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Half a billion years ago on Earth, if you had gone wading in shallow seas,
you might have come across Dickinsonia.
This organism was first described by paleontologists in the 1940s,
and it’s been bugging them ever since.
They couldn’t make heads or tails of it…
and not just because it doesn’t have an obvious head or tail.
Was it an animal? An oddball protist? Maybe a lichen?
But in a paper published last week in the journal Science, researchers used a new technique
to examine biomolecules in Dickinsonia fossils, and they think it’s an animal.
In fact, it may be the oldest known member of the animal kingdom.
And Dickinsonia was big.
Some specimens grew up to 1.4 meters long.
It’s a member of a group of creatures that lived during the
Ediacaran period around 558 million years ago.
Before this period, life on Earth was microscopic.
So these were the first more complex living things that evolved.
Paleontologists had a hard time deciding what Dickinsonia was, though,
partially because they only had trace fossils.
Trace fossils aren’t parts of the organism, like bones, shells,
or woody tissues that can be mineralized and preserved.
They’re something that got left behind, like a footprint.
Because Dickinsonia was a soft-bodied organism that just decayed,
all that’s left is essentially an impression its body made in the mud.
Now, imagine that combined with a lot of heat,
pressure, and weathering which can break down organic molecules.
So paleontologists were stuck debating where Dickinsonia belonged in the tree of life
based on a rough approximation of what it looked like.
That is, until a team of researchers from Australia, Russia, and Germany took a different approach.
They discovered some well-preserved fossils in sandstone cliffs by the White Sea in Russia.
And these specimens were so intact that the researchers could
try to analyze hydrocarbon biomarkers.
Those are the molecular remnants of some organic chemicals that made up Dickinsonia’s soft body.
What they found was the molecular fossils of a type of fat that all animals make, because
it’s a key part of our cell membranes: cholesterol.
If Dickinsonia were a protist like algae or a lichen, the researchers said they would’ve
expected to find higher levels of other molecules such as stigmasteroids and ergosteroids.
But they didn’t. So this study concluded that, because of cholesterol, Dickinsonia is an animal after all.
But there’s still not a consensus among paleontologists,
and these biomarkers may not be conclusive evidence.
So we’ll need more research before this mystery is actually solved.
Now, birds are much more familiar to us than squishy flat things from half a billion years ago.
But there are still some mysteries about their evolution.
Take the syrinx, an organ found only in birds that they use to sing.
It’s a puzzle because scientists don’t really have a good idea how, or why, the syrinx evolved.
But in research published this week in PNAS, an interdisciplinary team of biologists combined
evidence from the fossil record and developmental biology, and examined the anatomy of vocal
organs in animals from cats to salamanders.
This fresh look gave them a better understanding of how birds may have gone from making sounds
with a different organ, a larynx, to making them with a syrinx.
The larynx is a muscular organ that sits at the top of the windpipe in many animals,
including birds. It’s multi-purpose.
It helps direct air towards the lungs, and prevents food from falling in the windpipe
and becoming a choking hazard.
And some animals, including mammals and a few reptiles, make sounds with the larynx,
which is why it’s also known as a voice box.
These animals have vocal folds, mucus membranes that vibrate when you pass air over them.
But birds can’t make sounds with their larynx.
They use the syrinx, which sits at the lower end of the windpipe,
just above where it splits off into the tubes that go into the lungs.
The syrinx also has membranes that vibrate, but not quite like vocal folds.
And it has two independently controlled branches.
This means that many birds, like the wood thrush, can sing two notes simultaneously.
And even though we can see and hear how the syrinx produces beautiful bell-like tones,
its evolutionary history is unclear.
Since reptiles use their larynxes to make noise,
scientists think that early ancestors of birds probably did too.
But sometime after they split from dinosaurs, they evolved an entirely new organ to do the same thing.
Which is kinda weird, because typically a new structure evolves because an existing
structure starts to take on a new function and changes form.
For example, bats didn’t evolve a third set of limbs to get wings.
Their existing arms and hands gradually changed over tens of millions of years to become better at flying.
In the case of the syrinx, a new structure developed but it didn’t serve a new function.
The larynx was already making noise!
These researchers suggested that the evolution of the syrinx may have been driven by changes
in body shape related to flight, or the increasing complexity of birds’ vocal communication.
But they’re not exactly sure how that transition played out.
It’s possible that ancestral birds went silent for a while, losing the ability to
make sound with the larynx, and eventually gaining that ability with the syrinx.
But these scientists think it’s more likely that they had two functional vocal organs at once.
The point is, there’s a lot we don’t know yet.
One thing we do, though, is that the syrinx is generating important questions about how
new structures evolve, and how evolution sometimes reinvents the wheel.
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