebrae
that form generally within a couple of days of development.
We found a very particular change
in the somites at the end of the chicken embryo tail
that snaps a whole bunch of puzzle pieces together.
If a mouse mutant has this same sort of change,
it has a short tail.
It has fusion of vertebrae in that tail.
It also has changes in the formation of spinal nerves,
all of which are exactly what we see in the modern bird tail.
So maybe the fruitless search for Cretaceous intermediate forms
is because there were no intermediates.
A single genetic mutation could have been responsible
for both shortening the tail and fusing it up at the end.
A single genetic mutation can actually have a huge impact
on what an animal looks like.
Human dwarfism is an example of a single genetic change.
You can look at dogs.
You may think that there must be hundreds if not thousands of altered genes
between a chihuahua and a Great Dane,
but scientists are finding out that all of the differences
between all the different dog breeds boil down to just three genes!
If we can understand how changes in DNA cause physical changes,
we can better understand how traits evolve,
and we can better interpret the fossil record.
Since the DinoChicken Project is an out-of-the-box type endeavor,
we've made some out-of-the-box discoveries.
One surprising finding is a correlation with ankylosing spondilitis -
a human disease where your vertebrae fuse together
and the disks between your vertebrae can erode.
It's a rather horrible, debilitating disease,
and it looks a lot like the fusion of bones
at the end of the chicken tail.
Since there's no good animal model for ankylosing spondilitis,
we're proposing to study it in the chicken.
Who knew?
(Laughter)
Following all of these clues in genetics and development,
makes me feel like a detective in my very own whodunit story.
Like detectives, we, as scientists, have hunches,
or hypotheses, as to how we think something happened,
or how something worked.
I can tell you, I've had many, many hypotheses,
and I'm almost always wrong!
But I don't mind; really, really, I don't,
because in every case I've been wrong,
the real biology has been far more amazing
than my imagination could have ever conceived.
Every day, I walk into lab and ask, "How will my mind be blown today?"
And I am rarely disappointed.
This reverence for nature I've had since I was a kid,
when I found out the world is actually billions of years old,
and we didn't just magically appear,
but we, like every other living thing on this planet,
have long and rich, varied and beautiful evolutionary histories.
It's been an incredible privilege working on this project,
and it's been an absolute privilege talking to you today.
Thank you.
(Applause)