Thanks to this month’s impossibly awesome Patreon supporters.
They help keep the show going and so can you!
As a special bonus for joining and existing patrons, we’re sending anyone who is an
Eons patron on November 1st an Eons scale bar as a small thank you for supporting the show.
In 1977, American biologist Carl Woese announced that he’d stumbled on a new form of life.
He’d been developing a new method for reconstructing
evolutionary relationships between organisms,
one based on comparing specific genetic sequences rather than on comparing physical traits.
One day, while analyzing sequences from methane-producing
bacteria, he discovered something remarkable.
While the microbes seemed like typical bacteria, their genetic sequences were wildly different.
Different enough, in fact, that he realized they must actually be part of an early branch
of life that had diverged billions of years in the past and had remained hidden all this time.
Eventually, in 1990, Woese redrew the tree of life.
He proposed that prokaryotes - single-celled organisms without a nucleus - should be split
into two separate domains: Bacteria, and the
new domain Archaea, which literally means ‘the old ones’.
Along with our domain, Eukarya, this left
us with the three-domain system that we’ve been using ever since.
Everything that's alive falls into one of these three groups.
If you’re from planet Earth, you’re either a eukaryote, a bacterium, or an archaeon.
The unexpected discovery of an entirely new
domain of life was pretty huge and surprising - even if they do just look like bacteria.
But, in recent years, it’s been their connection to us that's turned out to be particularly
full of surprises - ones that may mean we have to redraw the tree of life yet again.
Despite being an entire domain of life, archaea
have often been a bit of a blind spot for both scientists and non-scientists.
For a long time following their initial discovery, they were only known to thrive in very particular
and often extreme environmental conditions that are hard to replicate in a lab.
Think low-oxygen deep-sea sediment, or boiling and acidic hydrothermal vents.
Plus, microbes in general tend to be underappreciated
- especially considering their diversity, abundance, and importance in the history of life.
Most folks only think about microbes when
they're doing something to or for them – like, causing disease or making beer.
So archaea have been a blind spot within a blind spot.
But by examining the ones we can culture, and probing the molecules of the ones we can’t,
we've managed to figure out some pretty interesting things about them.
Like bacteria, they’re single-celled and don’t have a nucleus or other organelles.
And they reproduce asexually through a process
called binary fission, in which a cell divides into two daughter cells.
Also like bacteria, they usually have protective
cell walls and propeller-like structures for getting around.
Though, in both cases, these features are
built very differently from those found in bacteria.
As are their cell membranes, which have a
unique molecular composition compared to the other domains.
And we now know that archaea aren’t just confined to extreme habitats.
They seem to be pretty widespread in a range of terrestrial and aquatic environments.
Once we knew what to look for, they started turning up everywhere - even living in and on
our bodies, though none are known to cause diseases.
Tens of thousands of species have been identified to date.
And genetic analyses of these species cluster them into a complex family tree that’s
regularly added to, rearranged, and argued over by researchers.
But they’re all definitely part of the ancient domain Archaea,
billions of years removed from both bacteria and eukaryotes.
But perhaps not equally removed from both.
See, despite looking like bacteria, they’re
actually more closely related to us eukaryotes -- yeah, organisms with large,
complex cells that have nuclei, like animals, plants, fungi, and protists.
And a few shared traits from this ancient common ancestry are still visible.
For example, some of the enzymes they use to process, read and replicate their genetic code
are very similar to ones found in eukaryotes and very different from those in bacteria.
So in Woese’s three-domain system - the
currently accepted tree of life - they’re considered a sister group of ours.
Archaea and eukaryotes shared a common ancestor with each other more recently than they do
with bacteria, so they’re each other’s closest relatives at the domain level.
Which means that archaea are more closely
related to people, dinosaurs, mushrooms, and bananas than they are to any bacteria.
But as our understanding of archaea has grown, an even more radical idea has emerged…
What if eukaryotes don't just share an ancient common ancestor with archaea?
What if we actually evolved within archaea,
and should really just be considered a part of their domain?
Now, a big moment in the origin of eukaryotes was endosymbiosis.
A mysterious ancestral cell engulfed a bacterium and formed a symbiotic relationship with it.
This became the energy-producing mitochondria
that helped fuel the rise of complex eukaryotes, a feature we all still have today.
Could that ancestral engulfing cell have been an archaeon?
Did eukaryotes emerge from a fusion of an archaeon and a bacterium?
Are we actually just a branch of the Archaea family tree?
If we could determine which branch we trace
our ancestry to, that might help answer all of those questions.
But for that, we would need to find a specific
group of living archaea that eukaryotes were particularly related to.
And in 2015, a group of researchers announced they’d found exactly that.
They’d been sampling marine sediment near a field of underwater hydrothermal vents
called Loki’s Castle, about halfway between Greenland and Norway.
And they were able to isolate and sequence enough DNA from the sample to reconstruct a
92% complete genome of an archaeon from a new group they called Lokiarchaeota.
Lokiarcheota were a part of a distinct group
within the Domain Archaea, and many other members have since been discovered around
the world, including Odinarcheota, Thorarcheota, and Heimdallarchaeota.
In keeping with the Norse mythology theme, this group has been named the Asgard Archaea.
When researchers compare the genomes of those Asgard archaea to other archaea,
as well as to eukaryotes, their results generally
suggest that they’re our closest living archaeal relatives.
Some analyses even conclude that eukaryotes arose specifically within the Asgard group!
And their genomes include the instructions
for a bunch of proteins that were previously considered exclusive to eukaryotes.
The discovery of these ‘eukaryote-like’
Asgardians was another major clue in the origins of eukaryotes.
Their existence supports the archaeal origin
hypothesis for eukaryotes by bridging the gap between the two groups.
And some researchers suggest that we might
even trace our ancestry to their particular branch of the Archaea family tree.
Now the debate is far from settled, and we’re
finding more and more clues as our understanding of archaea continues to grow.
But if it does turn out that we descend from an ancient Asgard or Asgard-related archaea
that engulfed a bacterium, gained the mitochondria, and gave rise to all eukaryotes,
that would mean that Carl Woese was both right and also…kinda wrong.
Yes - he was correct in separating archaea
from bacteria, but maybe his three-domain system was wrong to separate them from eukaryotes.
Perhaps the tree of life only has two domains after all.
Hey, hello, it's Blake, hi! If you enjoyed this episode about
some of the tiniest lifeforms on our planet, there’s a lot more where that
came from on another show produced by our team: Journey To The Microcosmos.
Join hosts Hank Green and Deboki Chakravarti
as they dive into the tiny, unseen world that surrounds us.
Check it out over at youtube.com/Microcosmos!
Wanna know more about the mysterious origin of complex life?
Then check out our episode, “How Two Microbes Changed History”.
And may the blessings of Asgard be showered upon this month’s Eontologists!
Jacksy Weiss, Colton, Melanie Lam Carnevale,
Chase Archambault, Annie & Eric Higgins, John Davison Ng, and Jake Hart.
Become an Eonite at patreon.com/eons and you
can get fun perks like submitting a joke for us to read.
Here’s one from The Burgins.
What do you call a velociraptor that can play the piano?
Talon-ted!
And as always thanks for joining me in the Adam Lowe studio.
Subscribe at youtube.com/eons for more ancient adventures.
Eu-car-ya, Eu-care-ya, Eu-care-yotes, Eu-care-yah...
ar-KEE-ah!